Ceramic Defects Table

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TABLE OF POTTERY AND WHITEWARES DEFECTS Extracted from a Number of References and my Old Gray Head: See Reference List Below copyright©2007 John Taylor Jones, Ph.D If you do not find the defect you are looking for in this table see the Table Index in The Ceramic and Pottery Defect Series 1-10 (CPDS 1-10) DEFECT PROCESS CAUSES AND CORRECTIONS black coring bisk firing Insufficient oxidation of carbon compounds during firing. blackness maiolica colors Colors rely on glaze to supply flux to color. No flux results in blackening. Add borax frit softened glaze to color. Reducing atmosphere is a problem with some colors. Application is a factor too. Reference 3 REFERENCES black spots decorating Bone china and porcelain. Usually on foot of ware. Ware not completely vitrified. Due to carbon and lack of oxidation in kiln. See reference 3. REFERENCES bittiness glazing Thin glaze reveals body surface and feels rough. Calcium has been dissolved from frit and reacts with carbonate in processing water forming small spheres. This occurs if glaze is stored in a too warm environment. Prepared glazes from supplier can be kept dry until needed. Less soluble frits can be used. Acid can be added but that flocculates the glaze slurry. blistering bisk ware Over firing, internal gases are trapped in a viscous body. Flame impingement can cause local blistering. blistering decorating Rare, pinholes from impurities as far back as the raw materials are more common.

description

ceramic glaze defects and remedies

Transcript of Ceramic Defects Table

Page 1: Ceramic Defects Table

TABLE OF POTTERY AND WHITEWARES DEFECTSExtracted from a Number of References and my Old Gray Head: See Reference List Below

copyright©2007 John Taylor Jones, Ph.D

If you do not find the defect you are looking for in this table see the

Table Index in The Ceramic and Pottery Defect Series 1-10(CPDS 1-10)

DEFECT PROCESS CAUSES AND CORRECTIONS

black coring bisk firing Insufficient oxidation of carbon compoundsduring firing.

blackness maiolica colors Colors rely on glaze to supply flux to color.No flux results in blackening. Add borax fritsoftened glaze to color. Reducing atmosphereis a problem with some colors. Application isa factor too. Reference 3 REFERENCES

black spots decorating Bone china and porcelain. Usually on foot ofware. Ware not completely vitrified. Due tocarbon and lack of oxidation in kiln. Seereference 3. REFERENCES

bittiness glazing Thin glaze reveals body surface and feelsrough. Calcium has been dissolved from fritand reacts with carbonate in processing waterforming small spheres. This occurs if glaze isstored in a too warm environment. Preparedglazes from supplier can be kept dry untilneeded. Less soluble frits can be used. Acidcan be added but that flocculates the glazeslurry.

blistering bisk ware Over firing, internal gases are trapped in aviscous body. Flame impingement can causelocal blistering.

blistering decorating Rare, pinholes from impurities as far back asthe raw materials are more common.

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blistering (orboiling)

glazing Overfiring of the glaze resulting in boiling.Flame impingement can cause blistering.Slowing down the heat up cycle, extendingthe maturation time, and slowing cooling canoften correct this problem. It takes time tomature the glaze. Reducing the moltenviscosity of the glaze may help. Manganesecompounds decompose releasing oxygen thatcan exasperate this situation. If whitingdecomposition is a problem use a fritcontaining CaO rather than whiting.

blistering green ware Does your slip stink? Maybe something isdecomposing. Use formaldehyde or otherantibacterial agent to kill the bugs. Smallblisters can be caused by overfiring. Seebloating.

bloating green ware Does your slip stink? Maybe something isdecomposing. Use formaldehyde or otherantibacterial agent to kill the bugs.

bloating bisk & glostware

Overfiring is the prime suspect . If thepreheat was not long enough or high enoughin temperature to allow decomposition ofcarbonates, etc., before vitrification sets in,you can have bloating. Fast firing may notallow gases to escape from the interior ofparts, especially thick sections. Black Coringoccurs if the kiln atmosphere is notsufficiently oxidizing. Reduction can alsoincrease vitrification due to the formation ofFeO which can seal off areas before thecarbon containing materials are removed.Bloating sometimes occurs in refired ware.That can be like overfiring so refireschedules need to be controlled.

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blow-out bisk See lime popping. Or perhaps bird poopingif that is what is occurring in the rafters ofyour factory. Keep slip and slop containerscovered. Lime, plaster, concrete, cement dustcan present problems. Lime particles canhydrate after firing and swell. Pop! If thedefect doesn’t show up in glazing it isbecause of the good action of the glaze indissolving the particle or by keeping it fromhydration. Increase bisk firing temperature.Check the quality of grog. Check your scrapfor plaster mold bits. Iron pyrites can causeproblems.

blurreddecorations

underglaze On vertical surfaces with Co and Ni colors.Solvent action of covering glaze. Keep glazecoat thin. Lower firing temperature. Add10% china clay (alumina) to glaze. Shortenmaturation time.

casting, brittle cast ware Sodium silicate may be the culprit. Use someDispex® from Allied Chemical.

casting,flabby

cast ware Soda ash may be the culprit. Balance withsodium silicate and I suggest that you usesome Dispex® from Allied Chemical.

casting, moldreleaseproblems

cast ware Excess deflocculant. Molds can be dustedwith talc but the government frowns on this. Isuggest rubbing fuel oil or kerosene on themold but don’t over do it. A damp cloth isgood.

casting, run-off problems

cast ware Make sure the slip specific gravity is high.Carefully increase the amount of dispersant.

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casting, slow cast ware Finer dispersions build up a barrier layer nextto the mold which then slows down casting.Over dispersion of the slip hastens thisprocess. Particle size distribution and state offlocculation determined by thixotropymeasurements by a torsion viscometer areimportant. Low slip specific gravitydecreases the casting rate. Wet molds slowcasting. For a detailed analysis of thisproblem with appropriate slip controltechniques see References 1 & 3.REFERENCES

casting,wreathing

cast ware Surface skin effect causing fine lines on theinside of ware during pouring. Tapping themolds worked for us. Use some soda ashwith sodium silicate. I like to add Dispex®.Keep molds in good condition or replace.

black spots bone chinadecorating

blow-out (seealso limepopping)

bisk, glost,decorating

Lime particles popping out after they hydrateand swell. Colemanite in glazes has a similareffect causing colemanite splutter. Maycause enamels to roughen.

chittered edge plastic forming See also Cracking, edge. Sometimes calledfish cracks. Fettling tools on plate rollersfinishing machines and other plastic formingfinishing machines are not properly adjustedor not in proper condition. The ware isalmost completely dry which exasperates thesituation.

color, frizzled decorating Roughing or bubbling of overglazedecorations. Too rapid firing. Organicburnout. Reduce medium in color. Lowerbinder in medium. Fire more slowly. Increasekiln ventilation.

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colorvariation

glazing Can be caused by glaze thickness or kilnconditions or other conditions. SeeReference 3 or a detailed description.REFERENCES

colorvariation

overglaze Underfiring or overfiring is generally theproblem. Combustion products can blackenor change some colors. See reference 3. REFERENCES

colorvariation

underglaze Inconsistent color, glaze, application, firing.Sulphur from oil firing. See reference 3. REFERENCES

cracking bisk See dunting. Cracks can be from handling orthermal shock.

cracking cast ware Related to slip quality, mold sticking,meshing of slips during pouring in complexmolds, and particle orientation. Overdeflocculated slips can be a problem. So canscrap utilization.

cracking drying Case hardening occurs in clay bodies ondrying. This is a impervious surface layerthat inhibits the removal of water from theinterior of the body. In the first stage ofdrying, rapid water removal is allowed untilthe body particles touch. After that, waterremoval is diffusion controlled and thetemperature and humidity of the dryer mustbe controlled until the piece is nearcompletely dry. That means slower humiditycontrolled drying in the second stage. Thelast water in the piece is tightly held and maynot be released until firing.

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cracking general Due to stress buildup in the formed part. Maybe induced during extrusion, forming, andhandling operations. Clay has a memory andtries to return to some original state. Unevendrying causes clay cracking (see above). Thick sections dry slower than thin sectionsso slow drying is required so part design isimportant. Cracks can be found by paintinghot paraffin on the surface. Sometimes thoseareas can be repaired by pegging. Edgesshould be wet sponged to remove cracks aftercutting, fettling, and trimming to preventopening of cracks during drying or firing.

cracking edge Short clay. Too rapid or uneven drying. Toothin edges which can be corrected by design.Engobes can strain ware. Absorption of waterby dry ware on standing which expands thepart in certain areas producing strain whichoften show up after bisk firing as fine cracks.Improper trimming or fettling, especially ontoo dry ware or by using dull tools. Wetsponging can help. See chittered edge.

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cracking handle Stresses build during drying due todifferences in shrinkage of the handle and thepiece. Placing cast handles on plastic formedware, if the handles have a higher moisturecontent than the piece, works because theshrinkage is the same over the cup handleplacement distance. That is, a cast handlehaving 17% moisture on a cup having 15%moisture can result in the same shrinkageover the cup handle placing distance. Thatprevent strain. Putting a plastic formedhandle on a plastic part does not always workbecause the required moisture in the handle ishigher than the cup and the shrinkage of thehandle is too high. Cast bodies usually shrinkless than plastic bodies which has been asource of “good luck” for potters over thecenturies. (I made a graph of this when Iworked for Pfaltzgraff Company but I nolonger have the data we obtained.) Handlesalso crack or pop off if the operator isunskilled. Also, the sticking slip, if usedrather than just water, can be the problem.Too thick slip is not good. Also, if the cuphandles are not trimmed properly resulting inimproper fit, the handle will come off. Agood fit and proper bonding are easilydetermined by impact testing. A good fit isobserved if when the handle breaks duringtesting, the break is in the piece or in thehandle but not at the join. Handles formed insitu on cast ware are usually reliable buthandling can weaken the handle when it isremoved from the mold. Finishing operationscan also disrupt the handle.

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cracking spiral Over extension of clay during formingoperations. May show up on drying or firing.Can be confused with dunting. The fracturesurface is smoother and more shiny withdunting.

cracking surface Improper moisture distribution. Water orslurry left standing in the base of thrownware causes S-cracks. Remove all excesswater by sponge. Insufficient or nonuniformpressure during plastic forming can cause theclay to stay randomly mixed rather than in aclose-packed playing card structure. Thiscauses irregular shrinkage. Irregular dryingcaused by thick versus thin cross sections canproduce surface cracks. Proper drying or partdesign is needed. Fine grained bodies tend toform a fine network of surface cracksnumbers. Drying must be controlled. Thebody may need to be reformulated to reduceshrinkage. Grog additions may help. SeeReference 3. REFERENCES

crawling glazing Crawling of glaze off ware can be caused bytoo much shrinkage of the glaze, too-thickglaze application, or incorrect binder system.Some areas of the body may not be taking upthe glaze due to contamination by oil orparticle orientation in some areas. Solublesalts from the body can be a problem. Tiethem up chemically using barium carbonate.

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crawling glost firing Crawling during the glost fire can be becauseof the glaze not wetting the body in certainareas. See causes above. Ware placed tooclose too each other may be a problem.Viscous glazes are prone to crawl. Crawlingcan occur because of handling. Controlledslower heat up of the glaze is sometimesrequired to correct the crawling problem. Seereference 3 for more details. REFERENCES

crawling underglazecolors

Crawling can occur on underglazedecorations due to improper adhesion of theglaze to the decoration. Binders and fluxescan improve adhesion. Too much underglazemedium. Fire slowly up to 500oC. Seereference 3.REFERENCES

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crazing glazing,decorating, infield

Crazing occurs when the glaze is in tensilestress. That may because the glaze has too-high expansion or because of moistureexpansion of the body. It often shows up indecorating fires. The cure is usually to lowerthe thermal expansion coefficient of theglaze. Silica added to the glaze or to the bodyoften does the trick. Silica acts different inbody and glaze (vitreous state versuscrystalline state). Moisture expansion can bereduced by adding flux to the body or sealingthe complete surface of the piece with glazewhich requires pin firing. Various chemicalelements usually in the frit form can reducecrazing. Low-expansion boron frits are oftenused. For a complete list of ChemicalElements and how they affect glazes it TableForm go to CPDS 1-10. This informationcan be extracted from the references (as Idid). For calculating the thermal expansioncoefficient (and other properties) of glazesfrom the elements see the tables in references2, 3, and 4. Crazing can occur during duntingof ware. See the references and CPDS 1-10for other condition that may induce crazingsuch as too-thick glaze, underfiring body orglaze, overfiring porcelain glaze, impropergrinding, etc. REFERENCES

cut glaze glazing Crawling of glaze due to handling often inthe dipping process. Scratches on the surfacecauses crawling along the mark. Oil or dirtfrom fingers or processing conditions can befactors that cause local crawling.

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devitrification glost firing Crystallization of a non-crystalline glazesometimes resulting in a pinkish or blueishdiscoloration. Zinc silicates and calciumborates are suspect. Calcium may come fromthe body. The longer a glaze is molten, themore chance it has of crystallizing so controlthe time/temperature during maturation andcooling. Additions of alumina perhaps aschina clay can help as does lead bisilicate orlead frit.

dimples glost firing Also called orange peel. Often a glazeapplication problem and can be recognizedby looking at the surface of the glaze duringapplication. Spray guns too close or spraypressure too high. Firing of higher viscosityglazes will not cure this defect. Glazeadjustment may be required if the applicationproblem can not be cured.

discoloration(see alsostaining)

cast ware A mold spot may appear at the point the slipfirst hits the mold possibly causing a localseparation of body components. If the spotends up on the bottom of the piece, it maynot be important. Mica and iron particulatecan concentrate and cause spotting. For adetailed analysis see Reference 3.REFERENCES

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dropping,glaze

glost kilns Called droppers by my friends in England.An Italian designed tile factory was put outof commission at Interpace® Corporation(before I worked there) when glaze drippedon the tile from the kiln roof. The factory wasused to make tile grog after that fiasco. Alsoat Pfaltzgraff we purchased an old factorywhere fluorite was fired in a tunnel kiln. Thatkiln dripped but we could protect the warewith setters and by remodeling the kiln. Leadglazes can be a problem. Proper kiln designwhich allows the removal of glaze vapors isneeded for some glazes. The vapors must becondensed outside the kiln chamber andcollected as toxic waste. Have you triedzircon kiln washes?

dunting glost fire Cracking of ware due to thermal shock oftenassociated with quartz inversions. The " to $inversions (which are shown incorrectly inmy copy of reference 3) are crystobalite225oC, quartz 573oC, and tridymite 870oC. Asmooth often shiny fracture surface indicatesdunting.The first two inversions are often theproblem. Controlling the cooling zone isrequired. You can cool rapidly to theinversion but then you must creep throughthe inversion tippee toe. Glazes too-high incompression are prone to dunt. Now if youopen the kiln door letting cold air rush inover hot ware, the ware may suffer thermalshock. See reference 3 for a morecomprehensive study of dunting.REFERENCES

dunting decorating See above. Fast cooling may be the problem.Keep ware above kiln shelf.

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dunting hot water Pouring boiling or very hot water into aceramic vessel may cause failure sometimesexhibited by the base breaking off a coffeepot or teapot. A lower expansion body,thinner glaze, and proper design avoidingnon-uniform thickness may help. This is aserious problem in the field as someonecould be scalded. Testing such ware in thefactory is essential. See reference 3.REFERENCES

exploding drying or firing Too rapid drying or firing of damp wareresulting from steam pressure buildup. Oftenin flat or flaky pieces.

firing, fast bisk, glost,decorating fires

Time temperature differences are important.Problems can be trapped gases, carboncompounds not oxidized, insufficientmaturation of body or glaze, decorations notproperly sunk. Keep working on it!

flaking engobes Usually due to shrinkage differences betweenthe engobe and the body. Differences indrying shrinkage show up after drying.Differences in fired shrinkage show up afterfiring. Thinner coatings may improveadherence and fluxes can help. For a detailedanalysis of this problem see Reference 3.REFERENCES

flaking decorations

flaking, colors underglaze Too rapid application. Dirty surface. Binderproblems. Not an industrial problem.

glaze, raw glazing

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grit or otherrough surfacebits

glazing Thin glaze reveals body surface and feelsrough. Calcium has been dissolved from fritand reacts with carbonate in processing waterforming small spheres. This occurs if glaze isstored in a too warm environment. Preparedglazes from supplier can be kept dry untilneeded. Less soluble frits can be used. Acidcan be added but that flocculates the glazeslurry.

gloss, loss of glazing See crystallization. Underfiring, improper orover grinding of the glaze, new batts orsetters, thin glaze, kiln atmosphere or otherkiln conditions. See reference 3.REFERENCES

handle fall-off bisk ware See Cracking, handle.

handle fall-off green ware See Cracking, handle

handle fall-off gilded (orluster) waredecorating

Precious metals get into cup joint and hignexpansion pops off the handle. Stay a wayfrom the join.

ironing decorating Cobalt decorations have bronze streak aboveor in decoration. Add flux to color, applycolor thinner, thicken glaze coating. Repairware by applying flux or glaze over color andrefiring. See reference 3. REFERENCES

laminating cast ware

laminating dry pressing Caused by poor material flow or airentrapment. Particle size distribution andparticle fineness are to be suspected. Bindersand lubricants can help. The flow of propaneto displace air in die cavities helps too.

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laminating green ware Ware formed by extrusion can laminate if theextrusion auger is worn or not functioningproperly. Also if the extrusion is not de-airedlaminations or blisters will result. Freezing across-section of extruded ware will reveallaminations. Ware formed on plate rollersand other plastic forming equipment maylaminate because of problems in the initialextrusion. Improper programing of a plateroller can cause the clay not to kneadproperly. Particle orientation during formingoperations should be considered a problemsource.

lime popping bisk fire A white spot in a pock mark may be a limeparticle. Test with acid. May occur duringstorage.

livering casting slip Thixotropic slip on standing or in under-deflocculated state. Slip surface look likejelly. Re-disperse the slip.

low strength green ware Increase percentage of fine-grained clay.Increase ball clay. Add small amounts ofmontmorillonite (bentonite). Use smallamounts of organic binder such as gumarabic or dextrin. Decrease grog. Age thebody before forming. Careful handlingrequired.

low strength bisk Increase vitrification by adding fluxingminerals or increasing bisk firingtime/temperature. Ball mill batch materials.Make sure body is properly mixed and de-aired during mixing and pugging operations.Make sure adequate pressure is appliedduring forming. Study the fired structureunder a microscope or SEM.

matt, changein

glazing

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matt/glostchanges in

overglazecolors

Shiny overglaze colors fire matt. Underfiring.Severe overfiring. See Sulphuring. Kilncondensation because of improperventilation. Contaminates especially calciumcompounds. Color substitution error. Lack offlux. See reference 3. REFERENCES

matt, changein

underglaze

metal release glazing anddecorating

Usually refers to Pb and Cd. In glazes addlead as frit or lead bisilicate frit. Lower thelead content or eliminate it all together. Leadrelease in decorating is usually from theglaze but Cd is usually from the decorations.Underfiring. Too soluble glazes ordecorations. Keep 4% moisture in final-fire(usually the gild fire) decorating kilns. Fritused in decorations can be a factor. Seereference 3 for more details. REFERENCES

milky colors underglaze Glaze devitrification. Underfiring. Too rapidfiring. Calcium borate formation. (Add Al tocolor and glaze.) Harden color before glazingusing a hardening on fire. Glaze slurrycontaminants. Hole in that sieve?

molds joints casting Tight mold joints are required because eventhough you fettle and sponge a protrusioncaused by too open mold joints, the defectwill appear again after drying and firing.

morted ware cast ware Caused by glazing partially dried ware bydipping. Stresses cause thin layers to sluffoff. Morted ware in the sanitary wareindustry is cracked ware due to incorrectpouring.

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opacity,change in

glazing Matt glaze becomes shiny. Use tin oxide(expensive) or zircon opacifiers. Titaniumgives unwanted color. Insufficient dispersioncan be corrected by using a zircon frit. Overfiring.

overfiring glost firing See blistering and bloating.

peeling glazeapplication

Glaze not adhering to substrate or having toomuch shrinkage. Control particle size andbinders as well as slip viscosity and specificgravity.

peeling glost fire Glaze shivering off when the glaze is in toomuch compression due to too low thermalexpansion compared to the body. Easy to fix.What is harder to fix is when the glazeexpansion is too high. See crazing.

pinholes cast ware Dirty slip or reclaimed slip. Contaminatedraw materials. Air entrapped by blungerblades whipping around above the slip level.Oil in blungers or dust. Improperly processedor excessive scrap. New molds can causethis. High density, high viscosity slips. Toofast pouring rate. Air or oil in casting lines.Improper screening or magnetic separation.Is there a hole in your screen? Check often.

pinholes decorating Pinholes that occur during glazing may showup in decorating fires. See below.

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pinholes glazing Body contaminants from raw materials orreclaimed body, factory dust, coarse basalt,sponge, water contaminants, oil frommachinery or air lines (use filters), fibers,lignite, air entrapped in pug mills, etc., pastfrozen clay, un-pugged clay, fast firing.Materials such as manganese dioxide in claycan give off oxygen in glost and decoratingfires if they are not decomposed during biskfiring. REFERENCES

plasticity, lackoff

plastic forming Aging the body for 2-6 weeks can improveplasticity. Also make sure your body isflocculated. Plaster can be added to the slipto increase the degree of flocculation. Also,finer kaolins and ball clays can helpplasticity. There are purer ball clays that canbe added in small amounts to preservewhiteness. V-gum® or refinedmontmorillonite clays can help. Bodiescontaining alumina or nepheline syenite tendto deflocculate with time. If this occurs, thesolution is to put the body back into slip formso it can be flocculated. Moisture control isimportant. A soil harness tester (clayhardness tester) is useful for control.

ring, loss off glost fire Look for minute cracks. Body underfired.

run off cast warepouring

See wreathing.

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scumming drying, firing,aging in field

From soluble salts migrating to surface oredges. Ware dried on the lip may showscumming on the foot. Filter pressing of bodyremoves some salts. Salts can also be tied upby barium added as barium carbonate. Slowerdrying can diminish salt migration. Salts maycome from the water source and sanitaryware plants often treat their process water.Vanadium and iron salts cause scumming instructural clay products. There is moreinformation on this in CPDS 1-10.

REFERENCES

spatter glazing See Colemanite Splutter

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specking glazing Specking can be internal or on the surfacewhich may help identify the source. Ifspecking is on the surface it may not be inthe batch but was deposited on the ware afterforming. Harry Fraser says in his book that ifspecking is on both top and bottom surfacesit probably occurred early in the process.Processing equipment should be checkedcontinuously especially storage tanks,mixers, screens, magnetic separators,extruders, etc. The factory must be keptclean. If overhead structures are a source ofspecking, they must be cleaned or at leastbody and glaze slip tanks must be coveredand ware protected if stored in the open.Wind can bring in dust. Workmen can makedust, Vacuuming of machinery and kilndecks is a good idea. Kiln combustionsystems at times create particulates. Filters ingas and air lines may be required. We hadrust from old stagnant gas lines completelycontaminate ware during the first firings atone factory I was managing. Colored specsmay be attributed to compounds of Fe, Co(which goes a long way),Cu, Ni and otherelements. Decorating materials should beheld in suspect. See CPDS 1-10 for othersources. REFERENCES

spit-out decorating

staining,brown

casting molds From my experience it is caused by overdeflocculation by sodium silicate which canrelease lignin from ball clays. Mold life isreduced as is casting efficiency.

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staining,smoke

cast porcelainand stonewarereduction glostfiring

Usually occurs on mold side surface. UseDispex® from Allied Chemical instead ofsodium silicate and soda ash as dispersant.Note: Dispex can also be used effectivelywith sodium silicate to improve casting ofother bodies (which do not necessarily showsmoking). Using sodium-sulphate-free claysalso eliminates the problem. See Reference 3.REFERENCES

starved glaze glazing Contamination or improper glaze slip (slop)adjustment. Look for oil or handling marks.

starved glaze glost firing Too close setters or refractory, thin glaze,underfiring.

streaking glazing Called glaze run in the U.S. The glaze runsduring application. Control glaze slurry(slop) specific gravity (pint weight) andviscosity. Glaze too thick. To much binder.Control grind of glaze so it doesn’t settle outor separate during application. Correctdipping techniques (see reference 3). Thisproblem in colored glazes can result is somepretty but off-colored streaks.REFERENCES

stuck ware bisk fire Overfiring. Contaminants on ware or settersor kiln slabs.

stuck ware glost fire Overfiring. Glaze on foot of ware. Glaze onsetters or kiln slabs. Use setter wash and kilnslab wash.

stuck ware slip castingmolds

New molds. Dust molds with talc (nowfrowned upon). I have always preferred towipe the mold with a damp cloth containingkerosene of fuel oil. Slip adjustments canhelp. Use Dispex®. See casting.

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sulphuring glost fire Or feathering if frost-like on a window herein Idaho. Matt areas on a glossy surface thatdo not show crystallization. Usually calciumsulfate. This occurs with oil or coal firing.

twisting glazing Clay has a memory and hand-thrown potsmay shift slightly during firing. This candisplace the handle or spouts on teapots, etc.Clever potters put the spouts and handles oncrooked. The twisting then puts themstraight.

warping bisk fire See below. Overfiring or hot spots. Oftenware must be supported by setters or props.For a discussion of shrinkage, drag, andslump see CPDS 1-10.

warping glost fire Warping occurs during drying or firing. Clayhas a memory and stresses induced duringmaking can cause warping. Even drying isrequired to prevent warping. Warping inkilns can be caused by hot spots in the kiln orflame impingement. Often ware must besupported by setters or props. Overfiringcauses warping. Reference 3 has a goodsection on how to prevent slump. To buy anew or used copy click on the reference linkin REFERENCES.

white spot glazing Contaminants on the surface of a glaze,usually random and irregular shaped.Uniform shaped defects in the glaze may bedue to opacifier, colorants, or other glazeconstituents. Refiring can help. Seeblistering.

wreathing cast ware See casting. Use correct pouring proceduresand adjust slip thixotropy and specificgravity. Use Dispex® (Allied Chemical)

copyright©2007 John Taylor Jones, Ph.D

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REFERENCES (references are linked to Amazon.com)

1. Ceramics: Processing & Testing, John T. Jones and M. F. Berard, Iowa StateUniversity Press, Second Edition, 1993

2. Ceramic Glazes, Cullen Parmalee and Cameron G. Harmon, Cahners 1973

3. Ceramic Faults and Their Remedies, Harry Fraser, A & C Black, London, 1986

4. Ceramic Glazes, Felix Singer and W. L. German, Borax Corporation 1960(1964 printing)

If you do not find the defect you are looking for in this table see the Table Index in

The Ceramic and Pottery Defect Series 1-10 (CPDS 1-10)

copyright©2007 John Taylor Jones, Ph.D