Module - 10 Glass

7/25/2019 Module - 10 Glass

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The term glass refers to materials, usually blends of metallicoxides, predominantly silica, which do not crystallise whencooled from the liquid to the solid state.

It is the non-crystalline or amorphous structure of glass thatgives rise to its transparency.

Glass made from sand, lime and soda ash has been known inEgypt for 5000 years, although it probably originated inAssyria and Phoenicia.

The earliest manmade glass was used to glaze stone beads,

later to make glass beads (circa 2500 BC), but it was not untilabout 1500 BC that it was used to make hollow vessels.


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For many centuries glass was worked by drawing the moltenmaterial from a furnace.

The glass was then rolled out or pressed into appropriatemoulds and finally fashioned by cutting and grinding.

Around 300 BC the technique of glass blowing evolved inAssyria, and the Romans developed this further by blowingglass into moulds.

The various colours within glass derived from the addition ofmetallic compounds to the melt.

Blue was obtained by the addition of cobalt, whilst copperproduced blue or red and iron or chromium produced green.


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In the fifteenth century white opaque glass was produced bythe addition of tin or arsenic, and by the seventeenth centuryruby red glass was made by the addition of gold chloride.

Clear glass could only be obtained by using antimony ormanganese as a decolouriser to remove the green colourationcaused by iron impurities within the sand.

Thousands of tonnes of glass are recycled each year, but thisis mainly domestic waste, which cannot be used for theproduction of window glass as this requires pure materials.

Even architectural waste glass is of variable composition withcontamination from wire, sealants and special glasses,making it not usable as cullet in the manufacturing processwithout careful sorting.


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However, clean cullet from glass processing companies isrecycled, and new float glass may incorporate up to 30% ofrecycled material, significantly reducing overall energy andCO2emissions.

Re-melting clean cullet uses 25% less energy than making theequivalent quantity of new glass from raw materials.

In addition, there is saving on extraction of raw materials.

Excess recycled glass, not required for remaking bottles, hasbeen used for making decorative paving surfaces and can be

incorporated into the manufacture of bricks.


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Recent experimentation with pulverised domestic glass wastehas shown that when expanded by CO2from added calciumor magnesium carbonate or carbon black, the resultingmaterial can be used as a lightweight aggregate in concrete.

Expansion of 1520 times gives a granular product with adensity range 200900 kg/m3.


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It is the most prevalent type of glass, used for windowpanes,and glass containers (bottles and jars) for beverages, food,and some commodity items. Glass bakeware is often made oftempered soda-lime silicate glass.

Modern glass is manufactured from sand (silica), soda ash(sodium carbonate) and limestone (calcium carbonate), withsmall additions of salt-cake (calcium sulphate) and dolomite(magnesian limestone).

This gives a final composition of typically 7074 % silica, 12

16 % sodium oxide, 512 % calcium oxide, 25 % magnesiumoxide with small quantities of aluminium, iron and potassiumoxides.


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The addition of 25 % broken glass or cullet to the furnace mixaccelerates the melting process and recycles the productionwaste.

The production process is relatively energy intensive at15,000 kWh/m3(c.f. concrete: 625kWh/m3), but theenvironmental pay back arises from its appropriate use inenergy conscious design.


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The composition of alkaline earth silicate glass is typically5570% silica, 514% potassium oxide, 312% calcium oxide,015% aluminium oxide with quantities of zirconium,strontium and barium oxides.

Borosilicate glass is typically 7087% silica, 08% sodiumoxide, 08% potassium oxide, 715% boron oxide, 08%aluminium oxide with small quantities of other oxides.

A particular characteristic of borosilicate glass is that it has acoefficient of expansion one third that of standard soda lime

silicate glass, making it significantly more resistant tothermal shock in case of fire.


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Ceramic glass is characterised by a near zero coefficient ofexpansion, making it highly resistant to thermal shock.

The composition of ceramic glass is complex, comprisingtypically 5080% silica, 1527% alumina, with small quantitiesof sodium oxide, potassium oxide, barium oxide, calciumoxide, magnesium oxide, titanium oxide, zirconium oxide,zinc oxide, lithium oxide and other minor constituents.

The glass is initially produced by a standard float or rollingtechnique but subsequent heat treatment converts part of the

normal glassy phase into a fine-grained crystalline form,giving rise to the particular physical properties.

Ceramic glass has a high softening point compared to otherglass products used in construction.


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A furnace produces a continuous supply of molten glass atapproximately 1100oC, flowing across the surface of a largeshallow bath of molten tin contained within an atmosphere ofhydrogen and nitrogen, which prevents oxidation of thesurface of the molten metal.

The glass moves across the molten metal, initially at asufficiently high temperature for the irregularities on bothsurfaces to become evened out leaving a flat and parallelsheet of glass.

The temperature of the glass is gradually reduced as it movesforward until at the end of the molten tin, it is sufficientlysolid at 600oC not to be distorted when supported on rollers.


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Thickness is controlled by the speed at which the glass isdrawn from the bath.

Any residual stresses are removed as the glass passesthrough the 200 m annealing lehr or furnace, leaving a fire-polished material.

The glass is washed and substandard material discarded forrecycling.

The computer-controlled cutting is followed by stacking,warehousing and dispatch.

A typical float glass plant will manufacture 5000 tonnes ofglass per week.


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Float glass for the construction industry is made within thethickness range 225 mm.

Many surface modified glasses are produced, byincorporating metal ions into the glass within the floatprocess, or under vacuum or by spraying the surface with

metal oxides or silicon.

Body-tinted glass, which is of uniform colour, ismanufactured by blending additional metal oxides into thestandard melt.

The use of all-electric melting processes offers higher qualitycontrol and less environmental pollution than previouslyproduced from earlier oil- or gas-fired furnaces.


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Glass may be cast and pressed into shape for glass blocksand extruded sections.

Profile trough sections in clear or coloured 6 or 7mm castglass are manufactured in sizes ranging from 232 to 498mmwide, 41 and 60mm deep and up to 7 m long, with or without

stainless steel longitudinal wires.

The system can be used horizontally or vertically, as single ordouble glazing, and as a roofing system spanning up to 3 m.

Blue tinted versions are available for solar control or

aesthetic reasons.

The double-glazed system produces a sound reduction andheat insulation


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Glass may be cast and pressed into shape for glass blocksand extruded sections.

Profile trough sections in clear or coloured 6 or 7mm castglass are manufactured in sizes ranging from 232 to 498mmwide, 41 and 60mm deep and up to 7 m long, with or without

stainless steel longitudinal wires.

The system can be used horizontally or vertically, as single ordouble glazing, and as a roofing system spanning up to 3 m.

Blue tinted versions are available for solar control or

aesthetic reasons.

The double-glazed system produces a sound reduction andheat insulation


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Glass blocks for non-load-bearing walls and partitions aremanufactured by casting two half-blocks at 1050oC, joiningthem together at 800oC, followed by annealing at 560oC.

The standard blocks are 115, 190, 240 and 300mm squarewith thicknesses of 80 and 100 mm, although rectangular

and circular blocks are also available. Walls may be curved.

The variety of patterns, offering differing degrees of privacy.

For exterior and fire-retarding applications natural orcoloured mortar (2 parts Portland cement, 1 part lime, 8 parts

sand) is used for the jointing.

Walls may be straight or curved; in the latter case, theminimum radius varies according to the block size andmanufacturers specification.


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Glass blocks for non-load-bearing walls and partitions aremanufactured by casting two half-blocks at 1050oC, joiningthem together at 800oC, followed by annealing at 560oC.

The standard blocks are 115, 190, 240 and 300mm squarewith thicknesses of 80 and 100 mm, although rectangular

and circular blocks are also available. Walls may be curved.

The variety of patterns, offering differing degrees of privacy.

For exterior and fire-retarding applications natural orcoloured mortar (2 parts Portland cement, 1 part lime, 8 parts

sand) is used for the jointing.

Walls may be straight or curved; in the latter case, theminimum radius varies according to the block size andmanufacturers specification.


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Vandal- and bullet-proof blocks are available for situationsrequiring higher security.

For interior use, blocks may be laid with sealant rather thanmortar. Glass blocks are now available in preformed panelsfor speedier installation.


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Vandal- and bullet-proof blocks are available for situationsrequiring higher security.

For interior use, blocks may be laid with sealant rather thanmortar. Glass blocks are now available in preformed panelsfor speedier installation.


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Glass pavers

Cellular or foamed glass

Self-cleaning glass

Decorative coloured glass

Georgian wired glass

Toughened glass

Heat-strengthened glass

Laminated glass

Fire-resistant glass

Energy conservation through double or triple glazing

Module - 10 Glass

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