Clay in a glaze recipes. Why is it there?

What happens when there is too much?

If I put this on social, I might get 100 comments on the cause. Most would say it's applied too thick. That is not correct. At the top, it is too thick, but not at the bottom (where it is still cracking). Crawling during firing is almost certain.

Before offering any opinion, one needs to sanity check the recipe. This one, GA6-B, contains a lot of clay. Why do dipping glazes contain clay, usually kaolin, ball clay? To suspend the slurry, slow down drying, gel the slurry to make it thixotropic and harden the layer on drying. But their chemistry is also important, clays supply the all-important Al₂O₃ and SiO₂ to the melt (which would otherwise have to come from feldspar). Certain clays excel at suspending. 15-20% EP kaolin, for example, is all that is needed. It’s not that plastic, but sticky and thixotropic (Grolleg and New Zealand kaolins are similar or even better). 10% of a gelling ball clay might be enough. However, 50% of a silty non-plastic clay might be needed. When the clay has too much influence, glazes shrink too much as they dry and then crack like this. Ones lacking clay (or employing one that is not suitable) have poor application properties (for dipping) and are powdery. This glaze has 80% pure raw Alberta Slip. That is a plastic clay, no wonder this is happening! 50% of it needs to be roasted.

Related Pictures

What makes EP Kaolin unique? Are others as good?

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In ceramics, EPK, or EP Kaolin, is used in clay bodies and glazes. Top right: Pure EPK is fairly plastic on wedging. But during the throwing process, it splits at the rim like this. The forming properties that it does have thus seem to be as much related to its stickiness and cohesion as plasticity. Without the help of bentonite or ball clay this would not be able to host the addition of feldspar and silica to make a usable porcelain or stoneware. But EPK shines in what it does as a slurry. Left: This slurry has been mixed to only 1.15 specific gravity and forms a thixotropic gel that clings to the spatula in an even layer. Bottom: This low specific gravity gel is thick enough that it will hold this heavy spatula vertical! Considering that our typical glazes have a specific gravity of 1.45 yet are still quite fluid it is evident just how much EPK can gel (and suspend a suspension). It is thus no surprise that it is the kaolin of choice in ceramic glazes. Are there other sticky kaolins that gel like this? Yes. Grolleg and New Zealand kaolin.

Shrinking glaze = peeling glaze

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Dipping glazes peel like this because they contain clay and shrink as they dry (the fact that all of them don't do this is actually amazing). Success is a matter of the shrinkage being low enough, the drying being fast enough, the layer being thin enough, the bisque being absorbent enough, and the bond with the bisque being good enough. Glazes with high clay content, thick applications or multi-layering are the main offenders. Thixotropic slurries apply most evenly and are least likely to go on too thick. Dipping glazes having 15-20% kaolin or ball clay are easiest to slurry up and have the best application and drying properties. Mixing base layers as first-coat dipping glazes is also important.

The problem with this piece: The addition of 7.5% bentonite to make up for the otherwise low raw clay content in the recipe produced a recipe that does not pass a sanity check. When that was replaced with kaolin it worked. There is a crowbar approach to fix these without any other changes: Add CMC gum (e.g. 1%) to make them brushing glazes.

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Links

Materials	Alberta Slip 1900F Calcined
Materials	Alberta Slip 1000F Roasted
Materials	Calcined Kaolin This is kaolin powder that has been fired in a furnace to remove the 12% crystal water and render it non-plastic.
Materials	Alberta Slip Albany Slip successor - a plastic clay that melts to dark brown glossy at cone 10R, with a frit addition it can also host a wide range of glazes at cone 6.
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	Crawling Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
Glossary	Calcination Calcining is simply firing a ceramic material to create a powder of new physical properties. Often it is done to kill the plasticity or burn away the hydrates, carbonates, sulfates of a clay or refractory material.

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