Ceramic Glaze Defects

Ceramic glaze defects include things like pinholes, blisters, crazing, shivering, leaching, crawling, cutlery marking, clouding and color problems.

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Ceramic glaze defects include things like pinholes, blisters, crazing, shivering, leaching, crawling, cutlery marking, clouding and color problems. So many things can go wrong it can seem amazing at times that it is even possible to successfully fire anything! For hobbyists and potters, a few small pinholes or uneven coverage are not an issue, but in industry one tiny defect can route a piece to the reject pile.

Defects are seen differently depending on who you are. A worker on a production line sees a defect as a maladjustment on one of the machines (e.g. the kiln, the application or decoration equipment). He/she consults trouble-shooting books (made by the equipment manufacturers) to find a sample picture that looks similar to the problem being experienced and then tunes the machine as instructed. When the adjustments don't work the problem is deemed to be caused by an upstream issue.

The technicians in charge of batching and maintaining the rheology of the slurry have different trouble-shooting books (made by the mixing equipment manufacturers, material and additives suppliers), the text beside the pictures explains how the problem might be related to the batching errors, mixing and milling issues, specific gravity, additive balance, viscosity, thixotropy, etc of the slurry. If none of the suggestions work it goes upstream to the glaze supplier.

The technicians at glaze supply companies use the university textbooks, they understand about glaze chemistry and the materials that supply it. They understand how to compare, evaluate and use frits. They know how to balance the tradeoffs between getting the desired visual or physical property in the fired result and what sacrifices and weakness are intrinsic to getting that. They know what changes to make in the recipe to fix the problem.

Often a glaze defect on a production line is a combination of issues at multiple steps.

A potter needs to know all of the above, to be able to decide what stage of production is the issue and fix it there.

A hobbyist just expects everything to work, glaze is like paint. He/she is unlikely to even read the label on the glaze jar. Hobbyists simply buy another jar and try that. If that fails they buy one of a different brand. If that fails they take up painting or weaving! The hobby ceramic supply industry is set up so that glazes are brushed on and are highly likely to work on the clay bodies hobbyists use. So most people can blissfully make ware while seldom encountering glaze defects.

This website is targeted mainly at potters, so it is about knowing every stage of production. Potters use frits much less than industry, so they are open to more glaze defects. However, they also have control of every phase of production and can thus compensate as needed. For example, they fire periodic kilns, which means they have the freedom to fire higher and employ much longer firing cycles, this gives even raw glazes time and temperature to melt and time to heal defects (on a controlled cool).

Related Information

Glaze defects are more visible with certain glazes

Reactive glazes, especially darker colored types like the one on the left, distract the eye with their visual character, this tends to hide defects (like pinholes, blisters, bare spots, scratches). But matte glazes, especially whites like the one on the right, make tiny defects in the glaze more visible. Their mixing, application and firing require much more attention to detail to produce a defect-free surface.

Links

Glossary	Crackle glaze Crackle glazes have a crack pattern that is a product of thermal expansion mismatch between body and glaze. They are not suitable on functional ware.
Glossary	Dimpled glaze A ceramic glaze defect characterized by tiny holes in the surface, often too small to see with the naked eye.
Glossary	Glaze fit In ceramics, glaze fit refers to the thermal expansion compatibility between glaze and clay body. When the fit is not good the glaze forms a crack pattern or flakes off on contours.
Glossary	Surface Tension In ceramics, surface tension is discussed in two contexts: The glaze melt and the glaze suspension. In both, the quality of the glaze surface is impacted.
Glossary	Viscosity In ceramic slurries (especially casting slips, but also glazes) the degree of fluidity of the suspension is important to its performance.
Glossary	Volatiles
Glossary	Glaze Chemistry Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.
Glossary	Glaze Bubbles Suspended micro-bubbles in ceramic glazes affect their transparency and depth. Sometimes they add to to aesthetics. Often not. What causes them and what to do to remove them.
Glossary	Glaze Compression In ceramics, glazes are under compression when they have a lower thermal expansion that the body they are on. A little compression is good, alot is bad.
Glossary	Glaze Shrinkage Raw ceramic glazes contain clay to harden them on drying and to suspend the slurry. The more clay there is the more the glaze shrinks as it dries on the piece.
Glossary	Leaching Ceramic glazes can leach heavy metals into food and drink. This subject is not complex, there are many things anyone can do to deal with this issue
Glossary	Cutlery Marking Ceramic glazes that mark from cutlery are either not properly melted (lack flux), melted too much (lacking SiO2 and Al2O3), or have a micro-abrasive surface that abrades metal from cutlery.
Troubles	Foaming of Ceramicd Glaze Slurries Ceramic glaze slurries can sometimes generate enough foam that it becomes difficult to apply an even layer to a surface. What can you do?
Troubles	Crawling Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
Troubles	Runny Ceramic Glazes Glazes of high melt fluidity are likely to run if applied to thickly or have not catcher glaze
Troubles	Clouding in Ceramic Glazes There a many factors to deal with in your ceramic process to achieve transparent glazes that actually fire to a crystal-clear glass
Troubles	Uneven Glaze Coverage The secret to getting event glaze coverage lies in understanding how to make thixotropy, specific gravity and viscosity work for you
Troubles	Specking on Ceramic Ware Specking, or speckling, can be both a fault or feature in fired ceramic ware - caused or produced by metal-bearing contaminants to metallic additives
Troubles	Powdering, Cracking and Settling Glazes Powdering and dusting glazes are difficult and a dust hazard. Shrinking and cracking glazes fall off and crawl. The cause is the wrong amount or type of clay.
Troubles	Glaze Slurry is Difficult to Use or Settling Understanding glaze slurry rheology is the key to solving problems and creating a suspension that does not settle out, applies well, dries crack free.
Troubles	Staining of Fired Ceramic Glazes Ceramic glazes are glass. That means they are always easy to clean, right? Wrong - because the surface is not always glassy.
Troubles	Bleeding colors In ceramics, the edges of overglaze and underglaze color decoration often bleeds into the over or under glaze. How can this be avoided.
Troubles	Glaze Blisters Questions and suggestions to help you reason out the real cause of ceramic glaze blistering and bubbling problems and work out a solution
Troubles	Glaze Pinholes, Pitting Analyze the causes of ceramic glaze pinholing and pitting so your fix is dealing with the real issues, not a symptom.

By Tony Hansen
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