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Lead Glazes for Ceramic Foodware An ILMC Handbook

i

Lead Glazesfor Ceramic

Foodware

The

International

Lead

Management

Center

Richard L. LehmanRutgers University


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Lead Glazes for Ceramic Foodware

Richard L. Lehman Rutgers University

The International Lead Management Center Research Triangle Park, NC USA


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A publication of the International Lead Management Center Research Triangle Park, NC United States of America

Copyright 2002 The International Lead Management Center All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording or any information storage or retrieval system, without permission in writing from the International Lead Management Center

Printed in the USA by International Lead Management Center, Inc. P.O. Box 14189 Research Triangle Park, NC 27709-4189 USA

Author: Lehman, R. L. (Richard Long), 1949 –

Lead Glazes for Ceramic Foodware 2002 First Edition


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ABOUT THE AUTHOR Dr. Richard Lehman is Professor in the Department of Ceramics and Materials Engineering where he conducts basic and applied research in new and traditional forms of glass, ceramics, and polymers. He has particular interest in lead-containing glasses and glazes, glass raw materials and melting reactions, and the chemical durability of glasses and glazes. Dr. Lehman is also active in the field of immiscible polymer blends and is Director of the Center for Advanced Materials via Immiscible Polymer Blends at Rutgers. In addition to research activities, Professor Lehman is active in graduate and undergraduate instruction and he participates in external consulting activities with local, national, and international industries and organizations. Prior to joining the Rutgers faculty in 1982, Dr. Lehman gained eight years experience at Johns-Manville and FMC Corporations where he worked in the fiberglass and industrial chemical fields. Dr. Lehman has over 100 publications and 25 patents on the processing and properties of materials and he is a Fellow of the American Ceramic Society. He received his BS., MS., and Ph.D. degrees from Rutgers University.


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ABOUT THE INTERNATIONAL LEAD MANAGEMENT CENTER

The International Lead Management Center was founded in July 1996 and is based in Research Triangle Park, North Carolina, in the United States of America. The Center was established by the international lead industry in response to the need for coordinated international action on the issue of lead risk management.

Although ILMC was founded and is sponsored by the lead-producing industry, liaison and cooperation has been established with the lead products applications sectors. ILMC expertise and advice is therefore available across the full range of issues associated with production, applications, recycling and disposal.

ILMC complements and supports existing international risk management activities and responds to the individual needs of countries who wish to introduce such projects in either industry or their local communities.

The Center welcomes inquiries and requests for either advice or assistance. Requests for assistance are assessed through the Center's considerable network of technical, metallurgical and occupational expertise. ILMC provides assistance by working with national governments and interested parties to identify the most appropriate risk management options.

ILMC activities are supported by an extensive and growing database containing consensus health materials and detailed lead risk information for mining, refining, manufacturing, recycling and associated case studies. The database will also maintain a register of those agencies and organizations able to assist with the funding of community projects.


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PREFACE

Glaze development and design has always been one of the more delicate and daunting tasks in traditional ceramic technology. So many oxides from which to choose and the properties of the glaze, always changing with firing conditions and body interactions, must match the underlying ceramic within close tolerances to give a professional, defect-free, and functional glazed surface. No wonder glaze formulation has maintained its artistic mystique even in the face of comprehensive scientific study and understanding. Lead glazes, in particular, offer a serious challenge to the formulator. Few other glaze constituents provide such outstanding properties to the glaze and forgive more processing transgressions, yet must be carefully formulated, handled, and processed to avoid exposing workers and end-users to lead. The ability to formulate and engineer lead glazes to be safe to all those involved, from the potter to the end-user, has been a major technological advance of the past 100 years.

In assembling this book, my goal was to collect a comprehensive body of technical data to enable all those interested in lead glazes for foodware to easily access relevant information in a single source. In striving for this goal several levels of information have been incorporated to address the needs of various readers. Detailed glaze compositions and lead extraction data are provided for the ceramic technologist, material handling and safety information for the producer, and an overview section for the non-ceramist. Hopefully this information assists in the safe use of lead-containing ceramic glazes and decorations in a range of industries and geographies from the large manufacturers of the industrialized world to smaller cottage industries in less developed areas.

Richard Lehman Princeton, New Jersey


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ACKNOWLEDGEMENTS

As with all books, this book was written and assembled only with a great deal of assistance. I wish to thank all of those who helped in the preparation of the notes and the assembly of the text. I am particularly grateful to the authors of the 1974 ILZRO Handbook on Lead Glazes for dinnerware who made much of the original material available for the present text. In addition, contributions from several key sources were invaluable, including Ferro Corporation, Lead Industries Association, US Borax, Hammond Lead and the American Ceramic Society.

Sections of Chapter 2 are adapted from, J. B. Wachtman, Ceramic Innovations in the 20th Century, pp 103 – 105. Reprinted with permission of The American Ceramic Society, PO Box 6136, Westerville, OH 43086-6136. Copyright 1999 by the American Ceramic Society. All rights reserved.

Sections of Chapter 3 are adapted from, F. Singer and W. German, Ceramic Glazes, Borax Consolidated Limited, London. Reprinted with permission of Borax Limited.

Sections of Chapter 6 are adapted from, Lead Industries Association Manual, Lead In The Ceramic Industries, Section 10. Reprinted with permission of Lead Industries Association.

Sections of Chapter 9 are adapted from, A. Huber, "Decoration of Porcelain, Earthenware and Bone China”, Ferro Corporation, West Wylie Avenue, Washington, PA 412-223-5900.. Reprinted with permission of A. Huber and the Ferro Corporation.

ISO Test methods in Appendix B are reprinted with permission of the International Standards Organization, Geneva, SW.

Sections of Appendix D are from Hammond Lead Company, reprinted with permission.


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Table of Contents About the Author iv About the International Lead Management Center v Preface vi Acknowledgements vii Chapter 1 1 Lead Glazes for Foodware – An Overview A stand-alone primer for the non-ceramist

Use of lead glazes in contact with food. 1 Measuring Lead Migration from Glazes 2 The formulation and application of lead glazes 2

Historical Development Ceramic Ware Raw Materials Safe Lead Glaze Compositions Glaze Application Methods Firing Method and Temperature Ranges Decorations

Chapter 2 10 Introduction

Foodware Safety 10 Lead Glazes 10 Lead Free Glazes 11 Glazes Function and Texture 12 Clear Glazes 13 Opacified, Matt, and Special Texture Glazes 13

Chapter 3 16 Lead Glazes and Their Development

Historical 16 Function of lead in glazes 16 The Use of Lead Frits 17 The Use of Boric Oxide in Lead Glazes 18 Applications for Lead Glazes 19


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Chapter 4 21 Lead Migration From Vitreous Surfaces

Factors Influencing Lead Migration 21 Effect of Glass Structure 22 Role of Specific Oxides 24

Chapter 5 25 Glaze Raw Materials

Frits 25 Description Types

Lead Oxides 26 Lead Monoxide -- PbO Lead Orthoplumbate or Red Lead, Pb304

Lead Hydroxides and Carbonates 28 Crystalline Lead Oxide-Hydroxide Pb(OH)2 2PbCO3 Pb(OH)2

Lead Silicates 30 Chapter 6 32 Materials Handling

Safe Handling Practice 32 Hygiene and Medical Monitoring 33

Proper Plant Hygiene Proper Instruction and Supervision Regular Medical Monitoring

Material Safety Data Sheets (MSDS) 34

Chapter 7 37 Glaze Compositions and Lead Migration Behavior

Summary of Experimental Results 37 Tests on Cone 3-5 Clear Production Glazes 39 Cone 5 Clear Glazes for Institutional Dinnerware 41 Effect of Coloring Oxides on Lead Release 42

Cu, Cr, and Co in a Clear Cone 4 Glaze Fe and Mn in a Clear Cone 4 Glaze Fe, Mn and Zircon in a Clear, Cone 02-4 Glaze Cu Fe, Mn and Zircon in a Clear, Cone 02-4 Glaze Cu, Cr and Co in a Low Temperature Cone 05 Glaze


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Effect of Commercial Stains on Lead Release 47 Pb-Sb Yellow Stain In Cone 08-04 Glaze Cr-Al Pink Stain in Cone 06-02 Glaze Sn-Sb Gray Stain in Cone 02-4 Glaze Co-Cr-Fe Black Stain in Cone 1 Glaze Cr-Al Pink Stain in Cone 02-4 Glaze Various Commercial Stains in a Cone 02-4 Glaze

Effect of Opacifying Oxides on Lead Release 54 Standard Glaze Fired to Cone 4 and 01 High Lead Glaze Fired to Cone 4 and 01.

Effect of Variations in Alkali, Alkaline Earth, Boric and Zinc Oxides and Beryl in a Cone 4 Glaze 56

Effect of Alkali Oxides Effect of Alkaline Earth Oxides Effect of Boric Oxide Effect of Zinc Oxide Effect of Beryl

Effect of Base Glaze Variations: Lead Silicate (PbO·1.3SiO2) 62 Additions of Alkali Oxides to Base Glaze Additions of Alkaline Earth Oxides to Base Glaze Additions of Opacifying Oxides

Effect of Base Glaze Variations: Lead Silicate (PbO·1.5SiO2) 68 Latin Square Experiment Knoop Hardness Effects of Al2O3, B2O3 and ZrO2

Repeated Extractions on the Same Glaze Surface 74 Repeated Tests on Clear, Cone 4-5 Glaze Repeated Tests on Low Temperature Cone 05 Glaze Repeated Tests on Low Temperature Uranium Red Glaze

Chapter 8 77 Effect of Glaze Processing Variables Introduction 77

Relationship Between Lead Release and Glaze Thickness 78 Effect of Glaze Thickness of a Cone 4 Fritted Lead Glaze 80 Effect of Glaze Thickness in Cone 07 Glazes 81 Effect of Different Bisque on Lead Release 82

Clear Cone 4 Glaze with Coloring Oxides Lower Temperature Glaze with Coloring Oxides Clear Cone 4 Glaze with Opacifying Oxides

General Effects of Varying Firing Time and Temperature 86


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Effect of Glaze Thickness, Firing Time and Temperature: Commercial Glazes 88

Factorial Experiment Design Results

Effect of Glaze Thickness, Firing Time and Temperature: Laboratory Fritted Glazes Effect of Under-Firing on Lead Release Properties of Underfired Production Glazes

Chapter 9 98 Decoration of Dinnerware

Requirements for Ceramic Colors 98 Composition and Preparation of Ceramic Colors and Inks 99

Overview Composition and Preparation of Pigments

Frits, Composition and Manufacture 101 Composition and Preparation of Media 102 Application of Ceramic Decoration 102

Overview Preparation of Printing Materials Preparations of Printing Inks Application

Firing 105 Summary 107

Appendix A: 109 Fritting Approaches to Control Lead Solubility

Solubility Tests On Frit Powders 115 Lead Extraction From Glazed Surfaces 119 Relation of Glaze Structure to Durability 120

Appendix B: 127 Tests For Lead Extracted From Glazed Surfaces

ASTM C738 - 94 (Reapproved 1999) Standard Test Method for Lead and Cadmium Extracted from Glazed Ceramic Surfaces. 127

1. Scope 2. Summary of Test Method 3. Interferences 4. Apparatus


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5. Reagents 6. Procedure 7. Report 8. Precision and Bias

ISO Standard 6486: Ceramic Ware, Glass-Ceramic Ware, and Glass Dinnerware In Contact With Food -- Release of Lead and Cadmium 131

0 Introduction 1 Scope 2 Normative References 3 Definitions 4 Principle 5 Reagents and Materials 6 Apparatus 7 Sampling 8 Procedure 9 Expression of results 10 Reproducibility And Variability 11 Test Report 12 Permissible limits

Appendix C: 146 Materials Handling

Proper Plant Hygiene 146 Proper Instruction and Supervision 148 Regular Checks By Plant Physician or Medical Director 148 Health Risks of Lead Compounds 148

Assessing Personal Exposure Health Hazard Information Physical Properties Major Health Effects Noted from Lead Exposure

Occupational Exposure to Lead: 152

Appendix D: 154 Sections of Materials Safety Data Sheets for Selected Lead Compounds Appendix E: 176 Pyrometric Cone Properties

Temperature Equivalents of Orton Large Pyrometric Cones 176


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Pyrometric Cone Table Notes: 177 Cone Position Diagram 178

Appendix F: 179 Glossary Of Terms Appendix G: 183 Bibliography and References


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CHAPTER 1

Lead Glazes For Foodware – An Overview

A stand-alone primer for the non-ceramist Uses of Lead Glazes in Contact With Food.

Lead glazes can be safely used on a wide variety of ceramic ware, such as earthenware pottery, stoneware, and a range of porcelain type bodies. See Appendix F for a glossary of these and other terms. In fact, the excellent

properties and wide processing latitude provided by lead oxide [PbO] in the glaze structure make it an ideal glaze component over a wide range of ceramic ware compositions and firing ranges. When properly processes, the lead oxide, which is typically present in concentrations ranging from a few percent up to nearly 50%, is chemically combined in the glass structure and the amounts that can be extracted by food

substances or other common acidic media are extremely low, well below established limits set by the FDA, ISO, and other regulatory groups. When isolated instances of high lead release have been measured for ceramic ware, the causes typically fall into one of three categories: [1] the glaze was improperly formulated and did not contain the proper mixture of ceramic oxides as described subsequently in this text, [2] proper firing practices were not followed and the lead oxide was incompletely combined with the silicate glaze matrix, or [3] decorations and/or coloring agents were inappropriately

Glazed earthenware bowl, cone 06


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used such that the lead in these agents was easily extracted, or they were applied on inappropriate parts of the ceramic ware.

Measuring Lead Migration from Glazes

Strict international standards exist that limit the amount of lead and cadmium that can be released from ceramic ware in contact with food. Such standards are defined by the International Standards organization [ISO] and are discussed in greater detail in later chapters. Generally, the tendency of lead to migrate from the glaze is greatest under acidic conditions and the standard ISO test defines a 24 h exposure of the ware to 4% acetic acid at room temperature, about 22o C. Such tests must be conducted under controlled laboratory conditions and numerous laboratories are equipped to conduct these procedures. Ware passes this test if the acetic acid contains less than 0.5 – 2 parts per million [mg/l] of lead after the test, depending on the size and shape of the ceramic ware. Flat plates pass if they release less than 0.8 mg/dm2 according to the established procedure. Well-manufactured foodware with well-designed lead glazes will easily pass these limits. In addition to the formalized procedures established by ISO, there are other methods for assessing the lead migration potential of foodware, ranging from alternative laboratory methods such as potentiometric electrode methods to much simpler test kits available to the consumer.

The Formulation and Application of Lead Glazes

Historical Development

According to written accounts and from the analysis of archeological artifacts, lead has been used in foodware from ancient Egyptian times to the present. The broad popularity of lead as a glass and glaze constituent stems from the numerous processing and property characteristics it imparts to the glassy state. Among other attributes, lead lowers the melting point of glazes, widens the processing range, imparts surface smoothness and brilliance, and

Atomic absorption instrument used in measurement of lead release.


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gives the glaze the ability to heal manufacturing defects such as blisters and pinholes during processing.

The main disadvantage of using lead in glazes is the high toxicity of lead when absorbed into the body. This can occur when the glaze is not properly designed and/or is improperly applied to the ware, or poisoning can occur in the factory where lead chemicals are being handling in the glaze preparation process. The historical use of highly soluble lead compounds, such as white lead, exacerbated this hazard. Beginning in the late 1800’s, new lead compounds were developed that were much less bio-

available and thus reduced the uptake of lead by pottery workers. Most successful of these approaches was the fritting of lead, a process by which lead oxide is chemically combined in a glassy matrix and crushed to a fine powder. These glassy powders, or frits, keep the lead compounds tightly bonded and have greatly reduced the hazards of working with lead. They are a key element of standard lead glaze practice today.

Ceramic Ware

A wide range of ceramic pottery processes exist today, ranging from the traditional methods of hand fabrication and glazing followed by one or more firings in simple periodic kilns -- much as has been done for centuries -- to modern efficient processes where dinnerware is automatically formed by jiggering machines, fired, glazed and fired again in high speed kilns. In all instances two of the most important variables, as will be repeatedly discussed in later chapters, are the nature of the underlying ceramic body and the temperature to which the ware is fired. Ceramic body types fall into several categories as indicated in the table:

Ceramic Body Typical Firing Description Earthenware Low, up to cone

03 Often called pottery, these materials are porous, usually colored or pale white and were originally prepared from natural mixtures of clay and other ceramic materials.

19th Century Pottery


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Stoneware Moderate to High Cone 2 – 10

Dense, impermeable ware, usually colored to pale.

Porcelain Moderate to High Cone 2 – 10

Dense, white ware, often with high translucency, made from highly refined raw materials.

Raw Materials

The raw materials used for lead-containing glasses in contact with food are principally the same raw materials used for other earthenware, stoneware and porcelain glazes. These materials, mostly beneficiated mineral materials, include clay as a source of SiO2 and Al2O3, flint [SiO2], feldspar [(Na,K)2O, Al2O3, SiO2], whiting [CaO], dolomite [(Ca,Mg)O], zinc oxide, talc, ulexite/colemanite [B2O3] and various frits. Frits are premelted glassy materials that contain certain percentages of important glaze oxides and which are crushed to a fine powder and sold as a commodity to the ceramic industry. Frits are desirable because they enable water-soluble materials to be combined in an insoluble glass matrix. Many alkali and borate oxides are thus rendered more useful as a glaze ingredient by fritting. Lead compounds are also often combined with silica and other ingredients and fritted. The lead in these frits is much less bio-available than the lead in more traditional and more soluble lead oxides and carbonates. Modern practice for manufacturing lead glazed foodware minimizes the use of highly soluble compounds such as white lead and favors the use of more insoluble compounds such as lead oxide or lead silicates.

Safe Lead Glaze Compositions

The formulation and use of lead in glazes in contact with food requires the development of a lead migration resistant composition and a proper firing cycle to permit the glaze raw materials to fuse together into a homogeneous acid resistant glassy surface coating. Although the fundamental design of safe lead glazes is not trivial and requires skill in glass and ceramic formulation technology, some general trends can be identified that will enable the layperson to understand the general approach.


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Glazes are glassy substances that are comprised of a silicate network modified with alkalis and other elements to produce the desired properties. Safe lead glazes are resistant to the attack of various food acids, which seek to extract lead and other modifier elements from the silicate network. Generally speaking, safe glazes have a large amount of silica and a low amount of alkali network modifiers. More specifically, oxides can be categorized by their role in the glaze and their general effect on lead release, as indicated below.

Glaze Network Formers

Glaze Stabilizers Glaze Fluxes

Strongly bind lead

Intermediate Promotes lead diffusion

SiO2 CaO PbO ZrO2 MgO Na2O Al2O3 Al2O3 K2O B2O3 B2O3

[note: Some oxides appear in more than one category and their role depends on the type and amount of other oxides in the glaze] The formulation of glazes is discussed in terms of the glaze molecular

formula and the relative number of moles of the various oxides. One mole of SiO2, for example, is equal to the number of grams corresponding to the molecular weight of the oxide. SiO2 has a molecular weight of 60.1, so one mole of SiO2 equals 60.1 grams. The weight of one mole of other oxides follows accordingly.

One class of foodware safe lead glazes contains low amounts of fluxes such that the moles of the stabilizers and fluxes, as noted above, is approximately 25% of the mole amount of the network formers. Examples of three such glazes, maturing from Cone 06 to 5, are given in the table.

Example Glazes for Foodware Surfaces

[Firing temperatures for corresponding large cone are at 150o C/h heating rate, no soak. Soaking will lower maximum required temperature. Use cones to precisely measure heat work imparted to ware]

Oxide Cone 5 Cone 1 Cone 06


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K2O 1.72% 2.48% 1.52% Na2O 3.06% 1.63% 1.60% CaO 7.64% 9.50% PbO 16.07% 15.65% 23.10% CaF2 18.33%

Al2O3 9.57% 9.57% 6.48% B2O3 6.03% 7.33% 10.70%

SiO2 55.90% 53.84% 37.30% ZrO2 0.98%

100.00% 100.00% 100.00% Firing Temperature, C.

1196 1154 999

Lead release, base glaze, 24 h, 4% acetic acid, 22o C, mg/l.

0.16 0.02 0.11

Glaze Application Methods

A glaze slip, or water suspension, is prepared from the raw materials selected for use in the glaze. In so-called raw glazes, the individual raw materials [clay, flint, feldspar, whiting, lead oxide] are dispersed directly in water to produce a suspension about the consistency of heavy cream. Fritted glazes usually contain all the necessary oxides precombined in the frit, except for the oxides corresponding to a 10% addition of clay or similar suspension agent that is added with the frit to the water. Binders such as sucrose or carboxy methylcellulose are sometimes added to

Applying glaze by the dipping process


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improve the dry strength of the glaze and avoid defects. The glaze can then be applied to the ware in a diverse number of ways, such as the brushing, sponging and dripping methods favored by artists and some potters, to the highly mechanized spraying and waterfall methods used in industry. Hand dipping of ware is a widely used practice [photograph] in large industrial plants and in small potteries. After sufficient drying at ambient or in a heated dryer, the ware is ready for firing.

Firing Method and Temperature Ranges

The firing of ceramics is defined and controlled by a seemingly archaic method of temperature measurement that relies on observation of the

softening and melting of miniature ceramic cone structures formulated from various ceramic formulations [see photo of partially and fully softened cones in a kiln with fired bowl]. This method, the method of cones, is in reality a highly accurate method for measuring heat work, the product of time and temperature, achieved during the firing process. It is more accurate, reproducible, and subject to fewer errors than many electronic methods based on

thermocouples and the like. Cones do, however, have an unusual labeling sequence as illustrated in the table of cones and corresponding temperature values given in Appendix E. Increasing temperature of firing is defined by decreasing cone numbers that begin with a zero (i.e. 010, 09, 08, etc.) until 01 is reached. Subsequent cone numbers commence with cone 1, 2, 3, 4, etc.

Most ceramic dinnerware is firing in the cone 06 [1000o C] to cone 10 [1305o C] range, with earthenware and other porous or highly fluxed compositions occupying the lower part of this range and stoneware and porcelain the upper part.

Firing methods have varied greatly since antiquity and current practice continues to demonstrate a great deal of diversity. The ancient methods

Pyrometric cones used to measure firing heat work. Slightly slumped free-standing cones at left, Fully down plaque cones at right.


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consisted principally of wood or coal firing in kilns insulated with refractory stones or simple bricks made of fireclay. Temperatures were usually low and most ware produced was earthenware. These kilns were often located on hills or facing windward directions to promote combustion. The ancient firing methods have gained renewed popularity in modern times among artists and specialty potters. However, the vast majority of commercial ware is produced in highly efficient periodic or continuous kilns fired with gas or electricity. Periodic kilns are filled with ware during a production shift and then fired over a period time ranging from eight to 36 h or more, depending on the size of the load and the type of ware. Continuous kilns fire ware continuously as it is loaded and pushed through the kiln on cars, belts or rollers; the thermal profile of the kiln and the speed at which the ware is pushed through determines the firing time/temperature cycle for the ware. An efficient process developed in recent decades is that of fast firing. In this process, the ware is advanced through a roller hearth kiln in a single layer and at great speeds. The speedy process results in fast heating and cooling, which requires special formulations to avoid the introduction of flaws during the process, and the complete firing cycle can be just an hour to two.

Decorations

Decorations represent an important area of technology in the manufacture of ceramic foodware. From an aesthetic and marketing perspective the use of decorations is critical to the generation of beauty, consumer appeal, and marketplace competitiveness. Principal types of decoration include decals, painted images, direct ink transfer images, and metalization. Decorations can contain lead, cadmium and other toxic materials that require careful application to foodware ceramics in order to avoid consumer ingestion. The three major issues that influence decorated foodware safety are the composition of the decorations, the area of the foodware to which they are affixed, and whether the decoration is underglaze or overglaze.

Decorations are mixtures of pigments and matrix media. The role of the matrix media is to provide a substance into which the pigments can entirely or partially dissolve and/or to provide a bonding agent that adheres the pigments to the ware. The formulation of pigments and the matrix media determines the stability of the decoration with respect to end-use conditions. Although a definite trend exists in the industry to reformulate decorations


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away from toxic materials, some colors (bright red for example) are difficult to produce without toxic materials [Cd in the instance of red]. Special methods, such as encapsulation, have been developed to address these issues.

When possible, it is desirable to position decorations away from direct food contact areas or other high-risk vicinities, such as the rim of a glass or cup that comes into contact with the consumer. Various regulatory guidelines either prohibit decoration within one centimeter of the drinking rim of such ware or strictly limit the lead release rates from these areas. One method to virtually eliminate consumer exposure to toxic decorations is to put the decoration under the glaze, i.e. to glaze over top of the decoration. This seals the decoration under a layer of glaze and precludes attack from food or dishwasher solutions. However, a major drawback of this approach is that often-sensitive decorations are required to withstand the harsh high temperature glaze-firing environment in contact with corrosive molten glaze. Special methods exist for fabricating overglaze decorations with durable coatings and fluxes such that they partially dissolve into the glaze when fired to combine the best features of overglaze and underglaze methods. These approaches have collectively become know as inglaze decoration.

Hand painted decoration


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CHAPTER 2

Introduction Foodware Safety

One of the most important contributions of ceramic technology has been the development of vitreous technology that has ensured the safe use of ceramic and glass foodware. Two general approaches to foodware safety have followed nearly parallel paths during the past 100 years; the careful formulation, control and processing of lead- and cadmium-containing glazes to assure low migration levels, and the formulation of a new category of glazes virtually free of the most toxic of the traditional constituents, principally lead.

Lead Glazes

The use of lead in ceramic foodware has an extensive history. According to written accounts and from the analysis of archeological artifacts, lead has been used in foodware from ancient Egyptian times to the present. The broad popularity of lead as a glass and glaze constituent stems from the numerous processing and property characteristics it imparts to the vitreous state. The addition of lead oxide to silicate glass or glaze compositions lowers the fusion point, widens the processing range, reduces surface tension, and permits greater flexibility in formulating a composition to achieve the desired properties of low expansion, a smooth surface, and high brilliance. Lead glasses and glazes are highly resistant to devitrification, have good chemical durability, and have the ability to heal body defects such as blisters, pinholes, drying cracks and other defects of the clay surface.


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However, if lead glasses or glazes are improperly formulated or fired, toxic amounts of heavy metals can be release via migration to food substances in contact with the defective vitreous surface. The problem of lead migration was recognized early in this century and an increasing level of research was focused on fritting of lead oxides to reduce the lead availability and on understanding the proper formulation of lead glazes to minimize migration by interdiffusion. This effort culminated in the 1974 Geneva Conference on Ceramic Foodware Safety at which state of the art technology was presented and the groundwork was laid for broad international standards on the test methods and permissible limits of lead and cadmium release from foodware surfaces. At the close of the century, the ISO completed its third issuance of international foodware standards that are the foundation for regional standards and for safe worldwide production and trade of ceramic products among developed countries.

Lead Free Glazes

An alternate approach to minimizing lead migration is the total avoidance of lead in the glass or glaze composition. Initial studies on leadless glazes pre-date the century, but significant formulation efforts commenced during

World War II when shortages of lead oxide prompted investigation of low-temperature opaque glazes. Substitutions were tested in which other fluxes replaced part or all of the lead oxide constituent. More recent emphasis on leadless glass and glaze development occurred in the 1980's and 90's as environmental, occupational safety, and lead migration regulations became stricter.

Three approaches to leadless compositions have been pursued. Direct substitution of bismuth for lead is the most obvious and produces adequate results. However, bismuth can impart a yellowish color under certain circumstances, the supply of bismuth is limited, the price is high, and the toxicity of bismuth itself may be an issue. A second group of leadless

HIGH GLOSS GLAZE ON CHINA TEA


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compositions uses zinc and strontium to provide the necessary fluxing. These glazes are glossy and fire well, but color development is poor. A third approach is toward alkali borosilicate [ABS] formulations. These glazes rely on alkali borate fluxing and a typical composition may contain approximately 10% B2O3 and 10% (Li, Na, K)2O by weight. The ABS glazes are becoming widely used, particularly on bone china due to the high expansion of the ware, but significant problems remain with its use. Higher firing temperatures are required to produce a smooth glaze surface, the leadless glazes react less aggressively at the body interface, defect rates are higher, and decoration is difficult. Continued development should result in increased performance and acceptance of this system.

Overall, the two approaches to safe ceramic foodware outlined here have produced outstanding results and have made a major contribution to world health. These methods continue to be practiced and developed on a worldwide basis with the desirable result that toxicity issues related to ceramic and glass foodware are rapidly decreasing. Indeed the isolated issues of lead poisoning associated with faulty ceramic ware have virtually vanished, and when such instances do occur they result from a failure to comply with good manufacturing practice

Glazes Function and Texture

Glazes on ceramic objects serve several purposes. A glaze seals the surface of the body to reduce exposed porosity, an important feature with regard to cleanability and safety for foodware surfaces. Food residues and bacterial contamination can readily adhere to porous ceramics but not to smooth, glazed surfaces. A glaze also adds hardness, environmental durability and wear resistance to a body as well as aesthetic qualities. Although the base ceramic ware is comprised of a mixture of glass and crystals that vary over a range, such as represented by porcelain and earthenware, glazes are nearly 100% glass. Varying amounts of crystals that act as opacifying agents or colorants are sometimes added and will alter the appearance of the glaze. The glass material in the glaze itself may also be colored by dissolved metal oxides. More than one glaze may be applied, especially if decorative glazes are being used. These various layers may be fired at different temperatures. One or more of the layers may also be fired in a single step along with the base ceramic ware. While this reduces the


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number of firing steps and saves on costs, it offers less flexibility of glaze material choice. Various categories of glazes will be subsequently discussed.

Clear Glazes

Glazes with no opacifiers can be used directly on a body or on an opaque glaze or engobe. The clear glaze gives hardness, scratch resistance, durability, and brilliance that an engobe cannot exhibit, or when a highly opacifying glaze is not adequate.

The constituent oxides of a clear glaze composition vary greatly, but usually include silica, alkali and alkaline earth oxides, alumina and lead oxide. This silica is the glass former, forming the basic structure of the material. The alkali oxides (usually Na2O or K2O) flux the silica, i.e. lowering the melting point to a temperature which is achieved by common kilns, as well as lowering the viscosity. The alkaline earth oxides (usually CaO or MgO) and alumina increase the environmental durability of the glass. Lead oxide takes part in network formation, as well as improving flow properties and altering optical properties.

Clear glaze can be colored while maintaining transparency using various species that dissolve in the glass matrix. The firing conditions, such as using a reducing environment, can affect the color that these species impart on the glaze. Table I. lists some of the ions used to color leaded glaze.

Opacified, Matt, and Special Texture Glazes

To mask the body and give various appearances to the surface of an object, many types of additives are used. Opacifiers are used to give the glaze an intrinsic color and characteristic apart from the body. The result is most often shiny due to the glass matrix around the opacifying additives, but to a lesser degree than in the clear glazes. An opacified glaze may be covered

CLEAR GLOSS GLAZE


14

with a clear glaze in another firing in order to impart a brilliance that may not be attainable by the opacifying layer.

The opacification can be caused by several different phenomena. The most common is addition of crystalline material that does not melt during firing, such as tin oxide. Light reflects off the boundaries between the crystals and the glass, or refracts through the crystals at different angles, creating a white appearance. The optical properties of the crystals, including impurities, will determine if any color results from these reflections. A similar optical situation occurs if the second material in the glaze is a gas, either air or some other gas evolved from the constituent materials during firing, such as carbon dioxide. Another less common situation occurs if two types of glass phases are present, also creating reflection and refraction, resulting in dispersed white light.

Other textures may be achieved using alternate additives. Matts impart a partially diffuse reflective surface. These tend to offer less wear resistance and more porosity. Other special textures are also possible using a wide

variety of materials, although the more exotic types are used for non-food surfaces. The mechanisms discussed above occur in these materials also, but usually on a more heterogeneous level. For example, very large bubbles, crystals, or macroscopic areas of different compositions create much more striking visual effects. In addition, the use of metallic particles will drastically change the effect on the incident light.

It is important to keep in mind that these unusual textures affect not only the appearance of the object, but other properties as well. Large bubbles and crystals affect the strength of the materials, often acting as stress concentrators. The chemical and mechanical durability, as well as surface porosity of such materials is also altered. Coating such a surface with a layer

TEXTURE GLAZE ON LOW TEMPERATURE STONEWARE


15

of clear glaze may be used to reduce these problems, but may result in an undesirable appearance.


16

CHAPTER 3

Lead Glazes and Their Development Historical

Lead compounds were probably some of the earliest materials used for the production of glaze coatings on earthenware. The incorporation of lead oxide produces a molten glaze that has a low viscosity at low temperatures due to its excellent fluxing properties. Furthermore the lead silicate that is formed has a high refractive index, producing a brilliance that cannot be achieved by alternative oxides. The result of these properties is that lead glazes can exhibit smooth brilliant finishes and have wide latitude during firing.

In earlier times lead sulfide ore (galena) was dusted on to damp clayware and fired to produce a crude form of lead glaze. This was later superseded by the production of raw glazes using lead compounds such as litharge (PbO) or red lead (PbS04) and white lead (2PbCO3. Pb(OH)2) suspended in water together with clay. The dense oxides of lead are granular in form and tend to settle out rapidly from suspension. However, white lead, the basic carbonate, has a flaky structure that suspends well in water and consequently it was widely used.

Function of lead in glazes

Lead oxide is a valuable component of many glasses and glazes. The importance of lead oxide in such glasses has received wide discussion. Some of the general characteristics of glazes, obtained from their lead oxide content, include:


17

1. Low Melting Range. PbO is one of the classic network modifiers, which has had widespread and continuous use in glazes from early times. The strong fluxing action of PbO allows the formulation of glazes, which mature at relatively low temperatures in comparison with their leadless counterparts.

2. Wide Firing Range. PbO in the glaze reduces viscosity and allows for satisfactory maturation over a wider firing range. Because of this greater margin of safety with variations in firing, these glazes are frequently referred to as being more foolproof.

3. Low Surface Tension. PbO imparts low surface tension, which is chiefly responsible for the smooth flow and generally high gloss of these glazes. Low surface tension is the property that contributes to the ability of lead glazes to heal over blisters, drying cracks, and other defects in the glaze surface. Low interfacial tension contributes to the good wetting and adherence of the glaze to the body. Low surface tension coupled with the wide softening range of lead glazes accounts for their superior maturing qualities.

4. High Index Of Refraction. Brilliant glaze surfaces are attained due to high index of refraction imparted by PbO.

5. Resistance To Devitrification. The presence of PbO in the glaze reduces any tendencies towards surface crystallization or devitrification of the glaze.

The Use of Lead Frits

The main drawback to the use of lead is its high toxicity when absorbed into the body. Unfortunately the oxides and the basic carbonate are all soluble in the hydrochloric acid present in the stomach, thus providing an easy route for assimilation of lead. Chronic lead toxicity was widely recognized in the pottery industry from the last century, but it was not until the latter part of the 19th century that efforts were started to try to eliminate this widespread problem. Although lead poisoning occurred mainly in the pottery works where the raw glazes were mixed and applied, it could also occur in the general population if unacceptably high quantities of the toxic metal were released from the glaze during normal household use.


18

Since lead was an essential constituent of many glazes, steps were originally taken to minimize the risk of assimilating lead compounds by improving cleanliness and hygiene in the potteries. Following these changes, development work was undertaken to produce lead compounds that would be largely insoluble in stomach acids. Fritting to decrease lead solubility was developed in the last decade of the 19th century and is still in use today. The

raw lead compound, normally red lead is fused together with silica to produce a silicate glass of low solubility. Obviously this process involves some hazards in both the mixing and handling stages as well as from lead fumes carried away with the combustion gases into the atmosphere. However, with careful pollution control and strict hygiene regulations within the factory it is possible to safely produce lead frits having a very low solubility in stomach acids. In addition to the preparation of frits, recent raw material practice for lead glazes has evolved to use other raw materials in which the lead oxide is reacted with other constituents to reduce health risks and also to assist in the reaction of the glaze. Tribasic lead silicate [3PbO - SiO2] is one such raw material. Illustrations of tribasic lead silicate granules and a typical frit powder are shown.

The Use of Boric Oxide in Lead Glazes

Since boric oxide is an essential component of many lead glazes but cannot usually be incorporated in the lead frit since it would raise the lead solubility of the frit to an unacceptable level, it is frequently included in the form of a separate "borax frit". The borax frit will also normally contain some other major oxides required in the final glaze. It is therefore possible to standardize on a small number of lead frits without limiting the range of lead

TRIBASIC LEAD SILICATE [LEFT] AND A LEAD SILICATE FRIT


19

glazes that may be produced. The lead silicate manufacturers can then concentrate on the production of a small number of frits that will meet the relevant regulations regarding lead release. A common lead frit of this type is the so-called lead bisilicate containing 2.2% Al2O3 and having the following molecular formula:

1.0 PbO - 0.074 Al2O3 1.82 SiO2

The use of this type of frit together with a borax frit and the usual mill additions effectively eliminated the problem of severe lead poisoning in the potteries. Lead silicate frits can be substituted for raw lead compounds from knowledge of the PbO content and decreasing the level of silica added from other sources to allow for the SiO2 present in the lead frit.

Applications for Lead Glazes

Lead cannot be used in glazes that are matured above about 1170°C due to unacceptable losses by volatilization. This problem is more acute when raw lead compounds are used to produce the glaze in comparison with low solubility lead frits. For example, white lead can lose as much as 10% by weight of its lead oxide content if fired at 1000°C for one hour. However, fritted lead glazes show lower levels of loss by vaporization and also improve the crazing resistance in comparison with raw lead glazes.

In addition to improving brilliance, the addition of lead to a glaze formulation lowers the expansion coefficient in comparison with alkali oxides and gives improved flow and elasticity to the finished glaze. The following two glazes are typical of boron free compositions:

HIGH TEMPERATURE LEADED GLAZE MATURING AT ABOUT 1160°C [units are moles of oxide]

K2O or Na2O

0.2

CaO 0.4 Al2O3 0.2 SiO2 2.5 PbO 0.4

LEADED EARTHENWARE GLAZE MATURING AT ABOUT 1080°C [units are moles of oxide]

K2O or 0 1


20

Na2O CaO 0.2 Al2O3 0.15 SiO2 2.5 PbO 0.7

For the vast majority of glazes that are required to mature at lower temperatures, boric oxide is added to produce a formulation of the type shown below:

TYPICAL EARTHENWARE GLAZE MATURING IN THE RANGE 960°C-1100°C

[units are moles of oxide] K2O or Na2O

< 0.4

CaO < 0.5 Al2O3 0.15 - 0.4 SiO2 2.0 - 5.0 PbO 0.2 - 0.5 B2O3 0.4 - 1.0

The addition of boric oxide in the form of a borax frit permits a reduction in the lead content of the glaze as well as decreasing the thermal expansion and firing temperature. This type of earthenware glaze containing both lead and borax can be extended to maturing temperatures well below 1000°C. However, the way in which the glaze must be formulated for lower maturing temperatures generally involves a penalty with respect to lead solubility.


21

CHAPTER 4

Lead Migration from Vitreous Surfaces Factors Influencing Lead Migration

The factors affecting lead solubility from both frits and finished glazes have been studied extensively since the time fritting was first introduced. Sir Thomas Thorpe investigated the solubility of lead frits and in 1889 published a paper, "The use of lead in the manufacture of pottery" in which he demonstrated that the ratio of SiO2 to PbO was critical if optimum insolubility is to be achieved. Thorpe examined the effect of the composition of lead frits on their solubility in on 0.25% hydrochloric acid that was used to simulate conditions in the stomach. He found that the lead solubility was least in the silicate having the mol ratio of PbO:SiO2 of 0.5. Additional work by Thorpe, and supplemented by subsequent work by J. W. Mellor concluded that if the ratio of the moles of basic oxides plus alumina were no more than half the number of moles of acidic compounds [primarily SiO2], then lead release from the glass would be low.

5.0][

min

2

��

SiOGenerallyoxidesacidicofMolesaaluofMolesoxidesbasicofMoles

Additional efforts have been made during the last century by researchers to further elucidate the relationship between the release of lead from lead-containing silicate glasses by interdiffusion in aqueous solutions and various experimental factors. The principal factors appear to be glass composition and pH. Some investigators have characterized oxides as "good" or "bad" based on statistical regression analysis of the general oxide contributions in


22

complex compositions, whereas other studies have examined possible structural mechanisms and proposed the existence of threshold compositional levels for the onset of rapid lead interdiffusion. The structural interpretation of such a threshold is based on the minimum number of non-bridging oxygen for each glass-forming atom to permit rapid lead interdiffusion. Lehman and Greenhut identified an unusual effect of small P2O5 additions to lead-containing silicate glasses. When small amounts of P2O5 were added to lead silicate (1:1) glass, the apparent release rate of lead ions into acidic solutions was greatly reduced. Microscopic observation of the leached glass surface revealed the presence of small lead phospho-silicate crystals. The results of that study prompted a much larger study in the beneficial effects of small phosphate additions were demonstrate for a wide range of soda-lime lead silicate glasses.

Effect of Glass Structure

A more general effect that relates to the early work on lead glazes is the effect of the ring structure of silicate glasses and the importance of preventing a continuous chain of ring breakages from occurring in lead glasses. It has been shown that six-member silicate rings predominate in silicate glass structures typical of commercial glazes, as shown below.

Si

SiSi

Si Si

Si

O

O

O O

O

O

ONa

SIX-MEMBER SILICATE RING STRUCTURE

WITH ONE NON-BRIDGING OXYGEN LINKAGE


23

If each silicon is shared between four such rings and each oxygen is shared between two such rings, then the total number of silicons in each ring is 6 x 0.25 = 1.5 and the total number of oxygen in each ring is 6 x 0.5 = 3.0. Thus each ring has the equivalent of 1.5 SiO2. To generate a broken linkage in the ring structure one unit of Na2O, PbO or other basic oxide is required as per the following relationship.

� Si - O - Si � + Na2O � 2[� Si - O- Na+]

This reaction breaks the ring structure of the glass and permits leaching and interdiffusion of the modifier ions such as lead, alkalis and alkaline earths. Therefore, a lead release threshold is expected when the mole ratio of glass modifiers to glass formers is greater than 1.0/1.5 = 0.67. Under these conditions each silicate ring will have at least one nonbriding oxygen unit -- a necessary condition for charge-assisted diffusion of modifiers through the network.

Lead Release from Silicate Glasses

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20

Moles Modifier/Glass Former

Lead

Rel

ease

, 60

min

, mg/

cm2

PSNPSKNPSCNPSACNPSBCNPSLinear (PS)Linear (NPS)Linear (CNPS)

LEAD RELEASE FROM SILICATE GLASSES WITH

DIFFERENT MOLE RATIO OF MODIFIER AND FORMERS.


24

Studies have shown that such a general relationship exists, as shown in the figure, Lead Release from Silicate Glazzes.

Role of Specific Oxides

In general terms it is possible to state that of the basic oxides normally encountered in glazes the alkali oxides Na2O and K2O increase lead solubility, MgO has little effect and CaO decreases solubility. Alumina is very beneficial in reducing solubility whereas boric acid has the reverse effect. Titania (TiO2) has also been found to have a marked effect on lowering the solubility of lead frits. A level of 1 - 2% may be used in high lead oxide frits although the coloring effect limits the level of titania that may be incorporated.


25

CHAPTER 5

Glaze Raw Materials

Many common raw materials are used to incorporate the various oxides into glaze bodies. The primary concerns are safety when fired, material cost, and required firing temperature. Non-lead containing raw materials in the glaze industry include silica (quartz), various alkaline earth and alkali compounds including oxides, hydroxides and carbonates, clay minerals including kaolin and feldspars, other many other metal oxides and compounds. This section describes the types of materials that can be used to include the lead oxide component in glaze, and their properties.

Frits

The largest change in the use of lead oxide in glazes in the last quarter century is the drastic increase in the use of frits, especially for small batches and artware. Frits have many advantages, which almost always outweigh the disadvantages. The major disadvantage is that in large quantities, frits may be more expensive than lead oxide compounds. However, unless large volumes of glaze are being used, frits are almost always much more beneficial and not of significant cost.

Description

A frit is simply a pre-manufactured powdered glass of a fixed composition. Powdered glass offers many benefits over crystalline oxide compounds. First, the glass softens at a lower temperature than the oxide compounds, facilitating firing. It acts as a compositionally independent flux in the system. During firing the frit will melt and mix with the other


26

compounds much more easily than if it existed as discreet compounds. Second, lead oxide already incorporated into a glass matrix with silica and other materials is much less soluble than crystalline lead compounds, decreasing possible routes of lead exposure to the body.

Types

Frits are available in a wide variety of compositions, including those containing lead oxide. It is also possible to have a frit manufactured for a certain glaze. This can reduce the need for inclusion of minor or even major oxide components, simplifying the production process by eliminating ordering, shipping, storage, weighing and mixing of such raw materials.

Lead Oxides

Lead Oxide -- PbO

Lead monoxide or litharge is the most important inorganic lead compound and typical commercial powder produced by sublimation consists of particles ranging from 0.25 - 0.50 �m for fine grades to 1.5 to 8 �m for coarser grades. Lead monoxide occurs in two polymorphic forms, a tetragonal form �-PbO is stable up to 489o C and the orthorhombic form �-PbO is stable above this temperature (In some countries the designation for the alpha and beta forms is reversed). The alpha form is red and the beta form is yellow, but differences in color caused by the state of crystallization or by contamination with Pb3O4 make color an unreliable indicator of crystal type. The density of litharge is 9.35 g/cm3 for �-PbO and 9.7 g/cm3 for �-PbO.

The ceramic industry is the leading consumer of litharge. Glasses of high lead content have greater density, lower thermal conductivity, higher index of refraction, greater brilliance, and greater stability and toughness than unleaded glass. The lead oxide content of commercial lead glasses generally ranges from 15% to 58%. Glasses with high lead contents have electrical capacities that compare favorably with those of mica. The unusual electrical properties are caused by the replacement of the very mobile alkali ions by the relatively immobile lead ion. Lead is widely used in optical, electrical, and electronic glasses and in fine tableware.

Glazes and vitreous enamels are glassy coatings used to protect ceramic bodies. Lead oxide is one of the major raw materials for glazes. It is such an


27

important component that glazes are often classified into leaded and leadless types. The use of lead glazes is limited to about 1150 °C since above this temperature lead compounds start to vaporize. The basic source of lead in a glaze is the monoxide, but red lead, Pb3O4, is also used. The oxides are converted into lead bisilicate frits to render the lead compounds insoluble. Several lead frits are manufactured, differing in the ratio of lead oxide to silica and alumina.

The advantages of lead in a glaze are numerous: wide range of compatibility; ready fusibility; relative insolubility in water of the available lead compounds; low viscosity and surface tension of its fused compounds; low price, considering its fusing power; and good solubility with colored oxides. Among its disadvantages are the high vapor pressure of lead compounds, which causes problems during firing; scratchability and to a lesser extent crazing of glazes; occasional attack of food juices or other liquids on housewares; and the possible danger of lead poisoning.

Vitreous enamels for metals are often formulated with litharge, chiefly for coating cast iron, but increasingly in aluminum for architectural applications. Lead monoxide is the starting material in the manufacture of lead silicate and lead carbonate.

Lead Orthoplumbate or Red Lead, Pb304

Of all the intermediate oxides only lead orthoplumbate, Pb3O4, is of commercial importance. It is a brilliant red pigment marketed under the names red lead in the United States and minium in Europe.

Red lead is the second most important lead pigment. It is used as an inhibitor in surface coatings to prevent corrosion of metals. It is also used in storage batteries and to a lesser extent in glass, lubricants, petroleum, and rubber. Red lead is the raw material for lead dioxide. It is often used in the manufacture of lead ferrite magnetic materials. Red lead is stable at ordinary temperatures. When heated above 500 °C it decomposes to lead monoxide. The reaction is reversible, and in practice red lead is made by controlled oxidation of lead monoxide.


28

Red lead is not attacked by acetic acid but is readily decomposed by nitric acid and hydrochloric acid to yield the corresponding lead(II) salt and lead dioxide.

Red lead is manufactured by heating lead monoxide in a reverberatory furnace in the presence of air at 450-500 °C until the desired composition is obtained. Orange mineral, a special brilliant orange grade, is made by thermal oxidation under carefully controlled conditions. The rate of the reaction is affected by the particle size of the litharge and can be increased by operating under pressure. The rate increases in the presence of small amounts of silver oxide and decreases when oxides of silicon, bismuth, zinc, or antimony are present. Commercially produced red lead contains 70 to 99% Pb3O4.

Lead Hydroxides and Carbonates

Crystalline Lead Oxide-Hydroxide

There appears to be only one crystalline lead oxide-hydroxide that has been formulated as 3PbO.H2O. The compound is formed by hydrolysis of lead acetate solutions or from reduced pressure evaporation of solutions of tetragonal PbO in large volumes of carbon dioxide free water.

Pb(OH)2

Lead hydroxide, an intermediate in wet preparation of the monoxide, is an amphoteric base. Pb(OH), is slightly soluble in acids and alkalis but insoluble in acetic acid. Lead hydroxide reacts with carbon dioxide or carbonates to form PbCO3. Basic lead carbonate, known as "white lead", is discussed subsequently.

When heated in air the hydroxide is dehydrated to lead monoxide. Dehydration begins at about 130 °C and is complete at 145 °C. In aqueous solution lead hydroxide is an amphoteric base. Lead hydroxide can be prepared by electrolysis of a lead salt or of an alkaline solution with a lead anode. It can also be prepared by adding alkali to a solution of lead nitrate or diacetate.


29

2PbCO3 Pb(OH)2

This compound, the most important basic salt of lead, is one of the pigments known as white lead. In recent usage all white pigments made from lead are called white lead: basic lead carbonate is described as basic carbonate of white lead, the basic sulfate as basic sulfate of white lead, and the basic silicate as basic silicate of white lead.

Basic lead carbonate is stable up to 100 °C, loses water between 120-155 oC. begins to lose carbon dioxide at about 190 °C. and decomposes at 400 °C.

In the presence of water and carbon dioxide it reverts to normal lead carbonate. Its tendency to blacken when exposed to hydrogen sulfide detracts from its value as a pigment.

Basic lead carbonate is manufactured by several methods. All processes are based on the production of soluble lead acetate, which is then treated with

EARTHENWARE BOWL GLAZED WITH CONE 05 LEAD FRIT GLAZE


30

carbon dioxide to form white lead. Lead acetate is made from lead metal or monoxide and acetic acid.

Lead Silicates

Lead metasilicate, which occurs in nature as the mineral alamosite, is the most common silicate of lead. Lead pyrosilicate, 3PbO.2SiO2 is the mineral barysilite. Lead orthosilicate, PbO.SiO2, and some other lead silicates including 4PbO.SiO2 and possibly PbO.2SiO2 are also known.

As in the PbO-B2O3 system, PbO and SiO2 form glassy compounds over a wide range of composition. Mixtures with up to 90% PbO stay glassy at room temperature if cooling is rapid.

Lead silicates have a wide range of applications, including ceramics, glasses, paints, rubbers, and other polymers. Commercial lead silicates are generally not well-defined compounds but rather mixtures of the two oxides in various ratios.

LEAD SILICATE PHYSICAL PROPERTIES Compound PbO.SiO2 2PbO.SiO2 3PbO.2SiO2 4PbO.SiO2 Common name

Metasilicate Orthosilicate Pyrosilicate Basic Silicate

Mineral Name

Alamosite -- Barysilite --

Crystal Structure

Monoclinic Prism

Prism Trigonal Several forms

Melting point, oC

765 - 770 740 -- 746 650 (decomp)

725

Specific Gravity

6.49 -- 6.7 --

Lead silicate is an important ingredient of enamels and glazes, and is used widely in flint and specialty glasses. Glasses containing lead bend light more than ordinary glasses and can be cut into facets with a gemlike effect. Lead silicate glass is opaque to ultraviolet radiation but transparent at other wavelengths so that it can be used as an optical glass. Other uses include electric and electronic bulbs, tubes, and other parts; radiation shielding; and


31

solder sealing glasses. Lead monoxide is the common source of lead in glasses and ceramics, but lead silicates are also used.


32

CHAPTER 6

Materials Handling Safe Handling Practice

Every type of lead material can be handled and controlled with safety if proper equipment is provided for the protection of the health of the industrial worker. This is borne out by the many plants that have as their primary function the processing and handling of lead and its compounds and do so with entire success and safety. A clean, well-ventilated plant with efficient material handling methods is essential for successful operations.

With hygienic plant controls, the general use of lead compounds in the ceramic industry is available with all of the recognized advantages of these versatile materials. Industrial health precautions are by no means confined to the use of lead and its compounds. Silica, beryllium, cadmium, antimony, selenium, tellurium and other elements and compounds present exposure problems. While we are primarily discussing the handling of lead compounds, it should be remembered that, basically, the same precautions are necessary in plants where no lead is being used. Dust of any kind is a health hazard.

Proper handling of lead compounds in the ceramic industry requires:

�� Proper plant hygiene,

�� Proper instruction and supervision of workers,

�� Regular monitoring by a health care professional.


33

Hygiene and Medical Monitoring

Proper Plant Hygiene

A. Adequate pre-employment examination of all plant workers. This should include previous working histories, and medical examinations.

B. Dispersion of dust should be minimized or eliminated if possible. All operations that disperse dust should be controlled by closed systems of local exhaust ventilation.

Lead compounds are packaged so that unloading, plant storage and movement to the location where the packages area to be emptied present no exposure problems. Dust hoods should be located where dry materials are charged or discharged from processing equipment and should surround the operation as completely as possible. Bins, elevators, chutes, covered conveyors, covered mixers etc., can usually be made dust-free by drawing off dust-laden air at critical points. By maintaining a slight negative pressure in the closed equipment, some air may enter the system, but dust will not escape. Relatively small air volumes are required to achieve dustless operation for closed equipment. Exhausted dust-laden air should be properly filtered before discharge outside the plant. Dust control and elimination are important to all industries today and there are many reliable and qualified manufacturers of equipment to do the job properly.

C. Only where local or general control is impossible, workers should be provided with respirators specifically approved for this type of protection.

D. Adequate washroom facilities should be provided. Hot water and soak should be provided, along with disposable hand towels. Shower facilities are recommended.

E. Proper locker room and shower facilities should be provided. Workers should have separate lockers for street clothes and work clothes to prevent contamination. Workers should shower prior to leaving the work premises.

F. A suitable and separate place should be provided for workers to eat.


34

No food should be eaten until the worker has washed and no eating should be allowed in the department or area where lead-containing materials are used. Workers should not smoke on the job while handling these materials.

G. Work clothing should not be taken home. Lead compounds will easily become attached to work clothing and commingling of work clothing with home clothing exposes others in the household to lead. The washing machine processes is inefficient. Washing work clothes with other clothing spreads contaminants.

Proper Instruction and Supervision

The purpose, correct use, and maintenance of respirators should be explained to the workers and enforced. Workers must be provided with respirators that fit the individual. Respirators assigned to one individual should be regarded as personal as a toothbrush and should not be interchanged. The use of respirators is no substitute for adequate plant engineering.

Regular Medical Monitoring

Most practicing physicians rarely have occasion to treat cases of heavy metal overexposure. Therefore, the appropriate health care professional should be informed of any such materials being handled by a worker. The health care professional will then be in a position to study the problem and outline a regular safety program. Monitoring of lead in the air, on surfaces, or regular blood tests may be deemed appropriate by health care personnel.

There are both governmental and private agencies in many cities competent to evaluate and advise on engineering and medical control of lead.

Material Safety Data Sheets (MSDS)

Lead compounds need to be handled with care to prevent accidental inhalation or ingestion of lead. Lead compounds vary considerably in the biological availability of lead. The frits and compounds in which lead is combined with other components, most notably silica, are the most stable in acidic environments and in general are the easiest to work with from a safety perspective. Lead oxides such as PbO and Pb3O4 are intermediate in durability and white lead, 2PbCO3 Pb(OH)2, is the compound most readily


35

dissolved in stomach acids, of the commonly used compounds, and must be handled carefully.

Materials Safety Data Sheets are available from all chemical suppliers for all compounds sold commercially, and virtually all MSDS sheets are now available over the internet. A simple search strategy of typing MSDS followed by the compounds name usually produces good results. The MSDS contain an abstract of useful information regarding the handling, storage, and disposal of the respective compound, as well as critical health and safety data. The general outline of the MSDS is as follows:

Section 1: Identification Section 2: Hazardous Composition/Ingredients Section 3: Hazardous Identification Section 4: First Aid Measures Section 5: Fire Fighting Measures Section 6: Accidental Release Measures Section 7: Handling And Storage Section 8: Personal Protective Measures Section 9: Chemical And Physical Properties Section 10: Stability And Reactivity Section 11: Toxicological Information Section 12: Ecological Information Section 13: Disposal Considerations Section 14: Transportation Information Section 15: Regulatory Information Section 16: Labeling Information


36

MSDS for seven lead compounds commonly used in the preparation of glazes for dinnerware are given in Appendix D. The seven compounds are:

�� Lead Bisilicate �� Litharge [PbO] �� Lead Monosilicate �� Lead Monoxide �� Red Lead [Pb3O4] �� Tribasic Lead Silicate �� White Lead [lead carbonate hydroxide]


37

CHAPTER 7

Glaze Compositions and Lead Migration Behavior

Summary of Experimental Results

A. Tests made on a large number of Cone 3-5 [~1160o - 1190o C] production, clear lead glazes show lead release values of less than 0.5 �g/ml [part per million, ppm] and in a number of cases less than 0.1 ppm. These values are important since 0.5 �g/ml reflects the lowest permissible limit for lead release from ceramic ware according to the current ISO standards, and 0.1 �g/ml represents the lowest significant values that can be measured with flame atomic absorption spectroscopy, the method of choice for manufacturing control measurements world-wide. Thus the Cone 3-5 clear glazes, which constitute the large part of dinnerware glazes, show low lead release as has been found in a number of other laboratories during the past three decades.

B. Data taken on typical Cone 5 clear glazes as used for institutional dinnerware also showed lead release values within ISO compliance limits.

C. A number of typical commercial stains, representing those most commonly used along with the appropriate clear base glazed, were tested. None of these affected the lead release of the base glaze adversely, showing these have been well designed.


38

D. Direct additions of various coloring oxides have shown the detrimental effects of copper oxide especially for the lower temperature glazes. This confirms earlier knowledge that led to the abandonment of the low temperature copper green glaze years ago. As is often noted in the literature, copper oxide should not be used either as a direct addition or component of a stain used in a lead glaze. It should also be noted that of more practical significance is the normal practice involving the use of prepared compositions or stains, which include the coloring oxides but are designed to be compatible with specific base glazes and their maturation.

E. The additions of various opacifying oxides (zirconium silicate, tin oxide, or titanium oxide) to lead glazes, in appropriate amounts to attain the desired opacity, do not affect the lead release adversely and for some glazes (showing higher lead release) such additions prove beneficial. Ions of high charge and small size, introduced as their oxides, fit into the interstices of the silicate structure and in many instances result in a strong contraction of the surrounding unsaturated oxygen ions, strengthen the structure and reduce lead release.

F. A study of variations in the alkali oxides and alkaline earth oxides in a typical Cone 4 glaze did not show any significant differences in lead release for the concentration used. An increase in lead release was noted with increase in boric oxide in the glaze. Additions of zinc oxide did not affect the lead solubility of the base glaze adversely. Essentially the same results were noted when these glazes were fired at Cone 01 [~1130o C]. The acid resistance of this typical Cone 4 lead glaze and the modifications studied is good. As noted below changes of this type may be reflected in the lead release from lower temperature glazes.

G. In a low temperature (Cone 07) lead silicate (PbO·1.3SiO2) base glaze small single alkali oxide additions increase the lead release with Li20 increasing it least, Na20 being intermediate, and K20 increasing the lead release most. These data show increasing lead release with increasing ionic radii of the alkali oxide added. A mixture of any two of the three alkali oxides (totaling 0.1 molecular equivalent) resulted in lower lead release than the same molar addition of any of the single


39

alkali oxides. The addition of the three alkali oxides (totaling 0.1 molecular equivalent) gave the lowest lead release. Hence the use of mixed alkali oxides would is preferred over single alkalis with respect to lead release.

H. The alkali earth oxide additions in the same low temperature base glaze gave similar trends to the alkali oxide additions in respect to lead release, showing (with exception of CaO in the data taken) an increase in lead release with increase of ionic radii of the oxide added. The best combinations of alkaline earth oxides, for the same total molar concentration, included those having smaller ionic radii, e.g., MgO-CaO or MgO-SrO or MgO-CaO-SrO.

I. A designed experiment on a Cone 03 low temperature lead silicate base glaze (PbO·1.5SiO2) involved varying additions of A1203, B203, and Sr02. Higher levels of A1203 definitely reduce the lead release whereas increasing B203 increases lead release. An attempt was made to correlate the acid resistance with calculated structural factors and glaze hardness (as an indication of the strength of the structure). The same experiment repeated with a higher Cone 1 glost fire gave different results thought to be due principally to the greater thermochemical action of this low temperature glaze on the body at the higher temperature. Zr02 additions were effective in increasing acid resistance at the higher firing temperature.

J. Depletion tests involving the repeated testing of the same glaze surface is of interest from the standpoint of continued service. A typical Cone 4-5 dinnerware glaze showed a very low initial value of 0.10 ppm and decreased to 0.06 ppm after eight cycles.

Tests on Cone 3-5 Clear Production Glazes

A number of glazed cup specimens were tested which represent current commercial production, i.e., Cone 3-5 clear glazes now being used. Three glazed cups were selected at random from a number of glazed specimens in each case representing a specific glaze. The data are given below:

Plant Code Cone Lead PPM (Av. for 3 cups)

A-l* 3** 0.06


40

A-2 3 0.39 A-3 3 0.33 B-1 4-5 0.04 B-2 4-5 0.07 B-3 4-5 0.10 B-4 4-5 0.07 C-1 1 0.08 D-1 4-5 0.04 D-2 4-5 0.02 D-3 4-5 0.16 D-4 4-5 0.08 D-5 4-5 0.07 D-6 4-5 0.11 D-7 4-5 0.06 D-8 4-5 0.08 E-1 4-5 0.27 E-2 4-5 0.10 F-1 4-5 0.13 G-1 4-5 0.45 G-2 4-5 0.20 H-1 3 0.23 I-1 4-5 0.10 J-1 4 0.07 J-2 4 0.15 J-3 4 0.11

Average 0.137 Standard Dev. 0.111 Coef. of Var. 81%

*1, 2, 3 etc. designate different glazes of subsequent tests on other samples of the same type glaze.

These data show lead release values of less than 0.5 parts per million for the Cone 3-5 clear glazes and corroborate a large body of data extending as far back as to work carried out at the National Bureau of Standards reported in 1939 that shows that clear glazes fired in this range produce low levels of


41

lead release. Indeed most work in past decades has concentrated on lead migration from colored glazes in organic acids. In the latter research most attention was given to several low temperature colored glazes (Circa Cone 05) and further reference is made to these in the discussion of the results of testing similar glazes in the present investigation. The low lead release characteristic of the Cone 3-5 clear glazes is seen further in this investigation from measurements on the base glazes to which various coloring oxide additions were made to evaluate the effects of these additions.

The experience of many laboratories many years has shown that the Cone 3-5 clear glazes, which constitute the large part of lead dinnerware glazes, show very low lead release (less than 0.5 part per million). Since no difficulty has been experienced with such glazes as regards lead migration, the principal effort of recent research has focused on the effects of various oxide additions, lower temperature glazes, and production parameters.

Cone 5 Clear Glazes for Institutional Dinnerware

An additional area of commercial interest is the glaze used for institutional dinnerware. Institutional dinnerware is traditionally a thicker and stronger dinnerware intended for the more intense use of hotels and other commercial or institutional users. The glost fire for such ware is traditionally higher than for other forms of dinnerware and Cone 5 is representative. A representative base glaze that was the average of five different commercial compositions was selected for this study and the oxide formula is given below.

Base Glaze 0.066 K2O 0.179 Na2O 0.261 PbO 0.494 CaO

0.340 A12O3 0.314 B2O3

3. 369 SiO2

All of the above base glaze composition was fritted except the necessary molecular equivalent of A12O3 and SiO2 to provide a ten percent (10%) clay mill addition. This glaze was then applied on hotel china bisque cups and fired to Cone 5 in a commercial glost fire. The lead release from this glaze and from commercial glazes on cups from two hotel china producers, using

the standard 24 h 4% acetic acid test, is given below: Lead PPM


42

Individual Data Average Base Glaze 0.18 - 0.14 0.16

Commercial HC Glaze 1 0.31 - 0.24 - 0.27 0.27 Commercial HC Glaze 2 0.12 - 0.08 - 0.05 0.08

As might have been expected from earlier data, the lead release from this clear Cone 5 glaze and from the two commercial hotel china glazes was within current ISO permissible limits.

Effect of Coloring Oxides on Lead Release

The objective of these studies was to investigate the effect of coloring oxide additions on the lead release of glazes since considerable literature data have showed that certain pigment additions can promote lead migration in the glazes [copper the most notorious example] whereas others tend to stabilize the glaze. The normal practice involves the use of prepared pigment compositions or stains, which include the coloring oxides but are designed to be compatible with the glaze and its maturation.

Cu, Cr, and Co in a Clear Cone 4 Glaze

This glaze employed two frits, one leaded and one leadless, and had the following composition:

Cone 4 Glaze 0.013 K2O 0.182 Na2O 0.290 A12O3 2.971 SiO2 0.572 CaO 0.360 B2O3 0.233 PbO

Glaze Batch Frit A 51.5 Frit B 25.1 Whiting 4. 0 Clay (Ajax PC)

4.7

Flint (Supersil)

14.7

100.0


43

Frit A 0.02 K2O 0.27 A12O3 2.49 SiO2 0.29 Na2O 0.57 B2O3 0.69 CaO

Frit B 1.00 PbO 0.25 A12O3 1.92 SiO2

Frit B represents the cone deformation eutectic for the PbO-Al2O3-SiO2 system and has high acid resistance in powder or dust form.

The above glaze and modifications of this glaze with various mill additions of coloring oxides were applied on bisque cups from three plants B, C and D. The coloring oxides and amounts used (as percent by weight mill additions on the basis of the dry glaze batch) are given in the table below. Each value for lead release given is the average for three cups. The test specimens were all fired to approximately five (5) hours to Cone 4 in a gas-fired laboratory kiln.

Lead PPM Bisque cups used from

Coloring Oxide

Oxide Addition Weight Percent

Plant B

Plant C

Plant D

Base Glaze – 0.07 0.10 0.03 CuO 0.1 0.08 0.05 0.03 CuO 2.0 1.58 1.19 1.14 Cr2O3 1.0 0.07 0.07 0.04 Cr2O3 3.0 0.04 0.03 0.03 Co3O4 0.01 0.04 0.08 0.11 Co3O4 0.1 0.01 0.01 0.11

With the exception of the modification of a two percent (2%) by weight mill addition of CuO to the base glaze, the lead release values for the base glaze and all the other modifications are too low to draw any conclusions. In large part all these values are less than one part in ten million.


44

Fe and Mn in a Clear Cone 4 Glaze

The same Cone 4 glaze using two frits was used in evaluating the effects of ten percent (10%) by weight mill additions of Fe2O3, MnO2 and equal parts of these two oxides on the lead release in comparison with that for the base glaze. These glazes were applied on bisque cups from Plant C. These were fired in five (5) hours to Cone 4 in a gas-fired laboratory kiln. The results were as given below:

Coloring Oxide Oxide AdditionWeight Percent

Lead PPM

Base Glaze – 0.10 Fe2O3 10 0.37 MnO2 10 0.97

Fe2O3-MnO2 5-5 1.49

These coloring oxides caused significant increase in lead release when compared to that from the base glaze. The mixture of the two oxides showed greater influence than either of the single oxides for the same total amount of oxides introduced. It is not obvious why Fe2O3 and/or MnO2 have an adverse effect on the acid. resistance of a lead frit. The fact that Fe and Mn can each readily assume different valences might be an important factor in promoting their mobility in these glazes.

Fe, Mn and Zircon in a Clear, Cone 02-4 Glaze

Base Glaze 0.09 K2O 0.09 Na2O 0.321 A12O3 3.064 SiO2 0.58 CaO 0.360 B2O3 0.24 PbO

Glaze Batch Frit D 90 Clay 10

Frit D 0.09 K20 0.09 Na2O 0.19 A12O3 2.80 SiO2


45

0.58 CaO 0.36 B2O3 0.24 PbO

Mill Additions, Weight Percent

BG BG-1 BG-2 BG-3 Fe2O3 – 4.5 9.0 5.0 MnO2 – 9.0 4.5 5.0 ZrO2·SiO2 – 0.5 0.5 2.0

The above glazes were applied on bisque cups from Plant C and fired in 4.5 hours to Cone 01 and in a second fire to Cone 4 in a gas-fired laboratory kiln. All of the glazed specimens showed good, bright glazes for both firing treatments. In general, the appearance of the glazes was slightly better for the higher cone temperature especially for BG-3. The average values of lead release from these glazes are given below:

Lead Migration, ppm Glaze Cone 01 Cone 4

BG 0.04 0.16 BG-1 0.81 0.07 BG-2 0.60 0.10 BG-3 0.15

These oxide additions show no detrimental effects on lead release of this type glaze for the normal Cone 4 firing. At the lower Cone 01 firing, the lead release was measurably greater than that from the base glaze with the possible exception of the BG-3 modification. This modification had less of the coloring oxides and more of the zircon additive; the latter would be expected to increase the acid resistance. The satisfactory behavior of BG-1 and BG-2 at the normal Cone 4 firing is of interest from the standpoint of the so-called Rockingham glazes.

Cu Fe, Mn and Zircon in a Clear, Cone 02-4 Glaze

The same base glaze was used as above with mill additions of Fe2O3, and MnO2 to note also the effect of CuO as follows:

Mill Additions, Weight Percent

BG-4 BG-5 BG-6 BG-7


46

Fe2O3 5.0 5.0 5.0 5.0 MnO2 5.0 5.0 5.0 5.0 CuO – 0.1 0.25 0.5

These glazes were applied on bisque cups from Plant C and given similar firings to Cone 01 and Cone 04. All of the glazed specimens were good in appearance. The average values for lead release were as given below:

Lead PPM Glaze Cone 01 Cone 4 BG-4 0.09 0.07 BG-5 0.06 0.08 BG-6 0.77 0.09 BG-7 1.45 0.10

The effect of CuO additions at levels up to 0.5 weight percent were not detrimental for the normal Cone 4 firing of this type glass, but increased migration rates were observed for lower temperature firing at Cone 01. Hence further tests were made using a lower temperature base glaze.

Cu, Cr and Co in a Low Temperature Cone 05 Glaze

The glaze used in the work is the Cone 05 glaze used on high talc-clay bodies typically found in the artware and pottery industries.

Base Glaze 0.071 K2O 0.113 Na2O 0.279 Al2O3 2.726 SiO2 0.455 PbO 0.675 B2O3 0.035 ZrO2 0.361 CaF2

Glaze Batch Frit C 90 Clay 10

Frit C 0.071 K2O 0.113 Na2O 0.131 Al2O3 2.429 SiO2 0.455 PbO 0.675 B2O3 0.035 ZrO2 0.361 CaF2


47

The above glaze and modifications of this glaze with various mill additions of coloring oxides as shown below were applied on bisque cups from Plant C. Each value for lead release is the average for three cups tested. The specimens were all fired in 4.5 hours to 1024o C in a gas-fired laboratory kiln.

Coloring Oxide

Oxide Addition Weight Percent

Lead Release ppm

Base Glaze – 1.20 CuO 0.1 3.10 CuO 2.0 12.00 Cr2O3 1.0 4.75 Cr2O3 3.0 3.31 Co3O4 0.01 6.90 Co3O4 0.1 1.17

The effects of the coloring oxide mill additions on the lead release from this low temperature glaze (maturing at 1024o C.) were marked. The lead release increased ten times by the two percent (2%) by weight mill addition of CuO over that for the base glaze.

This glaze was designed for the talc-clay type of artware and tile bodies. It is not used for dinnerware bodies. However, it was selected for this investigation because its maturing temperature is near the lower end of the temperature range under study. Furthermore, the coloring oxides are normally added as part of prepared stain compositions.

Copper-containing stains have had use also in artware glazes. However, they have not been used commercially for colored dinnerware glazes during the past several decades and were used only to a limited extent in earlier years. The industry has been aware of problems associated with such glazes for many years.

Effect of Commercial Stains on Lead Release

In this work segment commercial glaze stains were added to an appropriate clear base glazes and the effect of these additions on lead migration were assessed. These studies represented a wide range of


48

commonly used, typical commercial stains introduced in appropriate concentrations in compatible base glazes. Tests were conducted on these as described below:

Pb-Sb Yellow Stain In Cone 08-04 Glaze

Base Glaze 0.071 K2O 0.113 Na2O 0.279 A12O3 2. 726 SiO2 0.455 PbO 0.675 B2O3 0.035 ZrO2

0.361 CaF2

Glaze Batch

(Including Stain Addition) Frit C 90 Clay 10 Pb-Sb Yellow Stain, (GS-313)

5

Frit C 0.071 K2O 0.113 Na2O 0.131 A12O3 2.429 SiO2 0.455 PbO 0.675 B2O3 0.035 ZrO2 0.361 CaF2

The above base glaze and the same glaze with a five percent (5%) by weight mill addition of the commercial Pb-Sb Yellow Stain were applied on bisque cups from Plant C and fired to Cone 06 down. The average lead release from three cup specimens of each were as follows:

Lead PPM

Base Glaze 0.11

Base Glaze + Five Percent (5%) Pb-Sb Stain 0.04

The Pb-Sb stain addition did not affect the lead release from the base glaze adversely.


49

Cr-Al Pink Stain in Cone -06-02 Glaze

Base Glaze 0.01 K2O 0.13 Na2O 0.296 A1203 2.241 SiO2

0.28 ZnO 0.210 B203 0.192 ZrO2 0.58 PbO

Glaze Batch

(Including Stain Addition) Frit F 85 Clay 10 Zirconium Silicate

10

Alumina 5 Cr-Al Pink Stain (GS-515)

5

Frit F 0.01 K2O 0.13 Na2O 0.14 Al2O3 1.78 SiO2 0.28 ZnO 0.21 B2O3 0.58 PbO

The above base glaze and the same glaze with a five percent (5%) by weight mill addition of the commercial Cr-Al pink stain were applied on bisque cups from Plant C and fired in 4 hours to Cone 06 down.

Lead PPM

Base Glaze 1.52

Base Glaze + Five Percent (5%) Cr-Al Stain 0.62

The lead release from the above, low temperature fritted lead glaze was markedly reduced by the five percent (5%) by weight mill addition of the commercial Cr-Al pink stain.


50

Sn-Sb Gray Stain in Cone 02-4 Glaze

Base Glaze 0.09 K20 3.269 Si02 0.09 Na2O 0.326 A12O3 0.196 ZrO2 0.58 CaO 0.360 B203 0.072 SnO2 0.24 PbO

Glaze Batch

(Including Stain Addition) Frit D 87 Clay 10 Zirconium Silicate

10

Tin Oxide 3 Sn-Sb Gray Stain (GS-868)

5

Frit D 0.09 K2O 0.09 Na2O 0.19 Al2O3 2.80 SiO2 0.58 CaO 0.36 B2O3 0.24 PbO

The base glaze and the same glaze with a five percent (57c) by weight mill addition of the commercial Sn-Sb gray stain were applied on bisque cups from Plant C. The glazed cups were fired in a gas-fired laboratory kiln in four (4) hours to Cone 1 down. The average lead release for three cups of each is given below:

Lead PPM

Base Glaze 0.03

Base Glaze + Five Percent (5%) Sn-Sb Stain 0.05


51

The Sn-Sb stain mill addition did not affect the lead release from the above base glaze adversely. The difference between 3 and 5 ppm is not statistically significant at the 95% confidence level.

Co-Cr-Fe Black Stain in Cone 1 Glaze

Base Glaze 0.09 K2O 0.09 Na2O 0.321 Al2O3 3.064 SiO2 0.58 CaO 0.360 B2O3 0.24 PbO

Glaze Batch

(Including Stain) Frit D 90 Clay 10 Co-Cr-Fe Black Stain (GS-815)

8

Frit D 0.09 K2O 0.09 Na2O 0.19 Al2O3 2.80 SiO2 0.58 CaO 0.36 B2O3 0.24 PbO

The above base glaze and the same glaze with an eight percent (8%) by weight mill addition of the commercial Co-Cr-Fe black stain were applied on bisque cups from Plant C. These specimens were fired in a gas-fired laboratory kiln in four (4) hours to Cone 1 down. The lead release data are as follows.

Lead PPM Base Glaze 0.02 Base Glaze + Eight Percent (8%) Co-Cr-Fe Stain

0.03

The mill addition of the Co-Cr-Fe stain to the base glaze did not result in any significant increase in lead release.


52

Cr-Al Pink Stain in Cone 02-4 Glaze

Base Glaze 0.07 K2O 0.18 Na2O 0.338 A12O3 2.856 SiO2 0.29 CaO 0.260 B2O3 0.177 ZrO2 0.32 ZnO 0.14 PbO

Glaze Batch

(Including Stain Addition) Frit E 85 Clay 10 Zirconium Silicate

10

Alumina 5 Cr-Al Pink Stain (GS-515)

5

Frit E 0.07 K2O 0.18 Na2O 0.20 A12O3 0.29 CaO 0.26 B2O3 2.43 SiO2 0.32 ZnO 0.14 PbO

The above base glaze and the same glaze with a five percent by weight mill addition of the Cr-Al pink stain were applied on bisque cups from Plant C. These were fired in a gas-fired laboratory kiln to Cone 1 in four hours.

Lead PPM

Base Glaze 0.03

Base Glaze + Five Percent (5%) Cr-Al Stain 0.05

The Cr-Al stain mill addition did not affect the lead release from the base glaze adversely.


53

Various Commercial Stains in a Cone 02-4 Glaze

Base Glaze 0.09 K2O 0.09 Na2O 0.321 A1203 3.253 SiO2 0.58 CaO 0.360 B203 0.190 ZrO2 0.24 PbO

Glaze Batch

(Including Various Stains) Frit D 90 Clay 10 Zirconium Silicate

10

Stain 5

Frit D 0.09 K2O 0.09 Na2O 0.19 Al2O3 2.80 SiO2

0.58 CaO 0.36 B2O3 0.24 PbO

The stains as listed in the table below were each used in five percent (5%) amounts as mill additions to the base glaze. These glazes were applied on bisque cups from Plant C. The test cups were fired in four (4) hours to Cone 1 down in a gas-fired laboratory kiln. The lead release data for the base glaze and the various modifications are given below:

Lead PPM Base Glaze 0.11 + 5% Co-Al Blue (GS-1) 0.03 + 5% Co-Cr Green (GS-100) 0.09 + 5% Cu Turquoise (GS-104)

0.05

+ 5% V-Zr Blue (GS-119) 0.05 + 5% Sn-V Yellow (GS-309) 0.03 + 5% Pr-Zr Yellow (GS-322)

0.04


54

+ 5% V-Zr Yellow (GS-328) 0.04 + 5% Cr-Sn Maroon (GS-404)

0.01

+ 5% Cr-Sn Pink (GS-510) 0.04 + 5% Fe-Zr Pink (GS-521) 0.03 + 5% Cr-Fe-Zr Brown (GS-607)

0.03

+ 5% Zr Gray (GS-874) 0.04

None of the various commercial stain mill additions to the base glaze effected the lead release of the modified glazes adversely. This is of interest since these stains represented those most commonly used at present.

Effect of Opacifying Oxides on Lead Release

The objective of this study segment was to explore the effects of commonly used opacifying oxides in different concentrations as mill additions on the lead release of the glazes.

Standard Glaze Fired to Cone 4 and 01.

Base Glaze 0.09 K2O 0.09 Na2O 0.321 A12O3 3.064 SiO2 0.58 CaO 0.360 B2O3 0.24 PbO

Glaze Batch

(Not including opacifiers)

Frit D 90%

Clay 10

Frit D 0.09 K2O 0.09 Na2O 0.19 A12O3 2.80 SiO2 0.58 CaO 0.36 B2O3 0.24 PbO


55

The base glaze and various modifications to include opacifying oxide mill additions are given below along with the lead solubility data. The glazes were applied on bisque cups from Plant C and glost-fired on a 4.5 hour schedule to Cone 4 and a second firing to Cone 01 in a gas-fired laboratory kiln.

Lead PPM Opacifying

Oxide

Weight Percent Mill

Addition Cone 01 Cone 4

Base Glaze - 0.07 0.14 SnO2 5 0.05 0.12 SnO2 10 0.08 0.10

ZrO2·SiO2 5 0.02 0.13 ZrO2·SiO2 10 0.03 0.13 ZrO2·SiO2 15 0.09 0.17

TiO2 0.5 0.10 0.13 TiO2 2.5 0.21 0.16 TiO2 5.0 0.04 0.08

The glazes were all good in appearance with the exception of the yellowish coloration of the glazes with the higher levels of TiO2. The yellowish coloration was greater at the lower cone temperature. In all cases and at both cone temperatures the opacity increased (as would be expected) with greater amounts of any of the three opacifiers.

The lead release values were in all cases too low to draw any conclusions in respect to type and amount of opacifying oxide mill addition. However, it can be concluded that these additions to the base glaze do not affect the lead solubility of the base glaze adversely. They may in cases of base glazes having higher lead release prove beneficial.

High Lead Glaze Fired to Cone 4 and 01.

A second study at the cone 01 and cone 4 firing levels examined the effect of the opacifying oxides in a higher PbO, softer glaze as given below.

Base Glaze 0.071 K2O 0.113 Na2O 0.279 Al2O3 2.726 SiO2 0.455 PbO 0.675 B2O3 0.035 ZrO2


56

0.361CaF2

This lower temperature glaze contained 0.035 ZrO2 as part of the frit. The glaze batch included ninety percent (90%) of the above glaze as Frit C and a ten percent (10%) clay mill addition. The additions of opacifiers in kind and amount were the same as used in Example 1 and are shown below along with the lead release data.

The glazes were applied on bisque cups from Plant C. These specimens were fired to Cone 01 and Cone 4. The glazes were fairly good in appearance, showed the same general trends in respect to opacification and the yellowing of the higher levels of TiO2 additions. Results are given in the following table.

Lead PPM Opacifying Oxide

Weight Percent Mill Addition Cone 01 Cone 4

Base Glaze

(Contained 0.035 ZrO2 in

Frit. No mill addition other than clay)

0.14 0.13

SnO2 5 0.24 0.20 SnO2 10 0.10 -

ZrO2·SiO2 5 0.09 0.11 ZrO2·SiO2 10 0.13 0.03 ZrO2·SiO2 15 0. 08 0.16

TiO2 0.5 0.14 0.12 TiO2 2.5 0.23 0.11 TiO2 5.0 0.05 0.09

Again the lead release values were too low to form any conclusions in respect to the kind and amount of opacifier additions other than they do not affect the lead release of the base glaze adversely.

Effect of Variations in Alkali, Alkaline Earth, Boric and Zinc Oxides and Beryl in a Cone 4 Glaze

Base Glaze 0.09 K2O


57

0.09 Na2O 0.321 Al2O3 3.064 SiO2 0.58 CaO 0.360 B2O3 0.24 PbO


In all cases the glazes were applied on bisque cups from Plant C and fired to both Cone 1 and Cone 4 on a 4.5 hour heating schedule.

Effect of Alkali Oxides

The alkali variation consisted of altering the 0.18 molecular equivalents of alkali oxide in the base glaze, with the remaining constituents held constant:

0.58 CaO 0. 19 Al203 2.80 SiO2

0.24 PbO 0. 36 B203

The frits prepared had the following alkali oxide content:

Molecular Equivalents

Frit K20 Na2O Li2O A 0.18 – – B – 0.18 – C – – 0.18 D 0.09 – 0.09 E – 0.09 0.09 F 0.06 0.06 0.06

Lead release, ppm

Glaze Using Frit

Cone 01 Cone 4

A 0.35 0.12 B 0.22 0.10 C 0.09 0.08


58

D 0.10 0.16 E 0.07 0.11 F 0.05 0.08

All of these glazes were good in appearance at both cone temperatures.

These alkali oxide modifications in this typical Cone 4 lead glaze do not show any significant differences in lead release. The values for glazes A and B were somewhat higher when fired four cones lower at Cone 01. The acid resistance of this glaze and its various modifications is very good.

Effect of Alkaline Earth Oxides

Various alkaline earth oxides were substituted in part for the 0.58 molecular equivalents of CaO in the base glaze. The frits were made with the following portion of the base glaze held constant: 0.09 K2O 0.09 Na2O 0.19 A12O3 2.80 SiO2 0.24 PbO 0.36 B2O3

The frits prepared had the following alkaline earth oxide content:

Molecular Equivalents Frit CaO MgO BaO SrO G 0.48 0.01 – – H 0.38 0.20 – – I 0.48 – 0.01 – J 0.38 – 0.20 – K 0.48 – – 0.01 L 0.38 – – 0.20 M 0.38 0.10 0.10 – N 0.38 0.10 – 0.10 O 0.38 – 0.10 0.10 P 0.37 0.07 0.07 0.07

Glaze Using Lead Release, ppm Frit Cone 01 Cone 4 G 0.17 0.22


59

H 0.06 0.10 I 0.08 0.15 J 0.04 0.11 K 0.04 0.12 L 0.01 0.13 M 0.14 0.10 N 0.06 0.12 O 0.09 0.08 P 0.06 0.08

All of the glazes were good in appearance for both firing treatments.

The lead release in all cases was very low so it is difficult to draw conclusions on the relative effects of these different combinations of alkaline earth oxides in this typical Cone 4 glaze.

Effect of Boric Oxide

The same base glaze was used as above with three levels of B203 as follows: 0.18 eq., 0.36 eq., and 0.54 eq. The intermediate level was the normal level for this glaze. The constant portion for the three glazes, made from ninety percent frit (90%) and ten percent (10%) clay, was as follows: 0.09 K2O 0.09 Na2O 0.321 Al2O3 3.064 Si02 0.58 CaO 0.24 PbO

The glazes were applied on bisque cups from Plant D and fired in five (5) hours to Cone 46.

The results were as follows:

Glaze Eq. B203 In Frit

Lead Release,

ppm

Average

B-1 0.18 0.040-0.043-0.019

0.034

B-2 0.36 0.085-0.082-

0.080


60

0.072 B-3 0.54 0.092-

0.069-0.236

0.132

The average values show an increase in lead release with increasing B2O3 in the glaze frit. This is consistent with earlier findings.

Effect of Zinc Oxide

Three levels of ZnO were used in the same base glaze as follows: 0.00 eq. , 0.05 eq. and 0.10 eq. ZnO. The empirical molecular formulas for the three glazes, prepared from ninety percent (90%) frit and ten percent (10%) clay, were as follows:

Z-1 Z-2 Z-3 ZnO 0.00 0.05 0.10 K2O 0.09 0.085 0.08 Na2O 0.09 0.085 0.08 CaO 0.58 0.55 0.52 PbO 0.24 0.23 0.22

A12O3 0.32 0.32 0.32 B2O3 0.36 0.36 0.36 SiO2 3.06 3.06 3.06

The glazes were applied on bisque cups from Plant D and fired on a 4.5 hour heating schedule to Cone 26. The data were as follows:

Glaze Eq. ZnO In Frit

Lead Release

PPM

Average

Z-1 0.00 0.05-0.01-0.04

0.03

Z-2 0.05 0.08-0.01-0.03

0.04

Z-3 0.10 0.08-0.07-0.04

0.06

All glazes were good, bright, glossy glazes. There appeared to be an increase in lead release with increase in ZnO, however, these values are very


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small. It would be better to state that ZnO, within the concentration range in which it would be used, has little or no adverse effects on lead release.

Two additional glazes were prepared introducing 0.10 eq. and 0.15 eq. of ZnO in the frit at the expense of the other R2O/RO members other than PbO. The latter was held constant at 0. 24 eq. as in the base glaze.

Z-4 Z-5 ZnO 0.10 0.15 K2O 0.08 0.07 Na2O 0.08 0.07 CaO 0.50 0.47 PbO 0.24 0.24

A12O3 0.19 0.19 B2O3 0.36 0.36 SiO2 2.80 2.80

These glazes, fired to Cone 4, showed average lead release values of 0.13 and 0.11 ppm respectively. Again the addition of these two levels of ZnO did not affect the lead solubility of the base glaze adversely.

Effect of Beryl

Three glazes were prepared, using ninety percent (9O%) frit and ten percent (10%) clay, in which the frit contained 0.00 eq., 0.05 eq. and 0.10 eq. of BeO respectively. The empirical molecular compositions of these glazes were:

BE-1 BE-2 BE-3 BeO 0.00 0.05 0.10 K2O 0.09 0.085 0.08 Na2O 0.09 0.085 0.08 CaO 0.58 0.55 0.52 PbO 0.24 0.23 0.22

A12O3 0.32 0.32 0.32 B2O3 0.36 0.36 0.36 SiO2 3.06 3.06 3.06


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These glazes were applied on bisque cups from Plant D and fired on a 4.5 hour heating schedule to Cone 2. The lead release data were:

Glaze Eq. BeO In Frit

Lead Release

PPM

Average

BE-1 0.00 0.05-0.01-0.04

0.03

BE-2 0.05 0.12-0.15-0.14

0.14

BE-3 0.10 0.02-0.01-0.02

0.02

All three glazes were clear, bright, and glossy glazes when fired at Cone 2.

Effect of Base Glaze Variations: Lead Silicate (PbO·1.3SiO2)

This is a typical base glaze as was used formerly for tangerine or uranium red glazes. Various modifications were made of the lead silicate base glaze (PbO·1.3SiO2) as follows:

(a) Introduction of 0.1 molecular equivalent alkali oxides at the expense of PbO, simply as Na2O, K2O and Li2O, as combinations of two of these oxides, and as all three alkali oxides.

(b) Introduction of 0.1 molecular equivalent alkaline earth oxides at the expense of PbO, singly as CaO, BaO, MgO and SrO, as various combinations of two and three of these oxides, and as all four oxides.

(c) Introduction of opacifying oxides, TiO2, ZrO2 and SnO2 in small amounts.

All of the compositions discussed below and designated as glazes 1 to 29 were fritted entirely and water-quenched. The frits were poured at circa 1065o C. after 1.3 to 1.5 hours smelting periods. Slips with satisfactory spraying consistency were obtained by ball milling to pass a 200 mesh [75 �m] sieve with additions of methocel. The glazes were sprayed on bisque cups from Plant C. The specimens were glost-fired in a gas-fired laboratory kiln on a 4.5 hour heating schedule to Cone 07.


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Additions of Alkali Oxides to Base Glaze Composition of Glazes, Molecular Equivalents

Glaze Li2O Na2O K20 PbO SiO2 1 - - - 1.0 1.3 2 0.1 - - 0.9 1.3 3 - 0.1 - 0.9 1.3 4 - - 0.1 0.9 1.3 5 0.05 0.05 - 0.9 1.3 6 0.033 0.033 0.033 0.9 1.3 7 0.05 - 0.05 0.9 1.3 8 - 0.05 0.05 0.9 1.3

% Melted Oxide Composition Glaze Li2O Na2O K20 PbO SiO2

1 - - - 72.01 27.99 2 1.06 - - 71.25 27.69 3 - 2.17 - 70.45 27.38 4 - - 3.27 69.66 27.07 5 0.53 1.09 - 70.85 27.53 6 0.35 0.72 1.09 70.46 27.38 7 0.52 - 1.65 70.45 27.38 8 - 1.08 1.64 70.05 27.23

In the case of all specimens, the fired glazes were good in appearance. The lead release data are given below:

Lead PPM Glaze Individual Data Average

1 0.34 - 0.47 - 0.41 0.41 2 3.54 - 0.56 - 0.50 1.53 3 4.30 - 4.45 4.38 4 4.10 - 4.24 - 6.80 5.05 5 1.04 - 0.90 - 0.72 0.89 6 0.98 - 0.38 - 0.78 0.71 7 1.76 - 1.08 - 2.16 1.67


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8 1.80 - 1.54 - 1.90 1.75

For a constant alkali level of 0.1 molecular equivalent in this base lead silicate glaze, it is seen that all three single alkali oxide additions increase the lead release over that for the base lead silicate glaze with Li2O increasing it least, Na2O being intermediate and K2O increasing the lead release most. The effects of these alkali oxide additions on the structure and lead release should depend on their radii and field strengths and also on the sizes of the interstices that exist in the glass structure. The data show increasing lead release with increasing ionic radii of the alkali oxides (Li2O-0.60, Na2O-0.95 and K2O-1.33).

Consistent with earlier findings, it is seen that the mixture of any two of the three alkali oxides totaling 0.1 molecular equivalent resulted in lower lead release values than the same amount of any of the single oxide additions. The two binary additions containing K2O also showed higher lead release than the binary of the other two oxides. The addition of the three alkali oxides gave the lowest average lead release value for a given total alkali oxide addition to such a base glaze, the use of all three alkali oxides would be preferred over any two in respect to lead release.

Additions of Alkaline Earth Oxides to Base Glaze

The modifications of the base glaze by additions of alkaline earth oxides are shown below for glazes 9 through 23 inclusive:

Composition of Glaze, Molecular Equivalents

Glaze MgO CaO BaO SrO PbO SiO2 9 0.1 - - - 0.9 1.3 10 - 0.1 - - 0.9 1.3 11 - - - 0.1 0.9 1.3 12 - - 0.1 - 0.9 1.3 13 0.05 0.05 - - 0.9 1.3 14 0.05 - - 0.05 0.9 1.3 15 0.05 - 0.05 - 0.9 1.3 16 0.033 0.033 0.033 - 0.9 1.3 17 0.033 0.033 - 0.033 0.9 1.3 18 0.033 - 0.033 0.033 0.9 1.3


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19 - 0.033 0.033 0.033 0.9 1.3 20 0.025 0.025 0.025 0.025 0.9 1.3 21 - 0.05 0.05 - 0.9 1.3 22 - 0.05 - 0.05 0.9 1.3 23 - - 0.05 0.05 0.9 1.3

% Melted Oxide Composition

Glaze MgO CaO BaO SrO PbO SiO2 9 1.42 - - - 70.99 27.59 10 - 1.87 - - 70.59 27.44 11 - - - 3.58 69.43 26.99 12 - - 5.21 - 68.26 26.53 13 0.71 0.99 - - 70.79 27.51 14 0.71 - - 1.81 70.20 27.23 15 0.70 - 2.65 - 69.60 27.05 16 0.46 0.65 1.76 - 69.95 27.18 17 0.46 0.65 - 1.20 70.35 27.34 18 0.46 - 1.75 1.19 69.56 27.04 19 - 0.64 1.75 1.18 69.44 26.99 20 0.35 0.49 1.33 0.90 69.80 27.33 21 - 0.97 2.65 - 69.41 26.97 22 - 0.97 - 1.81 70.10 27.12 23 - - 2.63 1.78 68.84 26.75

Again the compositions were entirely fritted. The glazes were sprayed on cups from Plant C and were fired in a gas-fired laboratory kiln on a 4.5 hour heating schedule to Cone 076. All of these glazes were good in appearance. The lead release data were:

Lead PPM

Glaze Individual Data Average 9 2.86 - 3.08 - 1.86 2.60 10 0.84 - 0.94 - 1.08 0.95 11 3.85 - 1.45 - 7.55 4.28 12 5.60 - 5.40 - 6.70 5.90


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13 0.90 - 0.30 - 0.45 0.72 14 0.40 - 0.62 - 0.51 0.51 15 2.72 - 2.46 - 3.36 2.85 16 0.39 - 0.85 - 0.59 0.78 17 1.68 - 0.80 - 1.04 1.17 18 2.03 - 1.60 - 1.56 1.75 19 2.14 - 1.26 - 1.32 1.57 20 2.24 - 1.40 - 1.80 1.81 21 1.32 - 1.76 - 2.42 1.83 22 0.67 - 0.82 - 0.94 0.81 23 0.36 - 0.66 - 2.20 1.07

The alkaline earth oxide additions gave similar trends to the alkali oxide additions in respect to lead release. With 0.1 molar (molecular) equivalent additions of single oxides show an increase in lead release in the order: MgO, SrO, BaO or with increasing oxide ionic radii. The value for the 0.1 molar addition of CaO was anomalous in this experiment.

Additions of alkaline earths in equivalent molar pairs of MgO-CaO, MgO-SrO, CaO-SrO, CaO-BaO and SrO-BaO totaling 0.1 molar equivalent show a lead release below that of MgO above with MgO-BaO slightly above that of Mg0 above.

Additions of alkaline earths in equivalent molar triplets totaling 0.1 molar equivalents again show lead release values falling below that of MgO along, but above that of MgO-CaO and MgO-SrO.

Additions of equi-molar equivalent quadruple of the four alkaline earths totaling 0.1 molar equivalents falls below that of MgO alone, but above the values of the alkaline earth triplets.

From these data, it may be inferred that alkaline earth lead silicate glasses of best chemical durability with respect to lead release are to be obtained with oxide additions of MgO-CaO or MgO-SrO and triplet additions of MgO-CaO-SrO, that is, combinations of alkaline earth oxides having the smaller ionic radii.


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Additions of Opacifying Oxides

Composition of Glaze, Molecular Equivalents

Glaze PbO SiO2 TiO2 ZrO2 SnO2 24 1.0 1.3 0.03 - - 25 1.0 1.3 0.06 - - 26 1.0 1.3 - 0.03 - 27 1.0 1.3 - 0.06 - 28 1.0 1.3 - - 0.03 29 1.0 1.3 - - 0.06

% Melted Oxide Composition

Glaze PbO SiO2 TiO2 ZrO2 SnO2 24 73.49 25.72 0.79 - - 25 72.91 25.52 1.57 - - 26 73.18 25.02 - 1.80 - 27 71.88 24.58 - 3.54 - 28 72.98 25.56 - - 1.46 29 71.91 25.17 - - 2.92

The glazes were prepared as frits and applied on bisque cups from Plant C. The specimens were glost-fired in a gas-fired laboratory kiln on a 4.5 hour heating schedule to Cone 07. The glazes were good in appearance.

The lead release data were:

Glaze Individual Data Average 24 0.45 - 0.47 - 0.61 0.51 25 0.39 - 0.47 - 0.51 0.46 26 0.17 - 0.30 - 0.38 0.28 27 0.30 - 0.23 - 0.32 0.28 28 0.67 - 0.68 - 0.19 0.51 29 0.29 - 0.19 - 0.49 0.32

These data are to be compared with the value for the base glaze of 0.41 PPM lead release (similarly applied and fired). The ZrO2 additions (both levels) gave a somewhat lower lead release. Ions of high charge and small


68

size introduced as their oxides, e.g., TiO2 or ZrO2 may cause breakage of linkages. These cations of high field strength, when they fit into the interstices may cause a strong contraction of the unsaturated oxygen ions surrounding them and lead to a tight binding and an overall strengthening of the structure.

Effect of Base Glaze Variations: Lead Silicate (PbO·1.5SiO2)

Latin Square Experiment

A Latin square experiment was used to show effects of three levels of additions of Al2O3, B2O3 and ZrO2 to a base glaze of PbO·1.5SiO2. The levels of the additions selected were as given below:

Molar Equivalents (Al2O3) (B2O3) (ZrO2)

Levels A B Z 0 0.00 0.00 0.00 1 0.15 0.15 0.02 2 0.30 0.30 0.04

The molar formulas for the nine selected members are as follows:

PbO SiO2 A1203 B203 ZrO2 A0B0Z0 1.00 1.50 0.00 0.00 0.00 A0B1Z1 1.00 1.50 0.00 0.15 0.02 A1B2Z2 1.00 1.50 0.00 0.30 0.04 A1B0Z2 1.00 1.50 0.15 0.00 0.02 AlB1Z2 1.00 1.50 0.15 0.15 0.04 A1B2Z0 1.00 1.50 0.15 0.30 0.00 A2B0Z2 1.00 1.50 0.30 0.00 0.04 A2B1Z0 1.00 1.50 0.30 0.15 0.00 A2B2Z1 1.00 1.50 0.30 0.30 0.02

The compositions were fritted entirely with the exception of 0.15 equivalents of Al203 introduced as clay for the Al and A2 members. The A0 glazes, containing no A12O3 addition to the batch, were suspended using a gum-Methocel solution. The ZrO2 was introduced into the frit batches as milled zircon (ZrO2·SiO2). The frit batches were mixed thoroughly, then drip-fritted (through a small orifice in the crucible of 1/8 inch diameter) at


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temperatures of 1050o - 1165o C. The glaze batches were ball milled to pass 200 mesh. The glazes were applied on bisque cups from Plant C. The specimens were fired in a gas-fired laboratory kiln on a 4. 5 hour heating schedule to Cone 03. The glazes were all good, bright glazes with the exception of A2B0Z2 that was poorer in appearance. This glaze had high levels of additions of Al2O3 and ZrO2 and no B2O3.

The lead release data for the nine members, using the standard test, were as follows:

Individual Data Average A0B0Z0 0.23 - 0.19 - 0.17 0.20 A0B1Z1 0.82 - 0.75 - 0.95 0.84 A1B2Z2 1.34 - 0.99 - 0.61 0.93 A1B0Z2 0.43 - 0.20 - 0.25 0.29 AlB1Z2 0.22 - 0.28 - 0.13 0.21 A1B2Z0 0.20 - 0.17 - 0. 24 0.20 A2B0Z2 0.31 - 0.25 - 0.26 0.27 A2B1Z0 0.32 - 0.33 - 0.35 0.33 A2B2Z1 0.33 - 0.25 - 0.22 0.27

The individual data, with one exception, showed reasonably good agreement.

Knoop Hardness

Knoop hardness measurements were made on these glazes using a 1000 gram load. The objective was to relate this property with the strength of the structure and the R and P factors. These factors are calculated as follows:

B and Al Si, Of SumFormulaMolecular In Oxygens Of Sum

R �

ModifiersNetwork Of SumFormulaMolecular In Oxygens Of SumP �

R is a measure of the number of single-bonded oxygen. P is a measure of

the oxygens available per network-modifying ion. The R values are less than 2.5 and the P values are greater than 3.9 for normal glasses. In general, the structure becomes stronger and more resistant to acid attack as R is reduced. B2O3-containing glazes differ from aluminosilicate glazes. Al3+ always has 4


70

coordination, while B3+ may have 3 or 4 coordination. It has been suggested that in normal glazes all the B2O3 and A12O3 accept oxygen from the modifier oxides, the Al2O3 forming A1O4 tetrahedra, and the B2O3 broken B-O-B bonds at the glaze-maturing temperature. When the glaze is cooled slowly it is likely that BO4 tetrahedra form. Glazes with very high B2O3 content must contain BO3 triangles since there is insufficient oxygen brought in by the modifiers to break the B-O-B bonds; these glazes thus may have poor durability. Although increasing B3+ ions will reduce the R factor as rapidly as Al3+ ions, the resultant structure will not he as strong as if A12O3 had been added instead of B2O3; in fact the addition of B2O3 most probably will give a structure weaker than the parent lead silicate glaze and thus less durable.

These structural factors as calculated for the nine glazes are given in the following table along with average values for Knoop hardness and lead release (all 27 combinations are listed with data taken only on the nine selected members tested):

R Factor

P Factor KHN1000 Lead PPM

A0B0Z0 2.67 4-.00 389 0.20 A0B0Z1 A0B0Z2 A0B1Z0 A0B1Z1 2.49 4.40 417 0.84 A0B1Z2 A0B2Z0 A0B2Z1 A0B2Z2 2.37 4.79 406 0.98 A1B0Z0 A1B0Z1 2.49 4.40 417 0.29 A1B0Z2 A1B1Z0 A1B1Z1 A1B1Z2 2.37 4.79 419 0.21 A1B2Z0 2.31 5.35 41.4 0.20 A1B2Z1


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A1B2Z2 A2B0Z0 A2B0Z1 A2B0Z2 2.37 4.79 413 0.27 A2B1Z0 2.31 5.35 424 0.33 A2B1Z1 A2B1Z2 A2B2Z0 A2B2Z1 2.16 5.72 415 0.27 A2B2Z2

The Latin square design permits extracting information from a minimum number of samples but it assumed no interactions between the variables being studied. This approach can be most helpful in determining direction of future studies and elimination of unnecessary testing.

The average values for the different levels of the three oxides in the nine members were:

Lead PPM KHN1000 R Factor P Factor A0 0.67 404 2.51 4.40 A1 0.23 417 2.39 4.85 A2 0.29 417 2.28 5.29 B0 0.25 406 2.51 4.40 B1 0.46 420 2.39 4.85 B2 0.48 412 2.28 5.29 Z0 0.24 409 2.43 4.90 Z1 0.47 416 2.38 4.84 Z2 0.49 413 2.37 4.79

The data would suggest that A1B2Z0 (not tested) member would be the most acid-resistant of the 27 members. Higher levels of Al2O3 definitely reduce the lead release whereas increasing B2O3 levels increase it. For this type of glaze and the relatively low firing temperature, ZrO2 does not appear to improve the acid resistance.


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Analysis of variance for the individual lead release data shows the variance in respect to the A levels to be highly significant whereas variance in respect to the B and Z levels are significant. In contrast the levels of A, B and Z are insignificant as a source of variance for the individual hardness data. Therefore, only the lead release data are considered in respect to the structural factors.

The R factors decrease with higher levels of A and B and also for Z (to a much lesser extent). The P factors increase markedly for higher levels of A and B and decrease slightly for higher levels of A. The lead release data confirm what might be expected for the A12O3 and B2O3 additions. It would be of interest to try slower cooling of the glazes containing higher levels of B2O3 to form more BO4 tetrahedra and possibly attain greater strength and durability. Firing the same glazes to Cone 1 or 2 might also be informative in respect to the performance of ZrO2. Also other experiments might be designed with greater compositional variation to attain larger differences in the measured properties as well as the calculated structural factors.

Effects of Al2O3, B2O3 and ZrO2

The same Latin square experiment, designed to show the effects of three levels of additions of A12O3, B2O3 and ZrO2 to a base glaze of PbO·1.5Si02, was repeated except that a higher glost firing temperature was used. Again the glazes were applied on cups from Plant C. (The previous firing was to Cone 036). The specimens in this experiment were fired in a gas-fired laboratory kiln on a 4.5 hour heating schedule to Cone 16. The glazes were all good, bright, glossy glazes in appearance.

The lead release data for the nine members, using the standard test, were as follows.

Lead PPM Individual Data Average A0B0Z0 0.13 - 0.20 - 0.25 0.21 A0B1Z1 0.59 - 0.29 - 0.40 0.43 A0B2Z2 0.08 - 0.06 - 0.07 0.07 A1B0Z1 0.18 - 0.11 - 0.19 0.16 A1B1Z2 0.18 - 0.19 - 0.25 0.21 A1B2Z0 0.09 - 0.07 - 0.08 0.08


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A2B0Z2 0.27 - 0.27 - 0.23 0.26 A2B1Z0 0.54 - 0.60 - 0.45 0.53 A2B2Z1 0.18 - 0.31 - 0.17 0.22

The individual data showed fairly good agreement. The average values for the different levels of the three oxides in the nine members were:

Lead PPM

A0 - 0.23 B0 - 0.21 Z0 - 0.27

Al - 0.15 B1 - 0.39 Z1 - 0.27

A2 - 0.34 B2 - 0.12 Z2 - 0.18

Thus, of the 27 possible combinations of A0A1A2, B0B1B2, Z0Z1Z2, the above data indicate that A1B2Z2would be the most acid resistant although this combination was not one of the glazes tested. Of the glazes tested, A0B2Z2 appeared to be the most acid resistant. These two compositions would seem to indicate that for the higher firing temperatures, the low level of Al2O3 and the high levels of B2O3 and ZrO2 are favorable from the standpoint of acid resistance. Analysis of variance of these lead release data again indicates that changes in the levels of A12O3, B2O3 and ZrO2 all have significant effects on the resulting lead release of the glazes.

The effects of the oxides are not in agreement for the two firing temperatures and in part with what might have been expected from structural considerations. The Latin square designed experiment assumes no interactions in the glaze system, even though these several compositions had been tested independently instead of as thin glaze applications on a body. The effect of body solution from this corrosive lead silicate glaze, especially at the higher temperatures, is also not considered in this analysis. These are most likely the principal reasons for the lack in agreement on the effects of the levels of the oxides at the two firing temperatures.

If the lead silicate base glaze and the modifications as used here had been tested as fine powders, relatively high lead release values would be expected. It is important to note that these compositions applied as glazes and fired at relatively high temperatures (for the compositions involved) gave very low lead release values, when fired both at Cone 03 and Cone 1.


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Repeated Extractions on the Same Glaze Surface

Depletion tests involving the repeated testing of the same cup specimen is pertinent from the standpoint of continued service.

A cup from Plant B, which was glazed with a clear, Cone 4-5 glaze, was so tested. This was selected since this glaze is typical of the type used widely in the dinnerware industry and represents the great bulk of all dinnerware glaze surfaces. Using the standard test, the lead release data for repeated testing of the same cup were as follows:

Repeated Tests on Clear, Cone 4-5 Glaze Test Number Lead PPM

1 0.10 2 0.09 3 0.10 4 0.09 5 0.05 6 0.06 7 0.05 8 0.06

The lead release values are all very low; the difference between the high and low value is 5 ppm. Repeated leaching of the glaze appears to give decreasing lead release as the lead is depleted from the surface.

Repeated Tests on Low Temperature Cone 05 Glaze

This glaze was selected since it showed higher lead release due to the presence of a two weight percent mill addition of CuO. The glaze batch was: Frit C 90 Clay 10 CuO 2

Frit C 0.071 K2O 0.113 Na2O 0.131 A12O3 2.429 SiO2 0.455 PbO 0.675 B2O3 0.035 ZrO2


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0.361 CaF2

This glaze was applied on bisque cups front Plant C to 1875ºF. in a gas-fired laboratory kiln. The lead release data for repeated extractions on the same cup are given below:

Test Number Lead PPM 1 0.78 2 0.36 3 0.46 4 0.32 5 0.37 6 0.24 7 0.13 8 0.12

The progressive decrease of lead release from the same glaze surface (same cup specimen) on continuing testing is noted. The glaze surface involved here was good in appearance and of light green color. Similar trends for low temperature, copper-containing glazes were previously observed on glazes that incorporated copper-containing stains.

Repeated Tests on Low Temperature Uranium Red Glaze

A Cone 03-04 uranium red glaze was tested which gave the following values for the three cup specimens tested: 1.10-2.14-2.24; Average 1.83 PPM Lead. The individual cups giving the high and low values were subjected to repeated extractions using the standard tests. The results were:

Lead PPM Test No. Cup LV Cup HV

1 1.10 2.24 2 0.84 0.46 3 0.55 0.64 4 0.65 0.70 5 0.55 1.04 6 1.04 1.22 7 1.14 1.27 8 1.50 1.56


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9 1.37 10 1.28 11 1.41 12 1.58 13 1.53 14 1.92 15 1.80 16 1.74 17 2.18 18 1.66 19 1.82 20 1.62 21 1.62

As will be noted for both of these cup specimens, there is an initial decrease in lead release after the first extraction, - then the lead release progressively increased with repeated extractions. After eight repeated extractions, the lead release from the glaze on Cup LV exceeded the initial amount and this became progressively worse on continued testing up to seventeen tests after- which the lead release falls off again.


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CHAPTER 8

Effect of Glaze Processing Variables Introduction

In addition to the composition of glazes the way in which they are applied and fired has a significant effect on the appearance and properties of many glazes. Therefore, a study was undertaken to assess the effect of various production parameters on the lead release of a range of glazes as measured by the standard 4% acetic acid, 24 h test.

Production parameters varied in this study included:

�� The application of the glaze and thickness;

�� The characterization of the bisque body and the interaction of the glaze and bisque

�� The glost-firing parameters such as time, temperature and atmosphere.

While the manufacturing technologies for these parameters are well developed, the relationship between these parameters and the lead release performance is not well documented in the literature. The following summarizing points can be drawn from the data presented in the balance of this section.

In general, a trend was noted for increased lead release with increased glaze thickness. The lead release values for the typical Cone 4-5 dinnerware glaze, applied in different thicknesses, were too low to draw any conclusions.


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In contrast, tests on two Cone 076 low temperature glazes showed a marked increase in lead release with increasing glass thickness.

Tests on different production bisque cups from three plants, for similar glaze applications and firing treatments, suggested that variation in bisque properties would be reflected when the lead release values are sufficiently high to have experimental significance. The lead release values for the typical Cone 4-5 dinnerware glaze, applied on the different bisque, were too low to show any trends. However, when a low temperature (1875ºF) glaze was used along with modifications of this glaze (CuO and other coloring oxide mill addition) the variation in the bisque properties as affecting the glaze maturation was reflected in the trends on lead release values.

Variations in the time and temperature of firing of the typical Cone 4-5 dinnerware glaze again gave lead release values too low to consider that any significant trends were shown. Factorial designed experiments were run using a Cone 02-4 glaze and different levels of firing time, temperature, and glaze thickness. In these tests the time and temperature were the most important factors. The lead release was reduced by longer firing times and higher temperatures for this glaze-body system. In another designed experiment a low temperature glaze composition (Cone 07) was fired at higher than normal cone temperatures (Cone 02-5) for different time and glaze thicknesses. The importance of the time factor was emphasized here, permitting more body solution and improved acid resistance.

For well designed glazes, the lead release from the glazed surface was not increased substantially by firing considerably lower than the normal maturing temperatures.

Relationship Between Lead Release and Glaze Thickness

The data below are for production glazes, which were applied and fired at the plant. The test specimens received heavy, medium, and light applications of the respective glazes. The average thickness of the glaze was measured for three cup specimens of each glaze and each weight of application. All four of these glazes and their light to heavy applications were bright, glossy and of good texture. Data for the four production glazes are given below:

Cone 3 Clear Glaze #1


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Application

Thickness (in.)

Lead ppm

Heavy 0.006 0.44 Medium 0.005 0.39

Light 0.004 0.10

Cone 3 Clear Glaze #2

Application

Thickness (in.)

Lead ppm

Heavy 0.005 0.82 Medium 0.004 0.65

Light 0.003 0.59

Cone 01 All-Fritted Clear Glaze #1

Application

Thickness (in.)

Lead ppm

Medium 0.005 0.15 Light 0.004 0.09

Cone 01 All-Fritted Clear Glaze #2

Application

Thickness (in.)

Lead ppm

Heavy 0.008 0.60 Medium 0.004 0.31

Light 0.004 0.35

The above data show a trend towards increased lead release with increased glaze thickness.


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Effect of Glaze Thickness of a Cone 4 Fritted Lead Glaze

Glaze 0.09 K2O 0.09 Na2O 0.321 A12O3 3.064 SiO2 0.58 CaO 0.360 B2O3 0.24 PbO

The glaze was prepared from ninety percent (90%) Frit D and ten percent (10%) clay mill addition from the above glaze formula. This base glaze and various modifications involving different mill additions of opacifying oxides (previously described) were used here with different thickness of application. In these tests the base glaze and modifications were applied on standard bisque cups from Plant C using light, medium and heavy applications, - approximating 1.5 mils, 2.8 mils, and 3.5 mils respectively fired thickness. The specimens were fired on a 4.5 hour heating schedule to Cone 4 down in a gas-fired laboratory kiln.

Lead ppm Light

1.5 mils Medium 2.8 mils

Heavy Glaze

3.5 mils BG 0.07 0.14 0.01

BG-1 0.13 0.12 0.02 BG-2 0.03 0.10 0.04 BG-3 0.17 0.13 -- BG-4 -- 0.13 0.05 BG-5 0.12 0.17 0.08 BG-6 0.15 0.13 0.03 BG-7 0.03 0.16 0.06 BG-8 0.04 0.08 0.01

Again the lead release values are too low to draw any conclusions on the relation of lead release and glaze thickness.


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Effect of Glaze Thickness in Cone 07 Glazes

The scatter in lead release data for assumedly similar specimens is generally found to be greater when the acid resistance is relatively poor. Although the variation from specimen to specimen would depend upon a number of factors, in the following tests an attempt was made to relate the thickness of the glaze application to lead release. All the specimens were fired on the same 4.5 hour heating schedule to Cone 07. They involved different thickness of applications on the similar bisque cups from Plant C. The actual fired thickness was measured using a microscope with a filar head. The glaze compositions were selected to have poor acid resistance and were not of a type as would be used commercially. The compositions were:

Li2O K2O PbO SiO2 Glaze Molecular Equivalents

A 0.1 - 0.9 1.3 B - 0.1 0.9 1.3 % Melted Oxide Composition

A 1.06 - 71.25 27.69 B - 3.27 69.66 27.07

Sample

Fired Thickness of Glaze (mils)

Lead ppm

A-1 2.71 0.5 A-2 3.03 0.6 A-3 5.24 3.5

B-1 2.77 1.9 B-2 2.98 3.7 B-3 3.09 4.1 B-4 3.10 4.2 B-5 3.22 6.8 B-6 4.84 12.4 B-7 6.66 16.5


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From the above data, it can be concluded that for such compositions in this R2O-PbO-SiO2 system, the fired glaze thickness is a significant factor in respect to lead release.

Effect of Different Bisque on Lead Release

Some of the base glazes and modifications by mill additions of either coloring oxides or opacifying oxides were applied on bisque cups received from three different plants. The glaze application and firing treatment were held as constant as possible so that any differences in lead release might relate to variation in bisque properties and interfacial action. Several examples of the results of such tests are given below:

Clear Cone 4 Glaze with Coloring Oxides

The Cone 4 glaze used here had the following composition: 0.013 K2O 0.182 Na2O 0.290 A12O3 2.971 SiO2

0.572 CaO 0.360 B2O3 0.233 PbO

Glaze Batch

Frit A 51.5 Frit B 25.1

Whiting 4.0 Clay 4.7 Flint 14.7

The compositions of the two frits are as follows:

Frit A 0.02 K2O 0.29 Na2O 0.27 A12O3 2.49 SiO2

0.69 CaO 0.57 B2O3

Frit B 1.00 PbO 0.25 A12O3 1.92 SiO2


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The test specimens were all fired approximately five (5) hours to Cone 4 in a gas-fired laboratory kiln. Each value for lead release reported is the average for three cup specimens.

LEAD RELEASE DATA IN PPM FOR GLAZE APPLIED TO BISQUE CUPS FROM THE INDICATED PLANT

Glaze Plant B Plant C Plant D BG 0.07 0.10 0.03 BG-1 0.08 0.05 0.03 BG-2 1.58 1.19 1.14 BG-3 0.07 0.07 0.04 BG-4 0.04 0.03 0.03 BG-5 0.04 0.08 0.11 BG-6 0.01 0.01 0.11

With the exception of BG-2, which was a modification of the base glaze (BG) to include a two percent (2%) by weight mill addition of CuO, all the lead release values were too low to draw any conclusions regarding effects of different bisques.

Lower Temperature Glaze with Coloring Oxides

Glaze 0.071 K2O 0.113 Na2O 0.279 A12O3 2.726 SiO2

0.455 PbO 0.675 B2O3 0.035 ZrO2 0.361 CaF2

Glaze Batch Frit C 90 Clay 10

Frit C 0.071 K2O 0.113 Na2O 0.131 A12O3 2.429 SiO2

0.455 PbO 0.675 B2O3 0.035 ZrO2 0.361 CaF2


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The above glaze and modifications of this glaze with various mill additions of coloring oxides were applied on bisque cups from Plants B, C and D. Each value for lead release is the average for three cups tested. The test specimens were all fired in 4.5 hours to 1025o C. in a gas-fired laboratory kiln. The data are given below:


Glaze Plant B Plant C Plant D BG-LT 0.34 1.20 2.55 BG-LT -1 0.80 3.10 2.55 BG-LT -2 12.50 12.00 13.40 BG-LT -3 2.25 4.75 6.85 BG-LT -4 1.24 3.31 5.80 BG-LT -5 5.25 6.90 4.15 BG-LT -6 0.42 1.17 1.28

The effects of the coloring oxide mill additions on the lead release from this low temperature glaze were marked (previous note for these glazes on Plant C bisque only). With some exception, the same glaze applied on Plant B bisque gave the lowest value and on Plant D bisque the higher value with the Plant C bisque being intermediate. This suggests that the bisque properties and resultant body-glaze interaction would be reflected when the lead release values are sufficiently high to have experimental significance.

As previously noted this type glaze was designed for talc-clay type tile bodies and is not used for glazing dinnerware bodies.

Clear Cone 4 Glaze with Opacifying Oxides

Base Glaze 0.09 K2O 0.09 Na2O 0.321 A12O3 3.064 SiO2

0.58 CaO 0.360 B2O3 0.24 PbO

Glaze Batch

(Not Including Opacifier)


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Frit D 90 Clay 10

Frit D 0.09 K2O 0.09 Na2O 0.19 A12O3 2.80 SiO2

0.58 CaO 0.36 B2O3 0.24 PbO

The above base glaze and modifications of this glaze with various mill additions of opacifying oxides were applied on bisque cups from Plant B, C and D. The glaze cups were glost-fired to Cone 46 and a 4.5 hour heating schedule in a gas-fired laboratory kiln.


Glaze Plant B Plant C Plant D

BG 0.05 0.14 0.04

BG-1 0.04 0.12 0.01

BG-2 0.03 0.10 0.06

BG-3 0.08 0.13 0.05

BG-4 0.09 0.13 0.01

BG-5 0.04 0.17 0.02

BG-6 0.04 0.13 0.02

BG-7 0.04 0.16 0.01

BG-8 0.09 0.08 0.01

All the glazes were good in appearance. Because of differences in the fired color of the bisques from the three different plants any differences in opacity were difficult to evaluate. All of the lead release values were too low to draw any conclusions regarding effects of the different bisques.


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General Effects of Varying Firing Time and Temperature

The same Cone 4 glaze using a single lead frit and various modifications of mill additions of opacifying oxides were given additional firings to other Cone temperatures but in each case using a 4.5 hour heating schedule. In all cases the glazes were applied on bisque cups from Plant C using the same application procedure.

Base Glaze 0.09 K2O 0.09 Na2O 0.321 A12O3 3.064 SiO2

0.58 CaO 0.360 B2O3 0.24 PbO

Lead Release, ppm Glaze Cone 02 Cone 1 Cone 3 Cone 4 Cone 5 BG 0.06 0.09 0.01 0.14 0.04 BG-l 0.07 0.01 0.06 0.12 0.01 BG-2 0.08 0.02 0.01 0.10 0.09 BG-3 0.07 0.09 0.01 0.13 0.06 BG-4 0.07 0.01 0.02 0.13 0.11 BG-5 0.11 0.16 0.02 0.17 0.04 BG-6 0.14 0.06 0.01 0.13 0.09 BG-7 0.13 0.14 0.04 0.16 0.06 BG-8 0.06 0.04 0.01 0.08 0.05

The glazes were all fairly good in appearance. The modifications, involving mill additions of opacifiers, showed greater opacification at the lower cone temperature for the same level and kind of opacifying oxide. At all temperatures the opacification increased with increasing levels of each of the three opacifiers as would be expected. The high level of TiO2 gave most of the yellowing shown for this oxide addition and this was greater with the lower cone temperature firings.

Again for all the glazes, fired to the five different cone temperatures on the same heating schedule, the lead release values were too low to consider that any significant trends were shown.


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The same glazes as used above were applied on bisque cups from Plant C and fired to Cone 46 on slower and faster heating cycles than that used previously. The heating schedules were 2.5 hours, 4.5 hours and 6.5 hours in all cases to Cone 4.

LEAD RELEASE DATA IN PPM

Glaze 2.5 Hours

4.5 Hours

6.5 Hours

BG 0.04 0.14 0.27

BG-1 0.04 0.12 0.13

BG-2 0.02 0.10 0.03

BG-3 0.06 0.13 0.17

BG-4 0.04 0.13 0.07

BG-5 0.07 0.17 0.08

BG-6 0.04 0.13 0.13

BG-7 0.05 0.16 0.14

BG-8 0.04 0.08 0.14

Again the glazes were all fairly good in appearance. Opacification was greatest for the fastest firing schedule. The high levels of TiO2 additions showed greatest yellowing for the faster firing. The lead release values, although they might appear to be favored by the fastest firing, are too low in magnitude to form any conclusions. For this reason and also because commercial firings generally involve longer firing times, the statistically designed experiment reported later involved greater variation in these firing parameters.


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Effect of Glaze Thickness, Firing Time and Temperature: Commercial Glazes

Factorial Experiment Design

A factorial designed experiment was used to study the effect of production parameters on the lead release from a typical lead glaze using the standard tests. In such an experiment each variable is varied at all levels of all other variables. The variables or factors were:

�� Glaze thickness at three levels

�� Firing time at three levels

�� Firing temperature at three levels

�� Three-fold replication

In this work the same glaze composition and same body (bisque cups from Plant C) were used throughout the tests.

The glaze was a standard commercial Cone 02-4 composition, incorporated a single lead frit, as follows.

Glaze 0.09 K2O 0.09 Na2O 0.321 A12O3 3.064 SiO2

0.53 CaO 0.360 B2O3 0.24 PbO


The sample cups used were the standard bisque cups as obtained from Plant C.

The production parameters and the levels of each were as follows:

Experimental Factor Levels Fired Glaze thickness, �m

50 125 200


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Firing temperature, oC 1100 1140 1175 Firing time, hours 4 10 17

Three-fold replication was used throughout the experiment.

Results

The data are given on the following page. Statistical analysis of the data suggests that time and temperature are the more important factors influencing lead release from this glaze and thickness does not have a major effect. However, it should be noted that thickness was the most difficult parameter to control in this experiment. Lead release was reduced at longer times and higher temperatures of firing for this glaze body system. Since the lead release values are all very low, an experiment will be designed with a high solubility glaze.

EFFECT OF GLAZE THICKNESS, FIRING TIME AND TEMPERATURE: COMMERCIAL GLAZES

LEAD ppm 50 �m 125 �m 200 �m

Time, h Temperature, oC Temperature, oC Temperature, oC 1100 1140 1175 1100 1140 1175 1100 1140 1175

0.287 0.080 0.141 0.127 0.064 0.050 0.141 0.059 0.026 0.138 0.069 0.044 0.112 0.101 0.046 0.132 0.070 0.160

4 h

0.290 0.079 0.050 0.128 0.059 0.031 0.136 0.084 0.204 Av. 0.238 0.076 0.078 0.122 0.075 0.042 0.136 0.071 0.130 Range 0.152 0.011 0.097 0.016 0.042 0.019 0.009 0.025 0.173

0.060 0.044 0.031 0.002 0.014 0.040 0.021 0.044 0.034 0.012 0.034 0.033 0.010 0.022 0.020 0.016 0.025 0.029

10 h

0.002 0.040 0.017 0.028 0.040 0.014 0.021 0.047 0.029 Av. 0.025 0.039 0.029 0.013 0.025 0.025 0.019 0.039 0.031 Range 0.058 0.010 0.021 0.026 0.026 0.026 0.005 0.021 0.005

EFFECT OF GLAZE THICKNESS, FIRING TIME AND TEMPERATURE: COMMERCIAL GLAZES

[CONTINUED]

LEAD ppm 50 �m 125 �m 200 �m

Time, h Temperature, oC Temperature, oC Temperature, oC 1100 1140 1175 1100 1140 1175 1100 1140 1175 0.000 0.047 0.088 0.050 0.030 0.005 0.030 0.023 0.019 0.000 0.026 0.004 0.005 0.014 0.014 0.013 0.023 0.000

17 h

0.000 0.006 0.000 0.085 0.018 0.019 0.042 0.026 0.016 Av. 0.000 0.026 0.031 0.047 0.021 0.013 0.020 0.024 0.012 Range 0.000 0.041 0.038 0.080 0.016 0.014 0.029 0.003 0.019


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Effect of Glaze Thickness, Firing Time and Temperature: Laboratory Fritted Glazes

The previous factorial experiment used a commercial glaze in studying the effect of production parameters on lead release. In the present case a selected lead frit composition was investigated using the standard test. The variables or factors again were:

�� Glaze thickness at three levels

�� Firing time at three levels

�� Firing temperature at three levels

�� Three-fold replication

The glaze used in this experiment had the following molecular composition:

0.9 PbO 0.30 B2O3 1.50 SiO2 0.1 K2O

The glaze slip consisted of the fritted glaze composition suspended in a Methocel solution. Bisque cups from Plant C were used throughout the tests. The production parameter matrix was similar to the prior example but with a few differences in the firing times.

Experimental Factor Levels Fired Glaze thickness, �m

50 125 200

Firing temperature, oC 1100 1140 1175 Firing time, hours 4 8 12

The data are given on the following page. Summarizing the results, from the statistical point of view, the primary parameters affecting the lead release of this glaze-body combination is time with some effect due to temperature, some effects due to time and temperature, and with thickness (in this experiment) having a relatively insignificant effect.

The frit used in this experiment would be relatively poor in acid resistance if tested as a fine powder. Applied as a glaze on the bisque from 50 - 125 �m in fired thickness, and fired in from 4 to 12 hours to Cone 02 to Cone 5, the


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glazes do have good acid resistance. This reflects the body solution into the glaze providing a higher Al2O3 and SiO2 content. It would be expected that such a glaze fired at too low a temperature, which did not permit such body solution into the glaze providing a higher Al2O3 and SiO2 content, would be much poorer in acid resistance. The same glaze applied on bisque cups from Plant C and fired to Cone 01 showed extremely high lead release. Yet this glaze, fired at this low temperature, was relatively good in appearance. This is an extremely important consideration even though such a composition would not be used as a dinnerware glaze. As shown in the following section, satisfactory and well-designed glazes can be fired considerably lower than their normal maturing temperature without significant increase in lead release.


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EFFECT OF GLAZE THICKNESS, FIRING TIME AND TEMPERATURE: LABORATORY FRITTED GLAZES

Lead Release Levels (ppm)

Levels of Thickness 50 �m 125 �m 200 �m Temperature, oC Temperature, oC Temperature, oC 1100 1140 1175 1100 1140 1175 1100 1140 1175 Totals 0.06 0.08 0.06 0.04 0.07 0.06 0.05 0.03 0.06 4 h 0.07 0.03 0.01 0.11 0.02 0.06 0.05 0.05 0.15 0.07 0.04 0.010

. 0.05 0.00.

1 0.17 0.09 0.12 0.13

Sum 0.20 0.15 0.12 0.20 0.10 0.29 0.19 0.25 0.34 0.59 0.50 0.75 Avg. 0.07 0.05 0.04 0.07 0.03 0.10 0.06 0.08 0.11 0.20 0.16 0.25 Range 0.01 0.05 0.05 0.07 0.06 0.11 0.04 0.07 0.07 0.12 0.18 0.33 0.04 0.03 0.05 0.01 0.04 0.06 0.02 0.03 0.03 8 h 0.02 0.05 0.07 0.01 0.03 0.06 0.01 0.04 0.03 0.02 0.04 0.06 0.00 0.04 0.09 0.06 0.02 0.03

Sum 0.08 0.12 0.18 0.02 0.11 0.21 0.09 0.09 0.09 0.19 0.32 0.48 Avg. 0.03 0.04 0.06 0.01 0.04 0.07 0.03 0.03 0.03 0.07 0.11 0.16 Range 0.02 0.02 0.02 0.01 0.01 0.03 0.05 0.02 0.00 0.03 0.05 0.05

[continued on next page]


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EFFECT OF GLAZE THICKNESS, FIRING TIME AND TEMPERATURE: LABORATORY FRITTED GLAZES

[continued from prior page]

Lead Release Levels (ppm)

Levels of Thickness 50 �m 125 �m 200 �m Temperature, oC Temperature, oC Temperature, oC 1100 1140 1175 1100 1140 1175 1100 1140 1175 Totals 0.01 0.00 0.05 0.04 0.06 0.05 0.05 0.03 0.00 12 h 0.04 0.01 0.03 0.040

. 0.02 0.02 0.10 0.00 0.02

0.03 0.01 0.02 0.06 0.03 0.03 0.07 0.00 0.01

Sum 0.09 0.02 0.10 0.14 0.11 0.10 0.22 0.03 0.03 0.45 0.16 0.23 Avg. 0.03 0.01 0.03 0.05 0.04 0.03 0.07 0.01 0.01 0.15 0.06 0.07 Range 0.03 0.01 0.03 0.02 0.04 0.03 0.05 0.03 0.02 0.10 0.08 0.08 Totals Sum 0.37 0.29 0.40 0.36 0.32 0.60 0.50 0.37 0.46 1.23 0.98 1.46 Avg. 0.13 0.10 0.13 0.13 0.11 0.20 0.16 0.12 0.15 0.42 0.33 0.48 Range 0.06 0.08 0.10 0.10 0.11 0.17 0.14 0.12 0.09 0.30 0.31 0.36


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Effect of Under-Firing on Lead Release

Under firing tests were first run on the Pb-349 based glaze and later also on some commercial glazes. The tests on the Pb-349 based glaze are first reported. Again the Pb-349 frit was used in ninety percent (90%) amount along with ten percent (10%) clay in preparing the glaze. A large number of bisque cups from Plant D were glazed and fired on a five (5) hour heating schedule to Cone 4. Then other fires were made at Cones 1, 01, 03, 05, 07, and an additional low temperature firing to 815o C. The lead release data for the various cups fired to the various temperatures are given below:

Firing Cone or Temperature

Lead Release ppm

Average

46 0.115-0.072-0.184

0.124

16 0.106-0.115-0.060

0.094

016 0.061-0.105-0.077

0.081

036 0.284-0.151 0.218 056 0.065-0.092 0.079 076 0.057 0.057

This suggests that the underfiring of a glaze composed essentially of a ‘well-balanced’ frit does not show increase in lead release on underfiring.

Properties of Underfired Production Glazes

Five commercial glazes from Plants B, C, I, K and L were applied on bisque cups from the respective plants. These were fired on a 4.5 hour heating schedule to Cone 026 or approximately 6 cones below their normal maturing temperature.

Plant Lead Release ppm

Average

B 0.36-0.08-0.16 0.20 C 0.10-0.05-0.07 0.07


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1 0.08-0.02-0.05 0.05 K 0.65-1.28-0.95 0.96 L 0.17-0.01-0.11 0.10

All glazes were bright and glossy. Glazes B, I and L were clear, colorless glazes. Glaze C was a light blue glaze. Glaze K was a brown, Rockingham type glaze. Four of the five commercial glazes showed very low values and the fifth was less than one ppm, when these commercial glazes were fired 6 cones lower than their normal maturing temperatures.


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CHAPTER 9

Decoration of Dinnerware

The decoration of porcelain, stoneware, earthenware and bone china typically involves the use of ceramic colors, i.e., high-temperature resistant colored silicate materials. A wide palette of colors has been produced for the ceramics industry based in part on traditional methods but more recently on research and development of specialized compounds. Early application

methods consisted of brushing but more recent technology has introduced lithographic, screen, and offset printing. Decoration is an essential part of the aesthetic and technical design of tableware and other utensils, and plays an important role in product differentiation.

Requirements for Ceramic Colors

The requirements that ceramic colors need to meet depend on the desired properties of the finisher design and the process employer to produce this design. The design itself is selected according to two criteria: aesthetic and useful properties. To satisfy the aesthetic demands, the color should be as opaque as possible, glossy and pleasing to the eye. Richness of color is also

CONE 05 EARTHENWARE


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desirable and the design drawn up by the artist must be reproduced as faithfully as possible. End-user properties, such as dishwasher durability, scratch resistance, resistance to food and beverages, taste neutrality, and low toxicity, are important as well. Certain tableware items must also be heatproof and thermal shock resistant.

Composition and Preparation of Ceramic Colors and Inks

Overview

The color inks used today consist of a pigment, a frit (glassy binder to ensure adhesion of the pigment to the ware surface) and an organic medium to enhance applicability. Usually the pigment and grit are ground together to form a color powder. These powders have a typical particle diameter of 1 - 20 �m and a relatively high density of approximately 3 - 4 g/cm3. These color powders can be easily dispersed in organic media to produce liquid ink suitable for further processing.

Composition and Preparation of Pigments

Many pigments are available, but very few meet the exacting requirements of ceramic coatings. Not only must they exhibit a pure and intensive color tone, they must withstand firing temperatures without deterioration, be inert to the chemical attack of the hot glass melt and retain their color tone under oxidizing or reducing atmospheres. Consequently, only a few classes of pigments are suitable for coloring ceramic ware. These are mainly crystalline compounds such as spinels, rutiles, and varieties of corundum or crystals with lattice defects occupied by certain ions. The compositions differ depending on the required hue and shade.

To produce a green color, chrome oxide (Cr2O3) can be used, but the result is often not a pure saturated green color. Chrome/cobalt or aluminum/zinc spinels are preferred and can be used to produce a variety of green shades.

For blue, zirconium vanadium blue or different types of zircon silicate as well as Co-Al spinels are available to make up sky blue shades. The famous and beautiful intensive blue formed from cobalt requires higher temperatures (approximately 1200°C) to give the best results.


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Yellow shades are a bit difficult. Naples yellow can be produced with a lead antimonide but this compound is only stable to 900°C. Titanium nickel yellow (a rutile) and the bright cadmium sulfide can be used but these compounds also have limited thermal stability. Praseodymium doped zircon pigments are the one choice available for high temperature yellows.

The production of red pigments continues to be a challenge for the ceramist. Most red pigments are either toxic, unstable at high temperatures, or produce reds of low purity. The deep red cadmium selenium sulfide pigment is prized for its pure red color, but it is not thermally stable at elevated temperatures and it is highly toxic. Recent developments have ameliorated many of these problems by encapsulating the color center phase in zircon, thus stabilizing the pigment with respect to bleaching and toxic exposure. A colloidal dispersion of gold in glasses and glazes produces colors ranging from ruby red to deep purple with low consumer toxicity. Unfortunately, the temperature stability of gold purple is not excellent. Another possibility is the tin/chrome/calcium-silicate pigments, which produce a pinkish red shade but are quick to react to unsuitable frits. Iron oxide has been well known since antiquity to produce reddish colors when properly compounded and fired, although most of these colors tend towards rust or brownish colors.

In contrast to the spectral colors, black and white pigments are rather simple to produce. Black is obtained with chrome/iron pigments, which may be adjusted with the addition of cobalt oxide to give blue, or cuprous manganese or nickel oxide to produce truly neutral black. White pigments are readily produced from zirconium or tin oxide.

FIRING TEMPERATURES FOR DECORATION OF THE VARIOUS TYPES OF CERAMIC WARE

[TEMPERATURES IN CELSIUS] Decorating Process Ceramic Ware

Underglaze In-glaze Overglaze Ion Colored Glaze

Earthenware 750-820 1000 - 1040 Stoneware 1200 - 1260 Porcelain 1350 - 1450 1150 - 1250 800 - 840


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Bone China 750 - 820

In addition to the direct use of pigments, other coloring methods are available. The most significant of these alternative is ion coloring, which utilizes the color centers developed by elements or compounds dissolved in a glaze or glass to produce attractive colors. Good examples of ion coloring are the cobalt blues and the iron palette, which depending on the stage of oxidation produces colors ranging from blue-green through yellowish-brown.

Frits, Composition and Manufacture

A frit is a powdered glass that was melted to a certain composition for specific use in glazes. In decorating, the frit is combined with the pigment and serves to adhere the pigments to the substrate at the processing temperature. Frits are glassy powders with a melting range between 700 and 1200°C depending on the application temperature. Thermal expansion is an important property of the frit. The thermal expansion coefficient of the frit must be closely matched to that of the pigment and the substrate, otherwise

cracking and flaking may result. Most frits are lead borosilicate glasses to which other elements are added to satisfy property and processing requirements. For example, aluminum is added to improve resistance, while alkali metals are added to reduce the viscosity to permit easy spreading along the ware surface at low firing temperatures. Recent developments have focused

on lead-free glazes and decorations to avoid the issue to lead in the workplace and lead migration during use. Two main approaches are used to eliminate lead from glazes: use of an alkali borosilicate glass system in which lead is replaced by a combination of non-lead fluxes, principally alkalis and borates, but also including transition elements such as zinc; and bismuth borosilicate glass systems in which bismuth, an element chemically similar to lead, replaces the lead. However, with respect to end-use conditions, it is quite easy with proper formulation and processing to apply and fire glazes and decorations with high lead contents that meet FDA and ISO lead release specifications for foodware surfaces.

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Frits are usually manufactured in rotary tube furnaces, sometimes also in tanks. A batch of powdery raw materials is placed into the furnace, melted according to a certain time and temperature schedule, and then quenched in water. The resulting frit granulate is ground with the pigment to the required fineness.

Composition and Preparation of Media

The third important component in the production of a ceramic color ink is the medium, an organic polymer mixed with plasticizers and other agents to act as a vehicle for the pigment and frit. When mixed with the decoration color powder, the medium forms an easy-to-apply ink, which ensures accurate reproduction and subsequent fast dying. The medium must then burn off without residue during fining. The media are categorized according to the different drying processes such as evaporation of solvents or stiffening of melted thermoplastic media. In chemical film formation, the liquids react to form solid network structures. Evaporation of the solvent is the most common drying method.

The type of polymer used determines the choice of solvents. Common solvents are aromatic compounds, esters and ketones. Usually combinations of solvents are added to adjust the solvent power and set the required drying time. For screen printing it should be noted that the often desired fast drying could lead to a thickening of the ink on the screen. as the more volatile solvents evaporate and viscosity increases. Here the best possible compromise must be sought.

Application of Ceramic Decoration

Overview

The original artwork is reproduced by painting or printing the image onto the ware surface. The easiest ways of applying the colors are painting processes, like banding and spraying. These are generally only able to produce a relatively simple, plain surface covering. Should the desired decoration be more complex, besides hand painting, other possibilities are printing processes like screen-printing or offset printing. Indirect screen printing is the most important decoration technique used in the ceramic industry today.


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Preparation of Printing Materials

The designer s original artwork is either photographed by a reproduction camera or read by a scanner. After this each color to be printed is copied onto a screen or offset plate. To produce a motif requiring more than one color, a screen or offset plate must be prepared for each individual color. This can result in up to 20 screens or correspondingly 20 color printing passes. To keep costs within acceptable limits, it is important to keep the number of

printing passes required to produce the design to a minimum. Overprinting the basic colors of yellow, cyan, magenta and black to make the full spectrum of colors can be achieved to a certain extent. However, in practice many more colors are sometimes used to achieve detailed results, much like in the commercial paper printing process. Despite these limitations, current decorating practice can achieve good quality decorations with only four to nine colors instead of twenty or more colors commonly used several decades ago.

DecorationApplication Process

Indirect Direct

Screen Printing Offset PrintingPad Printing from

Engraving

Spraying

Screen Printing

Banding

Hand Painting

Wet Printing WithDecals

Transfer ScreenPrinting

Heat ReleaseTransfer

Wet

Dry

DECORATION APPLICATION FLOW CHART


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Preparations of Printing Inks

For the preparation of printing inks, the color powder must be dispersed in the medium as homogeneously as possible and without the formation of agglomerates. For this, the color powder is carefully dried and mixed with the medium before being dispersed with special equipment, usually a triple roll mill. This mixture is adjusted to required printing viscosity by the addition of a further medium.

Application

The most important printing process is screen-printing. In screen printing the color ink is pressed through a finely meshed screen by means of a squeegee. This process allows a thick layer of color to be applied. The screen is usually made of steel or polyester cloth. The screen gauze is clamped in a frame and a light-sensitive coating is applied. Those areas where no color is required, i.e. non-image areas, are covered by a screen stencil or a polymer film coating to prevent exposure in the subsequent lithography process. Then, the design is transferred to the screen by lithography and the exposed area is rendered water soluble and washed out. During the printing process, the washed out areas are filled with color ink and transferred to the ware. For screen printing, both flat-bed and cylinder printing machines are available. The latter allows greater precision and higher printing speeds and as such are more popular. For the screen gauze, steel and synthetic materials with different thread counts are used depending on what layer thickness is required. Current practice consists of either printing directly onto the ware (direct screen-printing) or on an intermediate vehicle like paper or silicon (indirect screen-printing).

Of the two processes, indirect screen printing onto paper (decal production) is the most efficient and popular process in the ceramics industry. This is because indirect screen-printing enables a large number of colors to be printed cost-effectively. The finished decal can be applied to any three-dimensional object such as a cup or a coffee pot, while until recently the printing of such items by direct printing was extremely difficult or impossible. Of course, the decal process involves several extra steps compared to the direct process, and recent developments in total transfer direct printing have brought it to a comparable level with the decal process for some applications.


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Another decorative printing process is transfer screen-printing. This process requires thermoplastic screen-printing inks. In this process, an image is screen printed hot so that the inks are liquid. A pad picks up this decoration and transfers it to the cold ware where the ink sets immediately and adheres to the ware surface. This process historically was limited to simple decorations of only moderate quality. Recently this process has become quite popular due to the increased quality of the process and its low cost.

Banding and spraying still hold certain significance as coating processes. Many plates and cups are decorated with colored edges. These are obtained by pressing rollers or a brush soaked in the color ink onto a rotating ceramic ware thus transferring the color to ware surface. If a plate or jug is to be completely covered with a color then spraying is the preferred method. The inks should be considerably thinner in consistency than for other application processes.

Firing

After the ware has been decorated, it is fired. This process can be roughly divided into four stages. The first stage is the burning out of the organic components. This usually takes place between 300 and 500°C. During the second stage the ceramic colored powder sinters onto the ware surface. In the third stage complete melting occurs to form a glossy coating. Cooling follows as the final stage in the process.

There are three ways in which a decoration can be situated with respect to the glazing of ware: overglaze, inglaze and underglaze. Overglaze decorations for porcelain are applied directly over the glost glaze and the decorations are fired between 800 and 840°C for two to eight hours. The resultant colors are not usually dishwasher durable, but the pallet of pigments that can survive this temperature is quite large and for that reason overglaze decoration is popular with designers.


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To ensure good dishwasher durability the trend is toward inglaze firing. This high-temperature fast firing is conducted at temperatures between 1150 and 1250°C for one and a half-hour. It is known as inglaze firing because at these temperatures the ceramic colors sink into and react with the glaze. In this way the decoration is less exposed to attack by cleaning detergents. Moreover, the high-melting frits used in this process are considerably more resistant than the low-melting onglaze fluxes.

Underglaze decoration is much more limited than the other two processes since the decoration must survive the full fury of the glost fire, thus severely limiting the palette and line sharpness possible by this method. In underglaze decoration, the decoration is applied to the ware surface before the glaze is applied on top. Naturally, this also results in excellent dishwasher durability as the ceramic colors are completely protected from chemical attack.

Thermal Stability of Overglaze Colors

700 750 800 850 900 950 100

Fe2O3

Cr2O3-SnO2

Sb2O3-PbO

CdS

UO2

CoO-Cr2O3

CuO

CoO

Cr2O3 - V2O3

Temperature, C


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Summary

A wide range of ceramic colors is available to the designer of ceramic ware, although the firing temperature of the ware considerably limits the palette. More varied colors are available for low temperature overglaze decoration of porcelain or general decoration of cone 06 earthenware. Recent technological advanced in decoration chemistry has expanded the palette of colors and reduced toxicity of certain pigments by encapsulating the pigments in zircon or other stable crystals. The inclusion pigments are a good example of this development. Furthermore, low lead and lead-free decorations have made substantial advancements in recent years.

Colors Imparted By Selected Ions InLead Silicate and In Alkali Alumino Silicate

Glasses And Glazes

Ion In Lead Glass/Glaze In Alkali AluminoSilicate Glass/Glaze

Ag++ Pale GreenAu++ Faint Violet (Colloidal)Co++ Intense Blue PurpleCr+6 Yellow Orange GreenFe++ BlueCu++ Intense Green

Fe+++ Yellow/Green Coral RedMn+++ PurpleMo++ Faint GreenNi++ Yellow Green Olive BrownPd++ Gray Black (Colloidal)

Pt Gray Black (Colloidal)Ti++ Faint YellowV4+ Faint YellowV6+ Intense Orange BlackW6+ Pale Yellow

Selenium Red RedCadmium selenide Orange


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Technology has also advanced in the area of applying decorations by various printing processes. The computerization of traditional paper printing methods has also improved the ceramic decoration printing processes. The traditional screen printing technology, either direct or decal, has been substantially improved so that many decorations can be printed with only four to nine colors and line sharpness and registration are also improved. New media have been developed wherein former drying times of 2 h have been slashed to just a few seconds. Offset printing is being continually perfected to give a more opaque covering. In certain sectors new application processes like heat release will gradually replace traditional manual application techniques.


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Appendix A:

Supplemental Discussion of Early Work Fritting Approaches to Control Lead Solubility

The use of lead in glazes was investigated at great length in England circa 1900 by Thorpe and his associates. Thorpe studied the effect of composition of lead frits on solubility in 0.25% hydrochloric acid; this was taken as equivalent to the strength of the acid in the gastric juices at body temperature. An average value for the gastric juices may be 0.17% but the 0.25% concentration was selected to compensate for the acid being at body temperature. Thorpe found that the lead bisilicate (PbO·2SiO2) was least soluble of lead silicate glasses tested. Thorpe recommended the following empirical relationship to predict the solubility of the lead frit:

Thorpe’s Ratio

60223

Oxides Acidic Of MolsAlumina Mols Oxides Basic Of Mols

�

�

For low solubility the above ratio should not exceed 2. Mellor (27) later stated the molecular relationship as:

)Max (5.0 RO

OAl RO

2

32�

�

If the ratio is equal to or less than 0.5, the frit will have a satisfactory low solubility. These ratios would hold for the compositional systems involved in this work.


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The British Government Home Office Circular of December 14, 1899, stated that no glaze composition containing lead or a compound of lead (other than galena) shall be regarded as satisfying the requirements as to insolubility, which yields to a dilute solution of hydrochloric acid more than four percent (4%) of the dry weight of a soluble lead compound calculated as PbO when determined as given below (Thorpe’s two percent (2%) solubility limit was raised to four percent (4%) as a result of arbitration).

A weighed quantity of dried material (particle size not specified) is to be continuously shaken for one (1) hour, at the common temperature with 1000 times its weight of an aqueous solution of hydrochloric acid containing 0.25% HCl. This solution is thereafter allowed to stand for one (1) hour, and to be passed through a filter. The lead salt contained in an aliquot part of the clear filtrate is then to be precipitated as lead sulfide and weighed as lead sulfate. It is interesting to note that the lead bisilicate, approximating sixty five percent (65%) PbO and thirty five percent (35%) SiO2 has sufficient insolubility to enable the mill mixture of the ordinary glaze to pass readily this Government test.

Rix in 1902 summed up the experience up to that time and stated that the lead should be fritted and that the lead frit should be insoluble in dilute hydrochloric acid. Rix stated that lead in the fritted form could be substituted for the raw glaze without change in the glaze composition and without change in the required firing temperature; the fluidity, of course, is affected by the character of the lead frit. The early potters realized that the use of fritted lead assisted in the immunity of the worker and frits were developed with low solubility. Rix also noted that production losses were greater when leadless glazes were substituted owing to the greater care required in the manipulation and firing of these glazes.

Koerner in 1906 confirmed Thorpe’s conclusions. He showed that increasing the silica in lead silicate frits reduced the solubility. He also found that the introduction of alumina greatly reduced the solubility of lead monosilicate.

Bartel in 1918, in an extensive investigation of lead solubility of fritted glazes, also evaluated lead silicates from PbO·1.5SiO2 to PbO, 4SiO2 with increments of 0.25 SiO2. The most acid resistant composition in this system PbO·Si02 was found to be PbO·2.5SiO2. Bartel tested many lead frits


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prepared from red lead, the carbonates of Na, K, Ca, Ba and Mg, ZnO, Zettlitz kaolin and pure sand. The test used a fine powder of the frit (through a given size sieve with no residue), exposure to four percent (4%) acetic acid for one-half (1/2) hour, adding a measured amount of K2Cr2O7 to precipitate any Pb as PbCrO4, then determining the excess K2Cr2O7 by an chemical process. Reference was made to the difference between such finely ground frit material and the layer of glaze on the ware, and also to the influence of other oxides in the glaze on the lead solubility.

Petrick in 1920 noted that when alkalis or borax are used in lead glaze frits they make the same soluble. Lead frits containing no alkalis or borax, on the other hand, are insoluble. By making two frits, one containing all of the lead oxide with no alkalis or boric acid, and another containing all of the alkalis and boric acid, the amount of soluble lead oxide is greatly reduced. Petrick recalculated a number of commercial glaze frits preparing a high lead and high alkali frit in the place of the alkali-lead frit.

Frit - Number VII

0.774 PbO 0.055 CaO

0.044 K2O 0.134 A12O3 2.374 SiO2

0.127 Na2O

Frit - Number XII

0.75 PbO

0.20 CaO 0. 12 A12O3 2. 4 SiO2

0.05 K2O

Frit Number VII, of which dilute HCl dissolves 1.0% lead oxide, was used for the preparation of low melting glazes and frit Number XII, of which dilute HCl dissolves 0.55-0.60% lead oxide, was used for the preparation of the higher maturing glazes. Using combinations of these frits the soluble lead oxide was decreased from 31.7% to 0.76% in one glaze, from 23.05% to 0.3% in the second, and in another from 20.72% to 0.25%. Pandermit (natural calcium borate) may be used in the place of the leadless frit. By preparing a tin glaze by the use of the two frits given above, the percentage


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of soluble lead oxide was reduced from 4.18 to 1.03%. The presence of CaO reduces the solubility of frits. The amount of lead oxides soluble in dilute acid according to Thorpe’s method should be less than 1.00%.

Vargin and Fradkova in 1934 reported on decreasing noxious effects of lead glazes by an understanding of the influence of the frit constituents on the acid stability of the frit. The influence of SiO2, A12O3, B203, PbO and CaO on tire acid stability of frits used for preparing lead glazes was studied.

Harkort in 1934 studied the acid solubility of lead frits using a four percent (4%) acetic solution as the agent and a controlled size of the frit powder. The soluble lead was determined by the molybdate method. He showed with this test that the lead monosilicate powder released lead in large amounts, and that by introduction of alumina in the frit a practically complete lead “fastness” was attained. Alumina was demonstrated to be more powerful in this respect than is silica, calcia, or magnesia. Titania was also found to exert a marked effect in lowering the solubility of lead frits and glazes. The addition of one percent (1%) to two percent (2%) titania is very beneficial but the coloring effect limits its use.

Harkort used the following frit in demonstrating how additions of boric oxide raises the solubility of lead frits:

0.2 CaO 0.1 Al2O3 1.0 SiO2

0.8 PbO

To avoid increasing the solubility of the lead, it was suggested that boric oxide be introduced as a borax frit. Alkali oxides are also detrimental. The needed amount of alumina to attain acid resistance does not increase the maturing temperature.

Rieke and Mields in 1935 reported on acid resistance of compositions in the PbO-SiO2, K2O-PbO-SiO 2, and Na2O-PbO-SiO2 systems. The resistance of these frits to four percent (4%) acetic acid was determined by the electrolytic separation as PbO2. The tests were conducted on that portion of the powdered flux which passed through a German standard sieve DIN Number 20 and remained on Number 40.


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It was found that a uniform increase or decrease of PbO in the binary system, PbO·SiO2. does not give a corresponding rise or fall in the lead solubility but shows alternately rapid rise in solubility, followed by a sharp rise in resistance. Lowest lead solubility occurred in the region corresponding to PbO·2SiO2 and PbO·4SiO2. With a constant alkali content the lead solubility decreased with falling PbO content, totally insoluble frits being obtained with ten percent (10%) to fifteen percent (15%) Na2O content and thirty percent (30%) to twenty five percent (25%) PbO content. Frits with five percent (5%) alkali content showed the greatest resistance with forty five percent (45%) to fifty percent (50%) PbO, yet further increases in the PbO content produced only slight increases in solubility. In the constant-alkali series, the solubility increased sharply with increasing PbO content from a point which varies in the two alkali-lead-silicate systems. Frits with high alkali contents showed this sharp rise with lower PbO contents more than low alkali frits. In the constant PbO content frits, the high lead fluxes showed a rapid rise in solubility with increasing alkali content. With a decrease in the PbO content the frits became more stable, even with increasing alkali content. The lowest PbO content series showed increasing stability with alkali additions up to ten percent (10%). Plotting the solubility of the frits of both series in the two 3-component systems, it is seen that the region of stable frits (lead solubility below one percent (1%)) in the Na2O series with low Na2O content, beginning with forty five percent (45%) SiO2, extends at fifty percent (50%) SiO2 over the whole range of composition investigated. The totally insoluble frits were found in the range of lowest PbO content (25-30%) and medium alkali content (10-15%).

Koenig, in work sponsored by the United States Potters Association, worked primarily on the development of highly acid resistant lead frits. The use of a highly acid-resistant lead frit in a fritted glaze was considered to be an important precautionary measure in handling the material in the plant.

The lead solubility of a lead frit in dilute acid is closely related to its chemical composition. For example, alumina strongly increases the acid resistance of the lead frit while the presence of alkalis and especially boric oxide reduces it. The detrimental effects of boric oxide are clearly marked. Lead borate shows complete solution of its lead content under test conditions used in this work. Lead borosilicates are highly soluble. A high boric oxide content is chiefly responsible for the poor acid resistivity of many


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commercial lead frits examined. The action of alkalis in decreasing the acid resistance is also pronounced, although it apparently is not as great as that of boric oxide. Increasing the alumina content is effective in compensating for high lead solubility resulting from boric oxide and alkalis in the frit. The same result can be achieved by increasing the silica content, although silica is not as powerful in this respect as is alumina. Zinc oxide, barium oxide, and calcium oxide are other glaze constituents effective in decreasing the lead solubility in the order given, with the latter having the greatest beneficial effect. Other effective oxides in increasing acid resistance include those of beryllium, titanium and zirconium.

The lead solubility in weak acid from lead bisilicate is progressively reduced by the addition of alumina. Of a series of lead aluminosilicates studied, the deformation eutectic composition (1 PbO 0.254 A12O3 1.91 SiO2) was recommended. This frit is about six times as acid-resistant as the lead bisilicate and is more readily prepared.

The solubility of the lead content of the frit in weak acid was shown to increase with decreasing particle size so the extent of grinding should be optimized to provide the desired overall glaze properties.

Highly acid resistant lead frits can and should be employed in dinnerware glaze formulation. Two recommended methods are as follows:

(1) The use of a single lead frit. This requires a high fritted content in the glaze, since the boric oxide and alkalis introduced in the lead fruit must be compensated for by larger amounts of alumina, silica etc. The trend towards fritting the entire glaze composition provides for highly acid resistant lead frits (using 8-12% clay in the mill batch) or less than one percent colloidal material (fast-fired glazes) to suspend the frit.

(2) The use of two frits. The lead frit has the maximum acid resistance attainable from the glaze constituents. The boric oxide and alkalis are introduced in a second (leadless) frit. (At the time this work was done this approach was most highly recommended. Since then the trend has been towards glazes with higher fritted content, circa ninety percent (90%) frit).


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Solubility Tests On Frit Powders

Harkort used a method analogous to that applied to glasses to test the lead solubility of frits. The frit was sized and the fraction between 144 and 400 mesh was washed with alcohol to remove all dust. A thirty-minute test with boiling four percent (4%) acetic acid was employed. The soluble lead was determined by the molybdate method. Koenig used a similar test in research sponsored by the United States Potters Association.

Currier developed a standard method for determining leachable lead in lead frits. The procedure is sufficiently simple for use by control laboratories whereby duplicate results can be obtained. It is based on leaching the lead at 40º ± 0.5ºC from frit -180, +200 mesh particles) with 0.137 N HCl and precipitating the lead as PbCrO4. The following data reported by Currier are of interest:

Frit Designation

PbO (%)

Al2O3 (%)

SiO2 (%)

Leachability (%)

A 65.0 2.0 33.0 1.58 1.58

B-1 61.3 7.1 31.6 0.39 0.38

B-2 61.4 3.0 35.6 0.29 0.30

C-1 64.8 0.12 34.9 1.68 1.68

C-2 64.9 1.17 33.8 1.86 1.88

C-3 64.8 3.34 32.1 0.64 0.64

C-4 64.8 5.47 30.1 0.85 0.85

C-6 64.7 1.97 33.2 0.95 0.94


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C-7 64.4 0.99 34.5 1.38 1.38

C-8 60.15 6.36 32.87 0.55 0.55

D-3 17.2 8.25 0.32 5.17 (B2O3) 0.32

*Percent of PbO in original sample.

Bennett and Vaughan reported on various chemical methods for the determination of lead in solution derived from attack of hydrochloric acid on lead frits. The accuracy of the chromate method was found to be as good as that of the sulfide-sulfate method, provided that certain interfering ions are absent. The chromate method has the advantage of simplicity and increased rapidity, complete determinations being carried out in twenty-four (24) hours. The method is not so reliable as the sulfate method if other ions are in the solution from which the lead is to be precipitated. but this is not likely to be the case where solubilities of lead bisilicate frits are being determined. If, however, other uses are found for the method, care must be taken to ensure the absence of interfering ions.

Later Bennett presented details on a method for determination of lead solubility, by precipitation of the lead as sulfide, followed by determination as chromate. The sulfide-chromate method was claimed to be a useful technique for determining the lead solubility of glazes containing elements liable to cause contamination of the lead chromate precipitate, when the straight chromate method is used. It is more rapid than the sulfide-sulfate method, and is capable of eliminating the same interfering elements. When using the straight chromate method the chief cause of contamination is SiO2, which is precipitated in a gelatinous form. Much smaller quantities of Al, Fe and Ti are precipitated. When Ba is added to the glaze in a soluble form, BaCrO4 is precipitated with the PbCrO4.

Lenk reported on additions to essentially a lead monosilicate frit to markedly improve its acid resistance when subjected to the British Home Office Test (boiling 0.25% HCl). A fourteen percent (14%) addition of the following frit to the frit batch reduced the solubility to one percent (1%):

0.915 Na2O 0.824 SiO2


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0.085 CaO 0.895 TiO2

0.313 F2

The addition had little effect On the softening temperature of the frit and it increased the maturing temperature Of typical glazes by not more than 30ºC. without adversely affecting the other properties. The simultaneous addition of A12O3 to the frit batch further decreased the solubility to 0.3%.

Some of the earlier solubility tests did not specify the particle size of the frit powder being tested. The importance of doing this was soon realized. Data were presented in reports of several investigations (2, 44 et al.) which showed the increase in solubility with increasing fineness of the lead frit particles. Aside from the intrinsic solubility of the specific frit composition, the degree of fineness of the frit powder was clearly the principal factor of the various test parameters. The rapid increase in lead solubility with decreasing particle size, not only required that this be controlled in testing, but also emphasized the necessity of not reducing the size beyond the necessary minimum in practice. The following data (44) are for the solubility of the same frit ground to different degrees of fineness:

% Composition Ground frit Particle size range A B C D Under 3 microns diameter

4.1 4.2 7.8 10.3

3-6 1.6 2.1 9.1 22.1 6-12 2.6 2.2 7.9 10.4 12-24 4.0 4.6 9.0 18.7 24-36 0.8 6.4 11.2 24.5 36-48 7.1 3.9 12.5 10.0 48-96 14.8 46.5 26.8 3.7 Over 96 65.0 30.1 15.7 0.3

Solubility % 1.1 1.8 3.4 5.5

Podmore made an experimental and theoretical comparison of the relationship between lead solubility and the specific surface of the lead frit. The purpose was to be able to compare solubilities of frits which has been


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ground to different degrees of fineness. The relationship suggested, the so-called Podmore Factor was:

Surface Exposed Specific

100 x Solubility Lead

Smith also studied the relationship between solubility and specific surface for lead bisilicate frits. It was shown that during the solubility test the dilute acid must penetrate the frit particles to some depth, and that the removal of lead is not simply a surface effect. An equation relating the solubility and specific surface was derived.

Norris et al. reported on the physical aspects of solubility determination and the relative importance of various test conditions. The relationship between the lead extracted and the “acid-contract” time was found to be essentially logarithmic for periods of a few hours. After this the rate of extraction proceeds at a rate greater than logarithmic. The steepness of the curves at periods of the order of 1-2 hours is such that the filtration time must be small and reasonably constant. The relationship between solubility and temperature was found to be linear over the range tested for normal frits, the coefficients in a number of cases being of such a value that fairly close temperature control is necessary for the accurate determination of solubility. The behavior of the “coated” frits tested were different, the coefficient increasing with temperature (British Patent Number 625,474 claims reduction of solubility of high lead frit particles by coating with a layer of silica). Norris and Bennett (44) reported the following data for the effect of temperature on solubility of eight (8) frits:

Solubility % PbO Temp., ºF A B C D E F G H

35 0.4 5.2 8.2 1.9 - 10.9 0.6 1.1 50 0.3 6.2 8.9 2.4 7.5 12.9 0.6 1.7 65 0.4 6.8 9.7 - 8.7 15.1 0.6 2.8 80 0.4 7.8 10.3 4.5 9.4 16.8 0.6 3.7 95 0.6 8.7 11.0 5.8 10.4 19.2 1.0 6.0 110 0.9 9.3 11.8 7.1 11.5 21.4 1.4 7.5

Norris et al. stated that when proper control of sampling total acid-contact time, and temperature is exercised, the variability associated with the


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physical aspects of this (British) method of test is roughly equal to that of the chemical estimation of the lead extracted. For solubilities of the order of five percent (5%) and below, the standard deviation for the whole solubility determination usually lies between 0.1 and 0.5%.

Lead Extraction From Glazed Surfaces

The literature contains voluminous references to the chemical durability and solubility tests on glazes and other glass surfaces. In large part this involved resistance to attack by water, acid and alkaline solutions. J. W. Mellor presented an extensive review of the earlier work up to 1934 on the durability of glazes.

The work of Geller and Creamer at the National Bureau of Standards, reported in 1939, is of special interest in that it was comprehensive and it involved the assistance of the United States Potters Association and the Food and Drug Administration. Various tests were considered which used as reactants, various fruit juices, citric acid and vinegar. One of these tests, using five percent (5%) acetic acid or white distilled vinegar and precipitation of lead by H2S, had extensive use prior to the development of the Dithizone Test. The results indicate that glazes of one color only, among those tested, had poor acid resistance. Two other glazes from one manufacturer, however, were found to be marginal, and a third may cause trouble if indicated corrective measures, such as a preliminary acid wash, should be neglected by the manufacturer. For other glazes tested in no case did they exceed one-half (1/2) part per million (intermediate shades of yellow, brown, blue and gray). Except for the maroon glaze of one manufacturer, the only specimens to give up more than two (2) parts per million of Pb were the tangerine (reds) and the greens. Some of the dark blues and yellows showed solubilities equal to, or in excess of, 1 part per million.

Weyl and Rudow reported on the use of the Dithizone Test to determine the lead solubility of earthenware glazes. The glazes were tested with four percent (4%) acetic acid. The effect of repeated tests on both a high-lead and a low-lead glaze was noted. A sharp fall in the quantity of lead release by the glaze was observed after the first treatment with acetic acid.


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Relation of Glaze Structure to Durability

Increasing attention has been given to glass structural theory and its application to glazes. A symposium on “Structure And Properties Of Glazes” reported in the 1956 Transactions of the British Ceramic Society is of great interest. Glaze properties, including durability, are considered on the basis of structural considerations.

Moor in showing how the properties of the glassy matrix (glaze) depend on its structure, reviewed the structural effects of introducing alkali oxide, and alkaline earth oxides in silicate-, borate-, and borosilicate glasses and also the structural role of alumina. Of main concern in this reference is the effect of various oxides on the structural features and resistance to attack by water (durability) as noted in part below.

The alkali ions are held comparatively weakly in the interstices of silicate glasses, and at ordinary temperatures the energy of their thermal vibrations may be sufficient to enable them to escape and to diffuse through the structure. If they reach the surface they are trapped by moisture, to form the hydroxide, and cannot return into the glass. The risk of deterioration increases with increased alkali content. With low alkali contents the alkali ions can constrain the structure sufficiently to form interstices in which they are closely surrounded by the number of oxygens corresponding to the most stable state of coordination. The effects of alkalis on deterioration depend on their field strengths and on their ionic radii, but also on the sizes of the interstices which can be formed or exist in the glass structure. The Li+ ion enclosed by four oxygens is held quite strongly but in a larger interstice is less strongly held and, being small, it can readily find its way through the rest of the structure. The Na+ ion enclosed by six oxygens is held moderately strongly, and it is conveniently housed in interstices which naturally occur in the silica network. But if it is freed it can also pass through the network fairly easily. The K+ ion is held fairly strongly when surrounded by eight to ten oxygens but it is too large to pass through the interstices and cannot diffuse nearly as rapidly as either the Na+ or Li+ ions. When mixtures of the alkalis are present, the effects depend on the proportions of the alkalis in relation to the proportions of the interstices of different sizes which can exist simultaneously in the structure.


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Replacement of some alkali by boric oxide gives improved resistance to attack by water due to fewer alkali ions being present and also, if the amount of boron is small, it will all become four coordinated with oxygen. The BO4 tetrahedra are linked direct to the SiO4 tetrahedra, as structure-building units, and the alkali ions which have donated the necessary oxygen are held close to the BO4 tetrahedra, in interstices surrounded entirely by “double- bonded” oxygens. The absence of any “single-bonded” O- ions in the groups of oxygen surrounding the alkali ions which have donated their oxygen to the four coordinated boron strengthens or stiffens the structure as a whole. It is important, in considering the effect of boric oxide, to ensure that there is enough alkali (or alkaline earth) for at least one-fifth of the boron to be four coordinated; also the total molecular B2O3 content should, preferably, be not greater than one-eighth of the molecular amount of SiO2 present. Boron present in excess of the amount represented by the two requirements has an increasingly harmful effect on the durability because the BO3-SiO2 linkages are very weak.

Replacement of some alkali, or even some SiO2, by A12O3 improves the resistance to attack by water. The effect is due to the formation of A1O4 tetrahedra, linked by “double-bonded” oxygens to SiO4 tetrahedra, the alkali or alkaline earth ions which have donated the necessary oxygens being held in close associations with the A1O4 groups. The alkali ions are thus less free than they would be in the absence of A12O3; also, each A12O3 molecule introduced causes one “gap” in the structure to be closed.

Divalent ions have marked effects on the durability as compared with the binary alkali silicates and borates, but increase beyond about ten percent (10%) has only slight further effect. Zinc oxide, by virtue of its ability to form ZnO4 tetrahedra, each closely associated with two alkali ions, causes these alkali ions to be held more strongly than they would be in ordinary interstices surrounded entirely by SiO4 groups. Each ZnO4 tetrahedron causes one gap to be closed, and in this way, as well as in holding the alkali ions more strongly, contributes considerably to improve resistance to attack by water.

Lead oxide behaves differently from the other divalent oxides. In a PbO-SiO2 glass the proportion of PbO can be increased to correspond with 2.5 PbO·SiO2, in which the SiO4 tetrahedra cannot possibly be all linked together


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by the oxygens at their corners. The Pb2+ ions must, therefore, be capable of forming “bridges” between the SiO4 groups to an extent which is far greater than the bridging action of other divalent ions. It has been suggested that, in silicate glasses, the Pb+ ion may form groups of the PbO4 type or, possibly, with the Pb2+ ion surrounded eight oxygen ions.

Moore (53) concluded that the maximum durability corresponds primarily, with the most compact and most strongly bonded structure, and this type of structure usually gives a maximum softening point and a minimum coefficient of thermal expansion.

Bloor (54) also discussed the structure of glazes in relation to certain properties, e.g., fusibility, surface tension, viscosity, thermal expansion, etc. Glazes were divided into two groups, those containing SiO2 and A12O3 but no B2O3 and those containing all three. The reason for this is that Si4+ and A13+ always have 4 coordination in glasses, while B3+ may have 3 and 4 coordination. For each glaze two factors were calculated:

group) secondin B (and Al and Si of Sum FormulaMolecular in Oxygens Of Sum

R �

Modifiers-Network of Sum FormulaMolecular in Oxygens Of SumP �

R is a measure of the number of single-bonded oxygens. P is a measure of the oxygens available per network-modifying ion. Normal glasses are said to be those whose R values are less than 2.5 and whose P values are greater than 3.9. For the glazes studied by Bloor (54) containing Si4+ and Al3+ (no B3+ ) the limits found for R and P were 2.64 to 1.96 and 3.5 to 21.6 respectively. The value R = 2. 64 and the high value for P were noted for high-lead glazes. In these cases part of the lead may be present as a network-former and the remainder as a network-modifier, or such exceptions may be due to the stabilizing effect of the Pb2+ ion. Plotting P values versus glaze maturing temperatures indicates that the effect of the number of network modifying ions outweighs the effect of the type of network-modifying ion on the fusibility of a glaze (which incidentally has been a long established observation in glaze technology).


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The B2O3-containing glazes differ from the aluminosilicate glazes. It was suggested that in normal glazes all the B2O3 and Al2O3 accept oxygens from the modifier oxides, the Al2O3 forming A1O4 tetrahedra, and the B2O3 broken B-O-B bonds, at the glaze-maturing temperature. When the glaze is cooled slowly it is likely that B04 tetrahedra form. Glazes with very high B2O3 content (R<2) must contain BO3 triangles since there is insufficient oxygen brought in by the modifiers to break the B-0-B bonds. These glazes would be very soft, and may well have poor durability. R is generally less and P more for the B2O3-containing glazes, which permit reduction in the number of network-modifying ions in the glaze.

Smith (42) in 1949 examined the structure of a lead bisilicate type glass from the basis of then current theory on glass structure; the latter was reviewed in this reference.

The molecular formula for the commercial lead bisilicate as then in use was given as:

0.9737 PbO 0.075 A12O3 1.802 SiO2

0. 0156 CaO 0. 018 Fe2O3

0.0107 K2O

For the above composition R = 2.51 and P = 4.67. Although P is greater than 3.9, R is not less than 2.5; this lead bisilicate, therefore has a structure composed of a spatial conglomerate of two-dimensional and one-dimensional networks. In order to make the structure more resistant to attack, R must be reduced to below 2.5. This can be done by the addition of glass-forming ions - the practical ones available being B3+, Si4+ and Al 3+.

Structures bonded in three directions only are obviously inferior in strength to those bonded in four directions, and hence - even though B3+ ions will reduce R as rapidly as Al3+ and more rapidly than Si4+, the resultant structure will definitely not be as strong as it would had SiO2 and Al2O3 been added instead of B2O3; in fact, the addition of B2O3 will most probably give a structure weaker than the original lead bisilicate. To increase the strength of the bisilicate structure it is necessary to add glass-forming ions with coordination numbers of 4. These are Si4+ and Al3+. It can be shown that Al2O3 reduces the R value of the frit much more rapidly than SiO2. The


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addition of A12O3 in small quantities (even of the order of five percent (5%) to the above frit brings its R value well inside the limit of R less than 2.5. This gives a structure with a special conglomerate of three-dimensional and two-dimensional networks and will be thus far more resistant to attack.

Ions of small charge and large size when introduced as their oxides, cause breakages of the silicon-oxygen linkages, the extra O2- ions thus may be taken up, and the large weakly-charged cations filling the interstices. The introduction of these ions will therefore weaken the frit structure. Ions of high charge and small size, e.g., Ti4+, Zr4+, when introduced as their oxides, also cause breakage of linkages. These cations of strong field, when they fit into the interstices, however, cause a strong contraction of the unsaturated oxygen ions surrounding them and lead to a tight binding. This results in an overall strengthening of the structure, up to a point, but if this limit is exceeded devitrification will occur. When it acts as a network modifying cation Al3+ will fall into this category. Thus the best method of increasing the strength of the frit structure is to add alumina. This rapidly reduces the R value of the structure, and absorbs the oxygen ions introduced by the PbO into the network. It is also possible to increase the strength of the frit by adding Si4+ ions and then to replace these extra Si4 ions isomorphously with the same number of Al3+ ions, and add Pb2+ ions to maintain electroneutrality. The R value of the structure will remain the same and so will its strength since the slight expansion of the framework, caused by the replacement of Si4+ by A13+ will be counterbalanced by the contraction caused by the Pb2+ ions also introduced. It may be that before the structure of maximum strength (with a fixed PbO content) is reached (either by the replacement of SiO2 by A12O3, or by merely adding A12O3, to the frit structure) it will be impracticable to continue this strengthening because of the detrimental effect on one or more of the physical properties of the frit. Smith (78) in a later paper stated it is to be expected that additions of alumina to lead bisilicate, up to the limited composition of twenty-one percent (21%) alumina and seventy-nine percent (79%) lead borosilicate, will steadily increase the strength of the structure and hence its resistance to chemical attack. This is not due to any chemical reaction which takes place, but to the structural change brought about by the introduction of the Al 3+ ions.

Ainsworth (55) proposed a diamond pyramid indentation test for investigating the structure of glazes based on a surface measurement. This


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provides a direct measurement of the strength property of the glaze. The measurement is sensitive to composition changes and should be capable of detecting changes of about one percent (1%) in alkali content, for example, over wide ranges of composition. The effects of all the oxides investigated were explained by considering three factors, the charge on the added cation, its size, and the way in which it goes into the structure.

Fajans discussed the role of Pb2+ in glasses. The Pb2+ ion does not possess the small size and high charge usually ascribed to network-forming cations. The interesting behavior of Pb2+, which can form a glass of composition 2PbO·SiO2, is attributed to its highly polarizable nature. The tetragonal structure of PbO in which four of the eight O2- surrounding one Pb2+ (18 + 2 outer electrons) are much closer to the latter than are the other four O2- is connected with the high polarizability of Pb2+. If one assumes that this lack of symmetry applies to the analogous situation of Pb2+ surrounded by O2- in a silicate, it is possible to understand the glass forming ability of Pb2+.

Tindall and Franklin, in the A. T. Green Book, discussed the durability of glass in relation to structure and composition - and, of special interest here - the behavior of overglaze colors. It was noted that the behavior of each ion in a glass is dependent on its immediate surroundings, so that the relationship between glass composition and chemical durability is a complicated one, particularly for the complex mixtures that form practical glasses and enamels. Owing to volatilization and the influence of surface forces, the composition of a glass surface will differ from that of the interior, as it will tend to contain a higher proportion of those cations that lower the surface energy, e.g., Pb2+, B3+. Even if the glass were homogeneous the conditions of the ions in the surface layer would differ from those in the interior, for the fields of the latter are completely screened by the surrounding ions, whereas those at the surface are incompletely screened at one side. Although the effect is reduced by a greater degree of polarization of the surface ions, considerable residual forces remain unsatisfied and act as absorption centers for which reactions are initiated. Normally these residual surface forces screen them- selves by absorbing moisture and grease from the atmosphere, cleansing materials etc.

The behavior of overglaze colors was studied by the British Ceramic Research Association. In view of the complicated relationship between the temperature, the concentration and nature of the attacking solution, and the


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glass composition which determine the rate of chemical attack, it was decided to investigate the overglaze colors using test conditions corresponding as closely as possible to actual usage conditions. The overglaze colors normally involve lead borosilicate or alkali-lead-borosilicate glasses containing appreciable amounts of coloring materials. The effect of inorganic detergents was in general in conformity with predictions that could be made on the basis of glass structural theories. The results obtained with organic washing agents were not predictable from structural theories. The results with weak organic agent and conditions of attack have their own individual relationship for glasses of a particular composition and these relationships may differ for different types of glass. The leadless glazes were on the whole much more susceptible to acid attack than were the lead glazes. The influence of added constituents in a lead borosilicate glass did not correspond to their influence in other glasses which have been investigated and reported. Small amounts of B2O3 lowered the chemical resistivity and A12O3 did not appear to have a beneficial effect, but a proportion of Na2O actually improved the performance of many lead-based compositions, even if it replaced SiO2-TiO2 gave no apparent improvement, but ZrO2 caused a noticeable improvement its effect being increased by the presence of TiO2. Since these overglaze colors contain a large amount of highly polarizable lead ions, it was suggested that more study of this type glass is needed which would give more insight into the nature of the glass structure.


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Appendix B:

Tests For Lead Extracted From Glazed Surfaces

Tests For Lead Extracted From Glazed Surfaces ASTM C738 - 94 (Reapproved 1999) Standard Test Method for Lead and Cadmium Extracted from Glazed Ceramic Surfaces.

1. Scope

1.1 This test method covers the precise determination of lead and cadmium extracted by acetic acid from glazed ceramic surfaces. The procedure of extraction may be expected to accelerate the release of lead from the glaze and to serve, therefore, as a severe test that is unlikely to be matched under the actual conditions of usage of such ceramic ware. This test method is specific for lead and cadmium.

1.2 The values stated in SI (metric) units are to be regarded as the standard. The values given in parentheses are for information only.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.


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2. Summary of Test Method

2.1 Lead and cadmium are extracted from the test article by leaching with 4 % acetic acid for 24 h at 20 to 24°C (68 to 75°F) and are measured by flame atomic absorption spectroscopy.

3. Interferences

3.1 There are no interferences when instrumental background correction and light sources specific for lead and cadmium are used.

4. Apparatus

4.1 Atomic Absorption Spectrometer equipped with light sources (hollow cathode or electrodeless discharge lamps) specific for lead and cadmium, instrumental background correction, and a 4-in. ( I 02-mm) single slot or Boling burner head. Digital concentration readout may be used. Use air-acetylene flame, instrumental background correction, and operating con-ditions recommended by instrument manufacturer. Using these conditions, characteristic concentration (concentration that gives 0.0044 absorbance) should be approximately (+20 %) 0.2 and 0.45 ppm for Pb measured at 217.0 and 283.3 nm, respectively. Characteristic concentration should be approxi-mately (+20 %) 0.02 ppm for Cd.

NOTE 1: 1 ppm = 1 �g/mL.

4.2 Lead Lamp, set at 283.3 or 217.0 nm.

4.3 Cadmium Lamp, set at 228.8 nm.

4.4 Glassware of chemically resistant borosilicate glass, to make reagents and solutions. Clean by rinsing with dilute nitric acid (10 % by volume) followed by copious quantities of water.

5. Reagents

5.1 Purity of Reagents—Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where such specifications are available.2 Other grades may be used provided it is first ascertained that the reagent is of sufficiently


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high purity to permit its use without lessening the accuracy of the determination.

5.2 Purity of Water—Unless otherwise indicated, references to water shall be understood to mean distilled water.

5.3 Acetic Acid (4 % by Volume)—Mix 1 volume of glacial acetic acid with 24 volumes of water.

5.4 Detergent Wash—Use detergent designed for washing household dishes by hand. Mix with lukewarm tap water according to product instructions.

5.5 Lead Nitrate Solution (1000-ppm Pb)—Dissolve 1.598 g of lead nitrate (Pb(NO3)2) in 4 % acetic acid and dilute to I L with 4 % acetic acid. Commercially available standard lead solutions may also be used.

5.6 Hydrochloric Acid (1% by weight)—Mix I volume of concentrated hydrochloric acid (HCl, sp gr 1.19) with 37 volumes of water.

5.7 Cadmium Solution (1000-ppm Cd)—Dissolve 0.9273 g of anhydrous cadmium sulfate in approximately 250 mL of 1 % HC1 (see 5.6) and dilute to 500 mL with 1% HCI. Commercially available standard cadmium solutions may also be used.

6. Procedure

NOTE 2: Take a method control through entire procedure. Use a laboratory beaker with dimensions similar to ware being tested.

6.1 Preparation of Sample—Take, at random, six identical units and the method control vessel and clean with detergent wash. Then rinse with tap water followed by distilled water. Dry, and fill each unit with 4 % acetic acid to within approximately 6 to 7 mm (A in.) of overflowing. (Distance shall be measured along the surface of the item tested, not in the vertical direction.) Record the volume of acid required for each unit in the sample (Note 3). Cover each unit with fully opaque glass plate (so that extraction is carried out in total darkness) o prevent evaporation of solution, avoiding contact between cover and surface of leaching solution. (If opaque glass is not available, cover glass with aluminum foil or other material to prevent exposure to light.) Let stand for 24 h at room temperature (20 to 24°C (68 to 75°F)).


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NOTE 3: If the sample unit is extremely shallow, or if it has an irregular brim, the analyst should be aware of evaporation of leaching solution. If such a loss is anticipated, record the headspace upon filling the vessel to 6 to 7 mm (/ in.) of the brim. Adjust to the same headspace with 4 % acetic acid after the 24-h leaching. Stir the solution and proceed with the determination.

6.2 Preparation of Standards:

6.2.1 Lead Standards—Dilute lead nitrate solution (see 5.5) with acetic acid (see 5.3) to obtain working standards having final concentrations of 0.0-, 1-, 2-, 3-, 5-, and 10-ppm Pb.

6.2.2 Cadmium Standards—Dilute cadmium stock solution (see 5.7) with acetic acid (5.3) to obtain working standards having final concentrations of 0.0-, 0.1-, 0.2-, 0.3-, 0.5-, and l.0-ppm Cd.

6.3 Determination of Lead by Atomic Absorption—Stir the leaching solution and remove a portion by pipetting into a clean flask. Use lead lamp (4.2) and concomitantly measure absorbance of lead working standards (6.2.1) and leach solutions. Dilute with 4 % acetic acid if leach solutions contain over 10-ppm Pb. Concentrate leach solutions containing less than l-ppm Pb by accurately transferring a minimum of 50.0 mL of solution to a 250-mL beaker and evaporating almost to dryness on a steam bath. Add 1 mL of HC1, then evaporate to dryness. Dissolve the residue in 4 % acetic acid by adding exactly 0.1 volume of solution taken for concentration, cover with watch glass, and swirl to complete dissolution. Calculate lead concentration (ppm Pb) of leach solution by comparison to standard curve.

6.4 Determination of Cadmium by Atomic Absorption— Proceed as in 6.3 using the cadmium lamp (4.3) and standards (6.2.2). Dilute with 4 % acetic acid if leach solutions contain over l ppm Cd. Concentrate leach solutions containing less than 0.l-ppm Cd as in 6.3.

7. Report

7.1 Report the type of units tested, the volume of acid used, and the lead and cadmium leached in parts per million for each unit tested.


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8. Precision and Bias

8.1 Precision—In an analysis of variance study from eight laboratories, the standard deviation between laboratories was 0.06 mg/L for lead and 0.007 mg/L for cadmium. The within laboratory precision had a standard deviation of 0.04 mg/L for lead and 0.004 mg/L for cadmium. The standard deviation for interaction between laboratories and samples is 0.06 mg/L for lead and 0.010 mg/L for cadmium. Reproducibility is defined as the square root of the sum of the three component variances. The reproducibility was 0.10 mg/L for lead and 0.013 mg/L for cadmium.

8.2 Bias—The bias of this test method is further limited by the ability to obtain representative samples of the statistical universe being sampled. An analysis of large populations (100 to 500) has shown that the lead and cadmium release data conformed to a Pearson III distribution with a coefficient of variation between 30 and 140 %, typically 60 %.

ISO Standard 6486: Ceramic Ware, Glass-Ceramic Ware, and Glass Dinnerware In Contact With Food -- Release of Lead and Cadmium Part 1: Method

Introduction

Lead and cadmium release from ceramic and glass ware surfaces is an issue which requires effective means of control to ensure the protection of the population against possible hazards arising from the use of improperly formulated and/or processed ceramic, glass-ceramic, and glass dinnerware used for the preparation, serving and storage of food and beverages. As a secondary consideration, different requirements from country to country for the control of the release of toxic materials from the surfaces of ceramic ware present non-tariff barriers to international trade in these commodities. Accordingly, there is a need to maintain internationally accepted methods of testing ware for lead and cadmium release, and to define permissible limits for the release of these toxic heavy metals.

The limits for lead and cadmium release specified in this standard are not intended to be regarded as the maximum amount of these metals to which exposure can be considered safe. They are levels which are consistent with good manufacturing practice in the respective industries, harmonize


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regulatory levels in principal world markets, and reflect a general objective of reducing overall exposure to these metals.

1 Scope

This part of ISO 6486 specifies a test method for the release of lead and cadmium from ceramic ware, glass-ceramic ware, and glass dinnerware intended to be used in contact with food, but excluding porcelain enamel articles.

This part of ISO 6486 is applicable to ceramic ware, glass-ceramic ware, and glass dinnerware which is intended to be used for the preparation, cooking, serving and storage of food and beverages, excluding articles used in food manufacturing industries or those in which food is sold.

2 Normative References ISO 1042 : 1983, Laboratory glassware - One-mark volumetric flasks ISO 3585 : 1991, Borosilicate glass -- properties ISO 3696 : 1987, Water for analytical laboratory use - Specifications

and test methods. ISO 385-2 : 1984, Laboratory glassware - Burettes - Part 2: Burettes for

which no waiting time is specified. ISO 4788 : 1980, Laboratory glass ware - Graduated measuring

cylinders ISO 648 : 1977, Laboratory glassware - One-mark pipettes ISO/DIS 8655-2 Piston and/or plunger operated volumetric apparatus

(POVA) - Part 2: Operating considerations ISO/DIS 8655-4 Piston and/or plunger operated volumetric apparatus

(POVA) - Part 4: Specifications

3 Definitions

For the purpose of this part of ISO 6486, the following definitions apply.

atomic absorption spectrometry (AAS): spectroanalytical method for qualitative determination and quantitative evaluation of element


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concentrations. The technique determines these concentrations by measuring the atomic absorption of free atoms.

atomic absorption: absorption of electromagnetic radiation by free atoms in the gas phase. A line spectrum is obtained which is specific for the absorbing atoms.

bracketing technique: Analytical method consisting of bracketing the measured absorption or machine reading of the sample between two measurements made on calibration solutions of neighboring concentrations within the optimum working range.

calibration function: Function relating atomic absorption instrument readings, either in absorption or in other machine units, to the concentration of lead or cadmium which generated the instrument reading.

ceramic ware: Ceramic articles which are intended to be used in contact with foodstuffs, for example foodware made of china, porcelain and earthenware, whether glazed or not.

cooking ware: Foodware, specifically intended to be heated in the course of preparation of food and drinks by conventional thermal methods and by microwaves.

dinnerware: Articles specially intended for the serving of food on the table, including plates, dishes and salad bowls, but excluding volumetric ware typically used for beverages, such as goblets and decanters.

direct method of determination: Analytical method consisting of inserting the measured absorption or machine reading into the calibration function and deducing the concentration of the analyte.

drinking rim: 20 mm wide section of the external surface of a drinking vessel, measured downwards from the upper edge along the wall of the vessel.

extraction solution: acetic acid, 4% (V/V), recovered after the extraction test and which is analyzed for lead and cadmium concentration.


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flame atomic absorption spectrometry (FAAS): Atomic absorption spectrometry that uses a flame to create free atoms of the analyte in the gas phase.

flatware: Ceramic or glass ware having an internal depth not exceeding 25 mm, measured from the lowest point to the horizontal plane passing through the point of overflow.

foodware: Articles which are intended to be used for the preparation, cooking, serving and storage of food or drinks.

glass ceramic: Inorganic material produced by the complete fusion of raw materials at high temperatures into a homogeneous liquid which is then cooled to a rigid condition and temperature treated in such a way as to produce a mostly micro crystalline body.

glass: Inorganic material produced by the complete fusion of raw materials at high temperature into a homogeneous liquid which is then cooled to a rigid condition, essentially without crystallization. The material may be clear, colored, or opaque, depending on the level of coloring and opacifying agents used.

hollowware: Ceramic ware having an internal depth greater than 25 mm, measured from the lowest point to the horizontal plane passing through the point of overflow. Hollowware is subdivided into three categories based on volume:

�� small: hollowware with a capacity of less than 1,1 litres.

�� large: hollowware with a capacity of 1,1 litres or greater;

�� storage: hollowware with a capacity of 3,0 litres or greater; cups and mugs: small ceramic hollowware commonly used for consumption of beverages, for example, coffee or tea at elevated temperature. Note: cups and mugs are vessels of approximately 240 ml capacity with a handle. Cups typically have curved sides whereas mugs have cylindrical sides.

optimum working range: Range of concentrations of an analyte over which the relationship between absorption and concentration is practically linear.


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reference surface area: The area that is intended to come into contact with foodstuffs in normal use.

test solution: The solvent used in the test to extract lead and cadmium from the article. Acetic acid, 4% (V/V)

vitreous enameled ware: Metallic articles coated with a vitreous inorganic coating bonded by fusion at temperatures above 500o C.

4 Principle

Silicate surfaces are placed in contact with 4 % (V/V) acetic acid solution for 24 h at (22 � 2)oC to extract lead and/or cadmium, if present, from the surfaces of the articles or test specimens.

The amounts of extracted lead and cadmium are determined by flame atomic absorption spectrometry (FAAS). In routine tests other equivalent analysis methods may be used.

5 Reagents and Materials

5.1 Reagents

All reagents shall be of recognized analytical grade. Distilled water or water of equivalent purity (grade 3 water complying with the requirements of ISO 3696) shall be used throughout.

Acetic acid, (CH3COOH), glacial, � = 1.05 g/ml.

Acetic acid test solution, 4% (V/V) solution. Add 40 ml of acetic acid (5.1.1) to distilled water, and dilute to 1 litre. This solution shall be freshly prepared for use. Proportionately greater quantities may be prepared.

Lead stock solution Prepare analytical stock solutions containing 1000 � 1 mg of lead per litre in the test solution (5.1.2). Alternatively, an appropriate commercially available standardized lead AAS solutions may be used.

Cadmium stock solution Prepare analytical stock solutions containing 1000 � 1 mg of cadmium per litre in the test solution (5.1.2). Alternatively,


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an appropriate commercially available standardized cadmium AAS solution may be used.

Lead standard solution Dilute the lead stock solutions ten-fold with test solution (5.1.2) to produce a lead standard solution which is 100 mg/l Pb, or 0,1 g of lead per litre.

Cadmium standard solution Dilute the cadmium stock solutions 100-fold with test solution (5.1.2) to produce a cadmium standard solution which is 10 mg/l Cd, or 0,01 g of cadmium per litre.

Notes:

Standard solutions may be kept in suitable, aged, tightly closed containers (i.e. polyethylene) for four weeks without loss of quality. New containers may be aged by filling with standard solution and allowing to stand for 24 h. The aging solution is discarded.

Use one-mark glass pipettes or precision piston pipettes with a fixed stroke, typically 1000 �l and 500 �l, and appropriate volumetric glassware (e.g. 500 ml to 2000 ml) to prepare proper calibration solutions by dilution of the standard stock solutions (5.1.5 and 5.1.6) with test solution (5.1.2). Keep the solutions in suitable and aged containers. Renew these solutions every four weeks.

5.2 Materials and Supplies

5.2.1 Paraffin wax, with a high melting point.

5.2.2 Washing agent, commercially available non-acidic manual dishwashing detergent in dilution recommended by manufacturer.

5.2.3 Silicone sealant, capable of forming a ribbon of sealant approximately 6 mm in diameter. This sealant shall not leach acetic acid, cadmium or lead to the test solution (5.1.2).

6 Apparatus

6.1 Atomic absorption spectrometer,

Atomic absorption spectrometer equipped with light sources [hollow cathode or electrodeless discharge lamps] specific for lead and cadmium,


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instrumental background correction, and a single slot [approximately 100 mm] or Boling burner head. Digital concentration readout may be used. Use air-acetylene flame and operating conditions recommended by the instrument manufacturer. Using these conditions, characteristic concentration [concentration that gives 0.0044 absorbance] should be approximately [�20%] 0.2 mg/l for Pb measured at 217.0 nm. Characteristic concentration should be approximately [�20%] 0.02 mg/l for Cd measured at 228.8 nm. Note: Where appropriate, a wavelength of 283,3 nm may be used for the analytical confirmation of lead.

6.2 Accessories

Assorted glassware as required, made of borosilicate glass as specified in ISO 3585.

Burette of capacity 25 ml, graduated in divisions of 0,05 ml, complying with ISO 385-2, class B or better.

Covers for the articles under test, e.g. plates, watch-glasses, Petri dishes of various sizes. Covers must be opaque if a darkroom is not available.

One-mark pipettes of capacities 10 ml and 100 ml, complying with ISO 648, class B or better. Other sizes as required.

One-mark volumetric flasks of capacities 100 ml and 1000 ml, complying with ISO 1042, class B or better. Other sizes as required.

Precision piston pipettes with a fixed stroke, typically 1000 �l and 500 �l.

Straight edge and depth gage calibrated in millimeters.

7 Sampling

7.1 Priority

When selecting samples from a mixed lot of foodware, articles having the highest surface area/volume ratio within each category should be given preference. Articles that are highly colored or decorated on their food contact surfaces should be especially considered for sampling.

7.2 Sample size


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It is desirable to develop a system of sampling control that is appropriate to the circumstances. In no case shall less than four items be measured. Each of the articles shall be identical in size, shape, color and decoration.

7.3 Preparation and preservation of test samples

Samples of ware shall be clean and free from grease or other matter likely to affect the test. Briefly wash the specimens at a temperature of about 40o C with a solution containing a non-acidic detergent. Rinse in tap water and then in distilled water or water of equivalent purity. Drain and dry in either a drying oven or by wiping with a new piece of filter paper. Do not use any sample that shows residual staining. Do not handle the surfaces to be tested after cleaning.

If an area of the surface of the sample is not intended to come into contact with foodstuffs in normal use, other than the interior of any lid, cover this area after the initial washing and drying with a protective coating such as paraffin wax or silicone which will withstand the effect of the test solution and which will not release any detectable levels of lead or cadmium into the test solution.

8 Procedure

8.1 Determination of reference surface area for flatware

Place a specimen on a sheet of smooth paper and draw a contour around the rim. Determine the enclosed area by a suitable means. One recommended method is to cut out and weigh the enclosed area and to determine the area by comparison of the weight with the weight of a rectangular sheet of known area. Record this area, SR, in square decimeters to two decimal places. For circular articles the reference surface area may be calculated from the diameter of the article.

8.2 Preparation of articles which cannot be filled.

Articles are normally filled to within 6 mm of overflowing as measured along the sloping side of flatware, or to within one mm of the rim as measured vertically for hollowware. Articles which cannot be filled in this manner to produce an acid depth at the deepest point of at least 5 mm are


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defined as non-fillable. Articles of this type may be tested by one of the following methods.

Standard articles may be fitted into a silicone rubber mold which forms a water-tight seal with the article and which encroaches no more than 6 mm from the rim and forms a depth of at least 5 mm but no more than 25 mm. Specimens prepared in this way are tested as fillable flatware articles.

A bead of silicone sealant may be formed around the edge of the article to permit filling of the article to a depth of at least 5 mm but no more than 25 mm. The bead shall encroach no more than 6 mm from the rim of the article. Specimens prepared in this way are tested as fillable flatware articles

The article may be coated on all surfaces except the reference surface with melted paraffin wax and subsequently tested by immersion in test solution. Specimens prepared in this way are tested as non-fillable flatware articles.

8.3 Extraction

8.3.1 Extraction temperature

Conduct the extraction at a temperature of (22 � 2)oC. When cadmium is to be determined, conduct the extraction in the dark.

8.3.2 Leaching

8.3.2.1 Fillable Articles: Fill each specimen with test solution (5.1.2), to 1 mm of overflowing measured vertically for hollowware or 6 mm from overflowing as measured along the surface of flatware. For flatware determinations measure and record the volume of acetic acid, 4%, used to fill the article. Cover the specimen. Leach for 24 h � 30 min.

8.3.2.2 Non-Fillable Articles: These articles, which have been masked with paraffin wax according to 8.2.c, are placed in a suitable vessel such as borosilicate glass of suitable size and test solution (5.1.2) is added in sufficient quantity to completely cover the sample. Record the amount of acetic acid added to an accuracy of 2%. Leach for 24 h � 30 min.

8.3.3 Sampling of the extraction solution for analysis


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Prior to sampling, mix the extraction solution by stirring or other appropriate method that avoids loss of the extraction solution or abrasion of the surface. Remove a sufficient amount of the extraction solution with pipette and transfer it to a suitable storage container.

Analyse the extraction solution as soon as possible since there is a risk of adsorption of lead or cadmium onto the walls of the storage container, particularly when Pb and Cd are present in low concentrations.

8.4 Drinking rim and other special tests

Note: This is an optional procedure for evaluating drinking rims.

Cups may be tested by marking each of four units 20 mm below the rim on the outside. Each cup is placed inverted in a suitable laboratory glassware container with a diameter between 1.25 and 2.0 times that of the cup. Add sufficient 4% acetic acid to the glassware container to fill to the 20 mm mark on the cup. Let stand for 24 h at (22 � 2)oC (in the dark for cadmium determinations) and protect from excessive evaporation. Before sampling the leachate, add 4% acetic acid to the glass container as necessary to re-establish the 20 mm level. Determine lead and cadmium by AAS and report the results as mg/article.

8.5 Calibration

Set up the atomic absorption spectrometer according to the manufacturer’s instructions using wavelengths of 217 nm for lead determination and 228,8 nm for cadmium determination with an appropriate correction for background absorption effects. Note: Where appropriate, a wavelength of 283,3 nm may be used for the analytical confirmation of lead.

Aspirate the zero member of the set of calibration solutions and adjust zero. Aspirate the set of calibration solutions, prepared by dilution of the standard solution with test solution (5.1.2) and prepare calibration curves over a linear range. Suggested ranges:

0,5 - 10,0 mg/l Pb

0,05 - 0,5 mg/l Cd

8.6 Determination of lead and cadmium


141

Set up the spectrometer as described previously. Aspirate distilled water and then acetic acid, 4%, and verify the absorbance is zero. Aspirate the extraction solution, interspersed with test solution (5.1.2) and record the absorbance values of the extraction solutions.

If the lead concentration of the extraction solution is found to be higher than 10 mg/1, dilute a suitable aliquot portion with test solution (5.1.2) to reduce the concentration to less than 10 mg/1.

Similar considerations apply to the determination of cadmium.

9 Expression of results

9.1 Bracketing technique

The lead or cadmium concentration, �o, expressed in milligrams per litre of the extraction solution, is given by the formula

� �� ooA A

A Ad�

�

�

�

��

�

��

��

��

1

2 12 1 1

where

Ao is the absorbance of the lead or cadmium in the extraction solution;

A1 is the absorbance of the lead or cadmium in the lower bracketing solution;

A2 is the absorbance of the lead or cadmium in the upper bracketing solution;

�1 is the lead or cadmium concentration, in milligrams per litre, of the lower bracketing solution;

�2 is the lead or cadmium concentration, in milligrams per litre, of the upper bracketing solution

NOTE: If the extraction solution was diluted, an appropriate correction factor, d, is used in the formula.

9.2 Calibration curve technique


142

Read the lead or cadmium concentration directly from the calibration curve or from the direct read-out.

9.3 Calculation of release of lead and cadmium from flatware

The lead or cadmium released per unit area from flatware, Ro, expressed in milligrams per square decimeter, is given by the formula

RV

Soo

R

�

��

where:

�o is the lead or cadmium concentration, expressed in milligrams per litre, of the sample extract solution.

V is the filling volume of the specimen, expressed in litres

SR is the reference surface area of the article, expressed in square decimeters.

For hollowware articles report the result to the nearest 0,1 mg of lead per litre and to the nearest 0,01 mg of cadmium per litre

For flatware report the result to the nearest 0,1 mg of lead per square decimeter and to the nearest 0,01 mg of cadmium per square decimeter. Also report the concentration of lead and cadmium in the leach solution to the nearest 0,1 mg of lead per litre and to the nearest 0,01 mg of cadmium per litre.

10 Reproducibility And Variability

Lead and cadmium release measurements from ceramic foodware are subject to analytical reproducibility errors and sampling variability. The material presented in this section are of scientific and technological interest but are not of normative or statutory value in the context of this ISO standard.

10.1 Reproducibility


143

Three types of determination errors occur in the analytical measurement of lead and cadmium concentrations. Each is listed in table 1 with an approximate value for the standard deviation of each1.

TABLE 1 -- SOURCES OF VARIATION IN ANALYTICAL DETERMINATION OF PB AND CD

Source of Variation Standard Deviation, Pb Determination, [mg/l]

Standard Deviation, Cd Determination, [mg/l]

Analysis, within laboratory 0,04 0,004 Analysis, between laboratories 0,06 0,007 Laboratory x Sample Interaction 0,06 0,01 Reproducibility 0,094 0,012

The statistical interaction term, row 4 in table 1, reflects the failure of the differences in sample analyses to be the same from laboratory to laboratory. A detailed discussion may be found in elementary statistical texts which address Analysis of Variance (ANOVA) methods. The reproducibility is the square root of the sum of the squares of the standard deviations from the three sources of variation.

10.2 Variability

Analytical reproducibility is quite good compared to the intrinsic variability of the extraction behavior of glass and ceramic surfaces. This variability, termed sampling variability, is by far the greatest source of experimental error. Moore2 has shown that the coefficient of variability for lead and cadmium release for large samples is typically 60%. Thus, the true average lead release value for a large population must be approximately 0,58 mg/l in order to avoid one of four test specimens from exceeding a 2 mg/l limit 1 in 10 000 times. The table 2 illustrates the effect of population mean

1 ASTM Standard Test Method for Lead and Cadmium Extraction from Glazed Ceramic Surfaces, C738-94, American Society for Testing and Materials, Philadelphia, PA 1994. 2 Moore, F., Transactions, Journal of British Ceramic Society, Vol. 76 (3), 1977, pp. 52-57.


144

and standard deviation values on the probability that 1 in 4 or 1 in 6 specimens will exceed a 2 mg/l limit value.

TABLE 2: PROBABILITIES OF EXCEEDING 2 MG/L LIMIT Population Mean Population

Std. Dev. Probability of 1 in 4 > 2 mg/l

Probability of 1 in 6 > 2 mg/l

0,4 0,24 <0,00001 <0,00001 0,8 0,48 0,13826 0,20005 1,2 0,72 0,75836 0,88122 0,4 0,12 <0,00001 <0,00001 0,8 0,24 0,00002 0,00004 1,2 0,36 0,32568 0,44627

11 Test Report

The test report shall include the following information:

�� a reference to this part of ISO 6486;

�� identification of the sample, including type, origin, and destination;

�� the surface area or the reference surface area and the filling volume or contact volume for non-fillable articles and test specimens;

�� the number of samples tested;

�� the test results, expressed as individual values for each specimen and the mean value for test sample groups. Test values for hollowware articles should be reported to the nearest 0,1 mg of lead per litre and to the nearest 0,01 mg of cadmium per litre. Test values for flatware should be reported to the nearest 0,1 mg/dm2 of lead and to the nearest 0,01 mg/dm2 of cadmium. NOTE: As supplementary information, the concentration of solutions from tests on flatware articles should also be included and reported to the nearest 0,1 mg/l of lead and to the nearest 0,01 mg/l of cadmium.

�� any unusual features noted during the determination;

�� any optional tests, or tests not included in this International Standard


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ISO6486: Permissible limits

12 Permissible limits

The permissible limits for lead and cadmium release are given in the following table.

Type of Ware n Permissible Limit Criterion Unit of measure

Lead Limit

Cadmium Limit

Flatware 4 Mean � Limit mg/dm2 0,8 0,07 Small Hollowware 4 All specimens � Limit mg/l 2,0 0,50 Large Hollowware 4 All specimens � Limit mg/l 1,0 0,25 Storage Hollowware 4 All specimens � Limit mg/l 0,5 0,25 Cups and Mugs 4 All specimens � Limit mg/l 0,5 0,25 Cooking Ware 4 All specimens � Limit mg/l 0,5 0,05


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Appendix C:

Materials Handling

Every type of lead material can be handled and controlled with safety if proper modern equipment is provided for the protection of the health of the industrial worker. This is borne out by the many plants that have as their primary function the processing and handling of lead and its compounds and do so with entire success and safety.

Industrial health precautions are by no means confined to the use of lead and its compounds. Silica, beryllium, cadmium, antimony, selenium, tellurium and other elements and compounds present extremely hazardous exposure problems. While this text primarily addresses the handling of lead compounds, it should be remembered that similar precautions are necessary in plants where other compounds are used that generate dust hazards.

Proper handling of lead compounds in the ceramic industry requires:

1. Proper Plant Hygiene

2. Proper Instruction And Supervision Of Workers

3. Regular Checks By Plant Physician Or Medical Director.

Proper Plant Hygiene

�� Adequate pre-employment examination of all plant workers. This should include previous working histories, chest X-rays and blood examinations.

�� All operations which disperse dust should be controlled by closed systems of local exhaust ventilation.


147

Lead compounds for the ceramic user are conveniently packaged so that unloading, plant storage and movement to the location where the packages are to be emptied present few exposure problems.

Dust hoods should be located where dry materials are charged or discharged from processing equipment and should surround the operation as completely as possible. Air velocity of 200 linear feet per minute at the hood face is the generally accepted standard requirement. Exhaust from hood face is the generally accepted standard requirement. Exhaust from hood ventilating systems must be discharged outside of the plant, preferably into cloth-screen dust arrestors having not less than one-half square foot of effective area of cloth per cubic foot of dust-laden air per minute passing through it.

Bins, elevators, chutes, covered conveyors, covered mixers etc., can usually be made dust-free by drawing off dust-laden air at critical points. By maintaining a slight negative pressure in the closed equipment, some air may enter the system, but dust will not escape. Relatively small air volumes are required to achieve dustless operation for closed equipment. Exhausted dust-laden air should be properly filtered before discharge outside the plant.

Dust control and elimination are important to all industries today and there are many reliable and qualified manufacturers of equipment to do the job properly.

�� Only where local or general control is impossible, workers should be provided with respirators specifically approved by United States Bureau of Mines for this type of protection.

�� Adequate washroom facilities should be provided. Hot water and soak should be provided, along with individual hand towels. The hand towels may be paper. Shower facilities are recommended.

�� Proper locker room facilities should be provided. Workers handling toxic materials should have separate lockers for street clothes and work clothes to prevent contamination.

�� A suitable and separate place should be provided for the workers to eat.


148

Workers should not eat or smoke on the job while handling toxic materials.

Proper Instruction and Supervision

The purpose, correct use, and maintenance of respirators should be explained to the workers and enforced.

There are several good types of approved respirators on the market. Be sure that workers are provided with respirators that fit the individual. Respirators assigned to one individual should be regarded as personal as a toothbrush and should not be interchanged. Respirators worn by a single workman usually tend to fit better and more comfortably after repeated use.

Spare respirator filters, head bands and other parts should be readily available. Respirators must be maintained in order to be effective.

Air supplied respirators are preferred where it is practical to use them as they are generally more comfortable and positive in their action.

Most authorities agree that the use of respirators is no substitute for adequate plant engineering. In practical plant operation, however, situations do arise during which engineering controls are being developed, or where repairs are necessary. The use of respirators in these situations is extremely important for proper plant hygiene.

Regular Checks By Plant Physician Or Medical Director

Physicians in the developed countries rarely have occasion to treat cases of heavy metal intoxication or poisoning since these events are few and isolated. Therefore, the plant doctor should be informed of any toxic materials being handled and instructed on symptoms and treatment for the specific hazards at a given plant. The plant doctor will then be in a position to study the problem and outline a regular safety program.

Health Risks of Lead Compounds

Human exposure to lead occurs through a combination of inhalation and oral exposure, with inhalation generally contributing a greater proportion of the dose for occupationally exposed groups, and the oral route generally contributing a greater proportion of the dose for the general population. The effects of lead are the same regardless of the route of exposure (inhalation or


149

oral) and are correlated with internal exposure, as blood lead levels. The main sources of information for this section are the US EPA's Integrated Risk Information System (IRIS), which contains information on the carcinogenic effects of lead, the Agency for Toxic Substances and Disease Registry's (ATSDR's) Toxicological Profile for Lead, and EPA's Supplement to the Criteria Document on Lead. Other secondary sources include the Hazardous Substances Data Bank (HSDB), a database of summaries of peer-reviewed literature, and the Registry of Toxic Effects of Chemical Substances (RTECS), a database of toxic effects that are not peer reviewed.

Assessing Personal Exposure

The amount of lead in the blood can be measured to determine if exposure to lead has occurred

Exposure to lead can also be evaluated by measuring erythrocyte protoporphyrin (EP), a component of red blood cells known to increase when the amount of lead in the blood is high. This method has been commonly used to screen children for potential lead poisoning.

Methods to measure lead in teeth or bones by X-ray fluorescence techniques are available.

Health Hazard Information

Acute Effects:

Death from lead poisoning may occur in children who have blood lead levels greater than 125 µg/dL and brain and kidney damage have been reported at blood lead levels of approximately 100 µg/dL in adults and 80 µg/dL in children.

Gastrointestinal symptoms, such as colic, have also been noted in acute exposures at blood lead levels of approximately 60 µg/dL in adults and children.

Short-term (acute) animal tests, such as the LC50 test in rats, have shown lead to have moderate to high acute toxicity.

Chronic Effects (Noncancer):


150

Chronic (long-term) exposure to lead in humans can affect the blood. Anemia has been reported in adults at blood lead levels of 50 to 80 µg/dL, and in children at blood lead levels of 40 to 70 µg/dL.

Lead also affects the nervous system. Neurological symptoms have been reported in workers with blood lead levels of 40 to 60 µg/dL, and slowed nerve conduction in peripheral nerves in adults occurs at blood lead levels of 30 to 40 µg/dL.

Children are particularly sensitive to the neurotoxic effects of lead. There is evidence that blood lead levels of 10 to 30 µg/dL, or lower, may affect the hearing threshold and growth in children.

Other effects from chronic lead exposure in humans include effects on blood pressure and kidney function, and interference with vitamin D metabolism.

Reproductive/Developmental Effects:

Studies on male lead workers have reported severe depression of sperm count and decreased function of the prostate and/or seminal vesicles at blood lead levels of 40 to 50 µg/dL. These effects may be seen from acute as well as chronic exposures.

Occupational exposure to high levels of lead has long been associated with a high likelihood of spontaneous abortion in pregnant women. However, the lowest blood lead levels at which this occurs has not been established. These effects may be seen from acute as well as chronic exposures.

Prenatal exposure to lead produces toxic effects on the human fetus, including increased risk of preterm delivery, low birthweight, and impaired mental development. These effects have been noted at maternal blood lead levels of 10 to 15 µg/dL, and possibly lower. Decreased IQ scores have been noted in children at blood lead levels of approximately 10 to 50 µg/dL.

Human studies are inconclusive regarding the association between lead exposure and birth defects, while animal studies have shown a relationship between high lead exposure and birth defects. (1,6)

Cancer Risk:


151

Human studies are inconclusive regarding lead and an increased cancer risk. Four major human studies of workers exposed to lead have been carried out; two studies did not find an association between lead exposure and cancer, one study found an increased incidence of respiratory tract and kidney cancers, and the fourth study found excesses for lung and stomach cancers. However, all of these studies are limited in usefulness because the route(s) of exposure and levels of lead to which the workers were exposed were not reported. In addition, exposure to other chemicals probably occurred. (1,2,5)

Animal studies have reported kidney cancer in rats and mice exposed to lead via the oral route. (1,2,5,6)

EPA considers lead to be a probable human carcinogen (cancer-causing agent) and has classified it as a Group B2 carcinogen.

Physical Properties

Lead is a naturally occurring, bluish-gray metal that is found in small quantities in the earth's crust. Lead is present in a variety of compounds such as lead acetate, lead chloride, lead chromate, lead nitrate, and lead oxide. Pure lead is insoluble in water; however, the lead compounds vary in solubility from insoluble to water soluble.

The chemical symbol for lead is Pb and the atomic weight is 207.2 g/mol.

The vapor pressure for lead is 1.0 mm Hg at 980 C.

Major Health Effects Noted from Lead Exposure Blood lead levels (µg/dL) Health numbersa

150 Death

100.0

Death (children) (125 µg/dL) Brain and kidney damage (adults) (100 µg/dL)

75.0 Brain and kidney damage (children) (80 µg/dL)


152

40.0 Increased blood pressure (40 µg/dL)

30.0 Slowed nerve conduction velocity (30 µg/dL)

20.0 Decreased IQ and growth in young children (20 µg/dL)

10.0 Preterm birth, reduced birthweight (10 to 15 µg/dL)

Occupational Exposure to Lead:

Lead has been poisoning workers for thousands of years. In the construction industry, traditionally most overexposures to lead are found in the trades, such as plumbing, welding and painting. Significant lead exposures can also arise from removing paint from surfaces previously coated with lead-containing paint, such as in bridge repair, residential renovation, and demolition.

Hobbies resulting in lead exposure may include firearm practice, soldering in jewelry making or stained glass work, and ceramics.

Exposure to lead may result when lead or any product containing lead is heated, especially above 500 degrees C (932 degrees F). Lead dust exposure may result from such operations as pouring powders containing lead, sanding or sandblasting surfaces coated with lead based paints.

Very small amounts of lead that may be unintentionally ingested via eating, drinking, or smoking on the job or through hobbies can be harmful. Good personal hygiene is important where lead is present.

Worker awareness and training are important so that employees can recognize the symptoms of exposure and get prompt medical attention. Jobs involving potential lead exposure should be targeted for detailed evaluation of their potential for lead exposure.

OSHA regulates lead for employees who work in the private sector. The OSHA lead regulations (29 CFR 1910.1025 and 29 CFR 1926.62) require:

If a worker works with lead, the employer must test the air for lead levels.


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If the lead level in the air is 30 micrograms per cubic meter of air or greater, the employer must offer employees routine blood testing.

An employee must be removed from all exposure to lead if the average blood level is 50 micrograms/deciliter or more on three tests. He or she cannot return to an environment where lead is present until the blood level falls to at least 40 mcg/dl.

The standard also establishes requirements for medical monitoring, respiratory protection, protective clothing, engineering controls and ventilation, work practice controls, hygiene facilities, and employee education.

OSHA requires employers to reduce airborne lead exposure below the OSHA Permissible Exposure Limit (PEL). The best way to do this is to simply replace lead and products that contain lead with less toxic materials. If this is not possible, the employer must provide change rooms and lockers, showers, and a thorough cleaning of work surfaces. There can be no smoking or eating in work areas. The employer must also provide training to employees on the hazards of lead and on the OSHA lead standard. Currently, there is no OSHA standard that provides a permissible limit for lead contamination of surfaces in occupational settings.

Lead dust can settle on your clothes and if these are not changed before going home, family members can be exposed. Lead can stay on your skin, hair, and on your shoes, lunch bucket, bookbag, etc. Young children are more sensitive to lead than are adults and can have health problems from exposure to less lead.


154

Appendix D:

Sections of Materials Safety Data Sheets

for Selected Lead Compounds [Note: The following abstracted sections from Materials Safety Data Sheets [MSDS] are intended to summarize some of the safety issues for compounds commonly used in lead glazes. Refer to the complete MSDS prior to handling or using any of these materials. Complete MSDS forms can be obtained from the raw material supplier or from the Internet. See URL: www.hammondlead.com]. Sections of materials safety data sheets for the following lead compounds are given in this appendix:

�� Lead Bisilicate �� Litharge [PbO] �� Lead Monosilicate �� Lead Monoxide �� Red Lead [Pb3O4] �� Tribasic Lead Silicate �� White Lead [lead carbonate hydroxide]


155

Lead Bisilicate


156


157


158

Litharge


159


160


161

Lead Monosilicate


162


163


164

Lead Monoxide


165


166


167

Red Lead


168


169


170

Tribasic Lead Silicate


171


172


173

"WHITE LEAD" -- 2PbCO3 Pb(OH)2

SECTION 1 - PRODUCT IDENTIFICATION

Trade Names and Synonyms White Lead Chemical Names and Synonyms Basic Lead Carbonate Chemical Family Basic Lead Chemicals

SECTION 2 - INGREDIENTS

Ingredien

t C.A.S.

Number % W/W

Min. %W/W

Max. Exposure

Limit LD50

oral, rat

Lead 7439-92-1 99.97 99.99 0.05 mg/m3

790 mg/Kg

SECTION 3 - PHYSICAL DATA

Boiling Point (deg C) N/A Specific Gravity 6.8 Vapor Pressure (mm Hg) N/A % Volatile (By Volume) NP

Vapor Density (Air=1) N/A Evaporation Rate

(Ether=1) NP

Solubility In Water N/A pH NP

Appearance White Odorless Powder

Melting Point (deg C) 550 - 790

Odor None

Form Solid-Powder

WHMIS Classification D2-A TDG Information

Shipping Name: NP UN Number: NP Class/Division: NP

NP - Not Pertinent U - Unknown

Packing Group: NP


174

SECTION 4 - FIRE AND EXPLOSION HAZARDS

Flash Point (deg C) and Method NP

Flammable Limits in Air (Vol %) Upper: NP Lower: NP

Means of Extinction: Class D - Water Fog, Flood, CO2, and Dry Chemical

SECTION 5 - HEALTH HAZARD AND FIRST AID DATA

Effects: May cause headache, nausea, abdominal pains, fatigue, muscle/joint pain, kidney disjunction, wrist-drop. Ingestion First Aid: Give water or milk. If conscious, induce vomiting. Effects: Dust or fumes may cause irritation.

Eye Contact First Aid: Flush eye with cool water for 15 minutes and seek immediate medical aid. Effect: May cause local irritation.

Skin Contact First Aid: Remove contaminated clothing and wash affected area with soap and water.

Skin Absorption NP

Inhalation See "Ingestion", CNS damage (results in fatigue, tremors, hallucinations, convulsions, delirium), weight loss, sleep disturbance.

Effect of Acute Exposure

See " Ingestion Effects " and " Inhalation Effects ".

Effects of Chronic Exposure

Possible anemia, central nervous system and kidney damage.

Carcinogenicity: IARC (Yes)

Mutagenicity: Yes Teratogenicit Yes Reproductiv Yes


175

y: e Effects:

SECTION 6 - REACTIVITY DATA

Stability Stable - Yes Conditions to avoid: Hydrogen peroxide

Water: No Corrosive: No

Acid: No Alkali: No

Oxidizers: Yes Reducers: No Incompatible

Materials Other: No

Hazardous Decomposition Products: Toxic lead oxide fumes will form at elevated temperature.

Hazardous Polymerization: May Occur: No Will Not Occur: X

Conditions to Avoid: NP


176

Appendix E:

Pyrometric Cone Properties Temperature Equivalents of Orton Large Pyrometric Cones

(1) 6OCº

108Fº

150Cº

270Fº Cone

Number 887 1629 894 1641 010 915 1679 923 1693 09 945 1733 955 1751 08 973 1783 984 1803 07 991 1816 999 1830 06 1031 1888 1046 1915 05 1050 1922 1060 1940 04 1086 1987 1101 2014 03 1101 2014 1120 2043 02 1117 2043 1137 2079 01 1136 2077 1154 2109 1 1142 2088 1162 2124 2 1152 2106 1168 2134 3 1168 2134 1186 2167 4 1177 2151 1196 2185 5 1201 2194 1222 2232 6 1215 2219 1240 2264 7 1236 2257 1263 2305 8 1260 2300 1280 2336 9 1285 2345 1305 2381 10 1294 2361 1315 2399 11 1306 2383 1326 2419 12


177

Pyrometric Cone Table Notes:

1. The temperature equivalents in this table apply only to Orton Standard Pyrometric Cones when heated at the rates indicated.

2. Temperature Equivalents are given in degrees Centigrade (ºC) and the corresponding degrees Fahrenheit (ºF) . Rates of heating shown at the head of each column of temperature equivalents are expressed in Centigrade degrees (Cº) and Fahrenheit degrees (Fº) per hour. These heating rates were maintained uniformly during the last several hundred degrees of temperature rise in the test. All determinations were made in an air atmosphere.

3. The temperature equivalents are not necessarily those at which cones will deform under firing conditions different from those under which the calibrating determinations were made.

4. For reproducible results, care should be taken to insure that cones are set in a plaque with the bending face at the correct angle of 82º from the horizontal, with the cone tips at the correct height above the top of the plaque. (Large Cones 2”, Small and P.C.E. Cones 15/16”. )


178

Cone Position Diagram

The illustration indicates a method of designating or recording the position or degree of bending of a cone on a plaque. Beginning at the left, the cone is in the original position with an 8 degree inclination. In the next figure the cone has deformed to the 1 o’clock position. In the next four figures, the cones have successively deformed to the 2, 3, 4 and 5 o’clock positions. The last figure shows the end of cone deformation -- 6 o’clock -- as far as a reading can be obtained. Temperatures on cones as given in the Temperature Equivalents Table were determined when a certain cone had reached the 6 o’clock position.


179

Appendix F:

Glossary Of Terms

Amphoteric -- Relating to an acid/base behavior intermediate between acids and bases. Aluminum oxide often behaves in an amphoteric way, sometimes behaving like an acid oxide such as SiO2 and other times behaving in a more basic way, such as MgO.

Bisilicate – A compound with two silica [SiO2] units in the formula, such as lead bisilicate, PbO . 2SiO2.

Bisque -- The kiln firing, or “fire”, in which the ceramic ware is matured before the glaze is applied.

Bridging – In the silicate molecular structure bridging refers to oxygens that are bonded to two silicon atoms, i.e. they bridge between the silicons.

Carboxy Methylcellulose – A cellulose polymer use as a volatile binder in ceramic glazes.

Cone – A small, tall pyramidal cone of a controlled ceramic material that has very specific thermal softening characteristics that can be used to monitor the time x temperature work product during a firing cycled.

Devitrification – Crystallization of a glass or glassy phase. Devitrified glazes are glazes in which crystals have formed.


180

Earthenware – Ceramic ware based on coarse or unrefined raw materials, often indigenous naturally-occurring mixtures of clay, flint and feldspar, that matures at low temperature [e.g. cone 06] and is porous and colored by impurities.

Engobe – A clay based coating applied to green ceramic ware to either mask the color or texture of the underlying body in preparation for glazing, or to produce special effects.

Frit – A glass that is formulated and melted from a specific composition of glaze oxides and then pulverized to a fine power for use in glazing. Frits are prepared to stabilize oxides that would otherwise cause problems during glazing.

Glost – The kiln firing, or “fire”, in which the glaze is matured.

Green – Not fired. A clay body before firing is said to be “green”.

Inglaze – A reference to decorations that react with the glaze and form a durable glaze/decoration combination.

Interdiffusion – The simultaneous counter movement ions or other molecular size particles in a material. Use in this book to refer to the movement of lead ions from the glass to the aqueous solution and the counter flow of hydrogen ions during acid leaching of glazes.

Knoop – A system for the measurement of hardness.

Leaching – The selective extraction of lead and other modifier ions from the silica glaze network during acid

Matts – Not glossy, i.e. possessing the property of diffuse light scattering.

Methylcellulose – See carboxy methylcellulose.

Nonbriding -- In the silicate molecular structure bridging refers to oxygens that are bonded to only one silicon atom.


181

Onglaze – See overglaze.

Opacifier – A substance which reduces the transparency of a glaze. Opacifiers are generally crystalline materials such as SnO2 or ZrO2 that scatter light and convert a clear glaze to an opaque glaze.

Overglaze – A reference to decorations that are applied over the glaze, i.e. after the glost fire. These decorations are then fired at a lower temperature to fix them to the surface.

Polymorphic – Of many forms. Polymorphic materials such as SiO2 have different structural forms depending on temperature.

Porcelain – Ceramic ware based on highly refined raw materials, mostly kaolin [china clay] and feldspar, and fired at high temperature [e.g. cone 2 – 12] to produce a strong, vitreous, fully dense, and translucent fired body that is white to off-white in color.

Potentiometric – relating to electrical potential, or voltage.

Rutile – a raw material containing principally TiO2, but often with other impurities.

Spinels – A specific ceramic crystalline structure, as that found in magnesium aluminate.

Stoneware – Ceramic ware based on coarse raw materials, often with significant amounts of grog [coarse non-plastic], and fired at high temperature [e.g. cone 6 – 10] to produce an opaque fired body low in porosity [impervious to water] and ranging from pale to moderately dark in color.

Ulexite/colemanite – sodium calcium borate compounds used in raw glazes, i.e. not fritted glazes.

Underglaze – A reference to decorations that are applied under the glaze to achieve the protection afforded by the glaze.


182


183

Appendix G:

Bibliography and References [1.] Adl, S, Rahman, IA, “Preparation of low melting temperature,

lead-free glaze by the sol-gel method”, Ceram. Int., volume 27, pp 681 – 687 (2001)

[2.] Agency for Toxic Substances and Disease Registry (ATSDR). Case Studies in Environmental Medicine, Lead Toxicity. U.S. Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA. 1992.

[3.] Agency for Toxic Substances and Disease Registry (ATSDR). Toxicological Profile for Lead (Draft). U.S. Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA. 1993.

[4.] Ainsworth, L. “A Method for Investigating the Structure of Glazes Based on a Surface Measurement”, Trans. Brit. Cera. Soc., 55, 661-73 (1956).

[5.] Anderson, O. “The Volatilization of Lead Oxide from Lead Silicate Melts”, J. Am. Ceram. Soc., 2 (10) 784-89 (1919).

[6.] Andrews R. and R. S. Murray, “Method of Improving Acid Resistance of a Glass Color”, U. S. Patent 2, 724, 662, November 22, 1955.

[7.] Azzoni, CB., Delnero, GL., Krajewski, A., Ravaglioli, A., “A Diffusive Model Of Pb+2 Release By Lead-Ceramic Glazes”, Journal Of Materials Science, Volume 16, pp. 1081 – 1087 (1981).


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[8.] Bartel, P. “Lead Solubility of Fritted Glazes”, Sprechsaal, 51, 25-43 (1918).

[9.] Bartel, P. “Literature on the Lead Questions as to Laws and Regulations”, Ber. Deut. keram. Ges., 3, 86 (1922).

[10.] Bennett H. and F. Vaughan, “Solubility of Lead Glazes, Part IV, Investigation of Certain Chemical Methods of Lead Determination”, Trans. Brit. Ceram. Soc., 52, 578-87 (1953).

[11.] Bennett, H. “The Solubility of Lead Glazes, Part V, Chemical Factors Affecting Solubility Determinations”, Trans. Brit. Ceram. Soc., 53, 203-17 (1954).

[12.] Berger, R. “Colors for Porcelain and Glazes”, Silihat J, 3 (5) 405 (1964).

[13.] Binns C. F. and F. Lyttle, “A Vermillion Color for Uranium”, J. Am. Ceram. Soc., 3 (11) 913-14 (1920).

[14.] Bishop, D. F. W. “Lead and Silicosis, Factory Precautions”, Trans, Brit. Ceram. Soc., 37, 17-26 (1937-38).

[15.] Bloor, E. C. “Glaze Composition, Glass Structural Theory, and its Application to Glazes”, Trans, Brit. Ceram. Soc., 55, 631-60 (1956).

[16.] Brandt, A. “Lead Poisoning in the Ceramic Industry, Part H”, Ber. Deut. keram. Ges. , 19, 46 (1938).

[17.] Burke, Francis M., “Leachability of lead from commercial glazes”, Ceram. Eng. Sci. Proc., 6[11-12]1394 (1985).

[18.] Carr, Dodd S.; Cole, Jerome F.; McLaren, Malcolm G, “Ceramic foodware safety: III, Mechanisms of release of lead and cadmium”, Ceramica (Sao Paulo), 28[N 148]151-5 (1982).

[19.] Cordt, F. W. “Production of Red Glazes”, Ceram. Ind., 21 (4) 172-75 (1933).

[20.] Cos, P. E. “Review of Glaze Making Aids”, Ceram. Age, 51 (3) 127-28 (1948).

[21.] Currier, A. E. “Standard Method for Determining Leachability of Lead from Lead Frits”, J. Am. Ceram. Soc., 30 (11) 335-38 (1947). Sponsored by the United States Potters Association.

[22.] E.J. Calabrese and E.M. Kenyon. Air Toxics and Risk Assessment. Lewis Publishers, Chelsea, MI. 1991.


185

[23.] Eggert, F. “The Influence of Admixtures on the Red Color of Uranium Glazes”, Email-Keramo-Techn., 2, 74 (1951).

[24.] Eisenlohr, H. “Acid-Resistant Colors: Testing”, Sprechsaal, 59 (39) 645 (1926).

[25.] Eska, H. “Weather-Proof Red Uranium Glazes”, Sprechsaal, 66 (4) 59-61; (5) 77-78; (6) 93-96; (7) 109-10; (8) 127-29 (1933).

[26.] Fajans K. and N. J. Kreidl, “Stability of Lead Glasses and Polarization of Ions”, J. Am. Ceram. Soc., 31 (4) 105-114 (1948).

[27.] Franklin C. E. L. and J. A. Tindall, “Attack of On-Glaze Colors by Acid, Alkali and Washing Agent”, Trans. Brit. Ceram. Soc. , 58, 589 (1959).

[28.] Franklin, C. E. L., J. A. Tindall and A. Dinsdale, “The Influence of Glaze Composition on the Durability of On-Glaze Colors”, Trans. Brit. Ceram. Soc. , 59, 401-23 (1960).

[29.] Frey, Emmo; Scholze, Horst, “Lead and cadmium release from fused colors, glazes, and enamels in contact with acetic acid and food under the influence of light”, Ber. Dtsch. Keram. Ges., 56 (10): 293-7 (1979).

[30.] Funk W. and H. Mields, “Resistance of Enamel Colors to Dilute Acids and a Method of Determining the Lead Solubility”, Ber. dent. keram. Ges. , 12, 535 (1931).

[31.] Funk W. and M. Mields, “Resistance of Colored Enamels to Dilute Acids and a Process for the Determination of Their Lead Solubility”, Ber. Deut. keram. Ges., 12 (11) 534-48 (1931).

[32.] Geller R. F. and A. S. Creamer, “Solubility of Colored Glazes in Organic Acids”, J. Am. Ceram. Soc., 22 (4) 133-140 (1939); See also Research Paper RP 1196, J. Res. Nat. Bur. Stand., 22, 441-52 (1939), and Ceram. Ind., 32 (3) 40 (1939).

[33.] Grindley, W. H. “Health and Working Conditions in the English Potteries”, Times Eng. Supp., 31, 416 (1933).

[34.] Haffner, H. “Austrian Regulation Governing the Solubility of Lead from On-Glaze Decorated Porcelain”, Ber. Deut. keram. Ges. , 39 (9) 460-61 (1962).

[35.] Haffner, H. “The Applicability of the German Lead and Zinc Law of June 25, 1887 (RGBI 273) to Decorated Porcelain, Earthenware,


186

Glass etc. 11, Ber. Deut. keram. Ges., 31, 259 (1954); See also ibid., 30, 7 (1953).

[36.] Haffner, H. “Variations in the Lead Release of Overglaze Colors”, Ber. Deut. keram. Ges., 38 (11) 515-18 (1961).

[37.] Hannon, GE., Hatton, KS., McCauley, RA., “Effects Of Processing Parameters On Lead Release And Color Of Copper-Containing Lead Glazes”, Am. Ceram. Soc. Bull., volume 63, pp. 1012 – 1013 (1984).

[38.] Harhort, H. Sprechsaal, 41, 621; 42, 637 (1931).

[39.] Harkort, H. “Lead Glazes and Methods of Testing”, Keram. Rundschau, 46 (41) 481-84 (1938).

[40.] Harkort, H. “Producing Lead Glazes Non-Injurious to Health of Workers”, Sprechsaal, 67 (41) 621-23; (42) 637-39 (1934).

[41.] Harkort, H. “The Manufacture of Lead Glazes by New Methods”, Trans. 3rd. Int. Ceram. Congr., p. 301 (1952).

[42.] Harrison, H. C., W. G. Laurence and D. J. Tucker, “An Investigation of the Volatility of Glaze Constituents by the Use of the Spectrograph”, J. Am. Ceram. Soc., 23 (4) 111-16 (1940).

[43.] Harrort, H., Untersuchungen uber die Herstellung bleifester, gesundheitsunschadlicher Bleiglasuren. Sprechsaal 67 621, 1934. Bleifeste Frittglasuren. Ker. Rund. 47 21, 42, 52, 1939.

[44.] Hayhurst, E. R. Industrial Health, Hazards, and Occupational Diseases in Ohio, State of Ohio Board of Health (1915).

[45.] Hibbert, R., Bai, ZP., Navia, J., Kammen, DM., Zhang, JF., “High lead exposures resulting from pottery production in a village in Michoacan State, Mexico”, J. Expo. Anal. Environ. Epidemiol., volume 9, pp. 343-351 (1999),

[46.] Hirayama, C. “Volatility of Lead Silicate and Lead Borate Binary Melts”, J. Am. Ceram. Soc., 43 (10) 505-09 (1960).

[47.] Kautz, K. “The Effect of Glaze Composition Upon the Colors Produced by Sodium Uranate”, J. Am. Ceram. Soc., 17 (1) 8-10 (1934).


187

[48.] Kerstan, W. “The Influence of Acid Resistance and Lead Release from On-Glaze Colors on Earthenware Glazes”, Sprechsaal, 87, 201 (1954).

[49.] Kettering Laboratory in the Department of Preventive Medicine and Industrial Health, College of Medicine, University of Cincinnati, Cincinnati, Ohio, Dr. Robert A. Kehoe, Director. Report dated July 12, 1961, on “The Extraction of Lead from Glazed Ceramic Surfaces”; Report dated January 31, 1962, on “Further Investigation of the Extraction of Lead from Glazed Ceramic Surfaces”; Report dated October 15, 1962; Report dated March 7, 1963; Report dated May 22, 1963.

[50.] Klug, J. “Red as a Glaze and Decoration Color”, Keram. Rundschau, 41 (33) 433-34 (1933).

[51.] Koch, W. J., C. G. Harmon and L. S. O’Bannon, “Some Physical and Chemical Properties of Experimental Glazes for Vitrified Institutional White Ware”, J. Am. Ceram. Soc., 33 (1) 1-8 (1950).

[52.] Koenig C. J. and A. S. Watts, “The Durability of Tableware Decorations”, J. Am. Ceram. Soc., 17 (10) 259-62 (1934).

[53.] Koenig, J. H. “Lead Frits and Fritted Glazes”, Ohio State University Studies Engineering Series, Vol. VI, No. 2, Eng. Exp. Sta. Bull. No. 95, July 1937; See also Ceram. Ind., 26 (2) 134-36, 27 (2) 108-112 (1936). Reports on investigation sponsored by The United States Potters Association.

[54.] Koenig, J., Lead Frits and Fritted Glazes. Cer. Ind. 26 1 34, 1936; 27 108, 1936. Ohio State University Studies, Engineering Series. Vol. VI, No. 2, July 1937. Engineering Experiment Station Bull. 95, 1937.

[55.] Koerner J., Sprechsaal, 39, 2, 41, 81, 125 (1906).

[56.] Koerner, J., Bleihaltige im Sinne des gesetzes ungiftige Glasuren. Sprechsaal 39 2, 41, 81, 125, 1906.

[57.] Kohl, H. “Cause of Fading of Decorations on Porcelain Ware During Cleaning”, Sprechsaal, 64 (16) 295-96 (1931).

[58.] Kohl, H. “Durability of Ordinary Porcelain Colors in Washing Machines and in the Laboratory”, Sprechsaal 64 (48) 887-90; (49) 907-08 (1931).


188

[59.] Krajewski, A., Ravaglioli, A., Carriveau, GW., “Tin lead ratio of late middle-age majolica glazes of some important Italian sites”, J. Mater. Sci. Lett., volume 11, pp 848 – 851 (1992).

[60.] Larcheveque, M. “Use of Lead Compounds in the Ceramic Industry”, Ceramique, 36, 221 (1933).

[61.] Leadless Glazes, Trans. Am. Ceram. Soc., 1, 101 (1899).

[62.] Lehmann, H. and M. T. Schulze, “Lead Poisoning in the Ceramic industry, Part 111, Ver. Deut. keram. Ges. , 19, 38 (1938).

[63.] Leiser, C. F. “Importance of Lead in Glass”, Glass Industry, 44 (9) 509-14; (10) 574-76, 594; (11) 630-32 (1963).

[64.] Lenk, S. “Attempts to Reduce the lead Solubility of Ceramic Glazes and Frits”, Keram. A., 8 (2) 57-59 (1956); See also Intercerarn, No. 4, 20-21 (1955).

[65.] Lorah, J. L. “Uranium Oxide Colors and Crystals in Low Temperature Glaze Combinations”, J. Am. Ceram. Soc., 10 (10) 813-20 (1927).

[66.] Malins, JP., Tonge, KH., “Reduction processes in the formation of lustre glazed ceramics”, Thermochim. Acta, volume 341, pp. 395 – 405 (1999).

[67.] McCauley, Ronald A., “Release of lead and cadmium from ceramic foodware decorations”, Glass Technol., 23[N 2]101-5 (1982).

[68.] Mellor, J. W. “The Behavior of Some Glazes in the Glost Kiln”, Trans. Ceram. Soc., (England) 12, 1 (1912-12).

[69.] Mellor, J. W. “The Durability of Pottery Frits, Glazes, Glasses and Enamels in Service”, Trans. Brit. Ceram. Soc., 34, 113-190 (1934).

[70.] Molera, J., Pradell, T., Salvado, N., Vendrell-Saz, M., “Evidence of tin oxide recrystallization in opacified lead glazes”, J. Am. Ceram. Soc., volume 82, pp 2871 – 2875 (1999).

[71.] Molera, J., Pradell, T., Salvado, N., Vendrell-Saz, M., “Interactions between clay bodies and lead glazes”, J. Am. Ceram. Soc., volume 84 (2001).


189

[72.] Monier-Williams, G. W. “Solubility of Lead Glazes”, Ceram. Ind., 5 (5) 440-44 (1925).

[73.] Monier-Williams, G. W. Rept. on Public Health and Medical Subjects, No. 29 Ministry of Health, London (1925); See also Chem. Age, 12, 155 (1925); Chem. and Ind., 44. 358 (1925); 45, 952 (1926, and 46, 174 (1927); J. Soc. Chem. Ind., 43, 771-73 (1924).

[74.] Moore, H. “The Structure of Glazes”, Trans. Brit. Ceram. Soc., 55, 589-600 (1956).

[75.] Newman, B. J., W. J. McConnell, O. M. Spencer, F. M. Phillips, “Lead Poisoning in the Pottery Trades”, U. S. Public Health Service, Bull. No. 116 (1921)

[76.] Nordyke, J. S. “Unsing Lead in White Wares and Other Ceramics”, Ceram. Ind., 77 (5) 70 (1961).

[77.] Norris A. W. et al., “The Solubility of Lead Glazes”, Part I “Physical Aspects of Solubility Determination”, Part 11, “A Modified Test Method”, Part III, “A Standard Specification for Lead Bisilicate”, Trans. Brit. Ceram. Soc., 50, 225-56 (1951). (See also Factory Orders, 1944 Ed., p. 124, London: H.M.S.O. (1944), and “Regulations for the Manufacture and Decoration of Pottery No. 2 (1913). Made under Section 79 of the Factory and Workshop Act (1901). Incorporated in the Factories Act (1937).

[78.] Norris, A. W. “Changes in a Glass When Used as a Glaze”, Trans. Brit. Ceram. Soc., 55, 674-88 (1956).

[79.] Orlowski, H. J. and J. Marquis, “Lead Replacements in Dinnerware Glazes”, Ohio State University Studies, Engineering Series, Eng. Exp. Sta. MI. No. 125, 1946; See also J. Am. Ceram. Soc. , 28 (12) 343-57 (1945). Investigation sponsored by The United States Potters Association; See also J. Am. Ceram. Soc., 28 (12) 343-57 (1945).

[80.] Parmelee, C. W. and E. C. Lyon, “A Study of Some Frit Compositions”, J. Am. Ceram. Soc., 13, 60-66 (1934).

[81.] Pearson, B. M. “Ceramic Glazes and Enamels”, Ceram. Age, 10 (6) 214 (1927).

[82.] Petrick, L. “The Lead Question in Ceramic Industries” Sprechsaal, 53, 405-08 (1920).

[83.] Podmore, H. L. “Lead Glazes Without Danger”, Pott. Gaz. Glass Tr. Rev., 73, 130 (1948).


190

[84.] Podmore, H. L. “The Specific Exposed Surface and the Lead Solubility of a Frit”, Trans. Brit. Ceram. Soc., 50, 262-68 (1951); See also Ceramics, 1, 243 (1949), and Pott. Gaz. Glass Tr. Rev., 73, 130 (1948).

[85.] Pottery Glazes-Regulations”, Chem. Tr. J., 121, 146 (1947); See also Chem. Age, London, 57,64 (1947).

[86.] Proceedings, International Conference on Ceramic Foodware Safety, pp. 8-17, 1975, Lead Industries Association Inc., 292 Madison Avenue, New York, NY 10017, USA

[87.] Purdy, R. C. and H. B. Fox, “Fritted Glazes”, Trans, Am. Ceram. Soc. , IX, 97 (1907).

[88.] Reumann, O. “Testing the Properties of Ceramic Glazes and On-Glaze Colors”, Keram. Zeitung, 15 (5) 268-72; (6) 334-36 (1963).

[89.] Rieke R. and L. Mauve, “Acid Resistance of On- Glaze Colors”, Ber. dent. keram. Ges. , 11 (9) 487 (1930).

[90.] Rieke, R. and H. Mields, “Acid Resistance of Lead Frits in Relation to the Composition”, Ber. deut. keram. Ges., 16 (7) 331-49 (1935).

[91.] Rix, W. P. “Lead Frits and Their Adaption to Pottery Glazes”, Trans. Am. Ceram. Soc. , IV, 201-207 (1902).

[92.] Rothman, S. C. “Installation for Control of Hygienic Exposure Arising from the Application of Pottery Glazes”, Ind. Med., 8 (1) 8-12 (1939).

[93.] Schweisheimer, W. “Lead Exposure in the Pottery Industry”, Ceramics, 6, 41 (1954).

[94.] Sharatt E. and M. Francis, “Durability of On-Glaze Decoration, Part H”, Trans. Brit. Ceram. Soc., 42 (9) 171-82 (1943).

[95.] Singer, F. “Low Solubility Glazes”, London: Borax Consolidated Ltd., p. 99 (1948).

[96.] Smith, A. N. “Lead Bisilicate Frits”, Part 11, “Theoretical Relationship Between the Solubility of Lead Bisilicate Frits and Their Specific Surface”, Trans. Brit. Ceram. Soc., 50, 257-61 (1951).

[97.] Smith, A. N. “Structure of Lead Bisilicate Frits”, Trans. Brit. Cer. Soc., 48 (2) 69-84 (1949).


191

[98.] Smith, A. N. “The Effect of Ammonium Chloride on Some Glazes” Trans. Brit. Ceram. Soc., 48, 85-97 (1949).

[99.] Stefanov, S., “Applications of borate compounds for the preparation of ceramic glazes”, Glass Technol., Volume 41, pp. 193-196 (2000).

[100.] Steger, W. “Comparative investigation of Earthenware Glazes”, Ber. Deut, keram. Ges. 17, 265 (1936).

[101.] Stewart, J. A. “Classification of Inorganic Enamels”, The Glass Industry, 46 (11) 654-57 (1965).

[102.] Stopford, W., “Lead-Exposure From Application Of Hobby Glazes”, Abstr. Pap. Am. Chem. Soc., volume 196, p. 15 (1988).

[103.] Tailey, R. B. ”Acid-Resistant Overglaze Colors”, J. Am. Ceram. Soc., 11 (5) 307 (1928).

[104.] Thomason, W. “Attempt to Ascertain Toxic Properties of Fritted lead Glazes”, Trans. Ceram. Soc. , (England), 9, 198-209 (1909-10).

[105.] Thorpe, T. E. The Use of Lead in the Manufacture of Pottery, 1899 Brit. Govt. Paper 8383-150093/1901 wt. 32982, DaS-4.

[106.] Thorpe, T. E. “The Use of Lead in the Manufacture of Pottery”, 1899, Government Paper 8383-150093/1901; See also “Report on Work of the Government Laboratory on the Question of Employment of Lead Compounds in Pottery”, 1901; Government Paper 9264-1500-61901; Report on “Lead in the Potteries”, London, 1910.

[107.] Tindall J. A. and C. E. L. Franklin, “Glass Structure and Color Durability”, The A. T. Green Book, pp. 82-93 (1959), Published by the British Ceramic Research Association.

[108.] Tite, MS., Freestone, I., Mason, R., Molera, J., Vendrell-Saz, M., Wood, N., “Lead glazes in Antiquity - Methods of production and reasons for use”, Archaeometry, volume 40, pp 241 – 260 (1998).

[109.] Tunstall, SC, Klein, R, DeCou, S, Amarasiriwardena, D, “Characterization of lead glazes and lead leaching properties of glazed ceramics from the Solis Valley, Mexico using inductively coupled plasma-mass spectrometry and diffuse


192

reflectance Fourier transform IR spectroscopy: Are we what we eat out of?”, Abstr. Pap. Am. Chem. Soc., volume 221, (2001).

[110.] Turbett, F. L. and H. B. Stephenson, “Raw Materials for Ceramic Glazes”, U. S. Patent 2,438,524, March 30, 1948.

[111.] U.S. Department of Health and Human Services. Hazardous Substances Data Bank (HSDB, online database). National Toxicology Information Program, National Library of Medicine, Bethesda, MD. 1993.

[112.] U.S. Department of Health and Human Services. Registry of Toxic Effects of Chemical Substances (RTECS, online database). National Toxicology Information Program, National Library of Medicine, Bethesda, MD. 1993.

[113.] U.S. Environmental Protection Agency. Deposition of Air Pollutants to the Great Waters. First Report to Congress. EPA-453/R-93-055. Office of Air Quality Planning and Standards, Research Triangle Park, NC. 1994.

[114.] U.S. Environmental Protection Agency. Integrated Risk Information System (IRIS) on Lead. Environmental Criteria and Assessment Office, Office of Health and Environmental Assessment, Office of Research and Development, Cincinnati, OH. 1993.

[115.] U.S. Environmental Protection Agency. Supplement to Criteria Document on Lead. 1990.

[116.] U.S. Environmental Protection Agency. Technical Background Document to Support Rulemaking Pursuant to the Clean Air Act--Section 112(g). Ranking of Pollutants with Respect to Hazard to Human Health. EPA-450/3-92-010. Emissions Standards Division, Office of Air Quality Planning and Standards, Research Triangle Park, NC. 1994.

[117.] Uranium Red Glazes, Keram. Rundschau, 46, 221 (1938).

[118.] Vargin, V. V. and L. L. Fradhova, “Lead Glazes for Faience”, Ceramic and Glass, 7 (5) 29-34 (1934).

[119.] Vargin, V. V. and L. L. Fradkova, Ceramics & Glass, 7, 29 (1931).

[120.] Vogt, A., Selter, D., Seebach, G., “Chemistry of art: Ceramic glazes with chromium and lead”, Abstr. Pap. Am. Chem. Soc, volume 221 (2001).


193

[121.] Weyl, W. and H. Rudow, “New Analytical Method for Determining the Lead Solubility of Earthenware Glazes Using Dithizone”, Ber. deut. keram. Ges., 16, 281 (1935).

[122.] WHO Food Additives Series No. 4, 1972

[123.] WHO Technical Report Series No. 505, 1972

[124.] WHO/Food Additives 77.44, Ceramic Foodware Safety, Sampling, Analysis and Limits for release (Report of a WHO Meeting, Geneva 8-10 June 1976).

[125.] WHO/Food Additives HCS/79.7. Ceramic Foodware Safety, Critical Review of Sampling, Analysis, and Limits for Lead and Cadmium Release (Report of a WHO Meeting, Geneva 12-14 November 1979).

[126.] Yillias, E. “Influence of Alkaline Cleaners on the Resistance of Underglaze Colors” Ber. deut. keram. Ges. , 11 (11) 602-06 (1931); See also R. Rieke, ibid., 11 (11) 607 (1931).

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