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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.

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In ceramics, EPK, or EP Kaolin, is used in clay bodies and glazes. EPK feels fairly plastic on wedging. But it splits like this during throwing (top right). It needs the help of bentonite or ball clay to be usable in porcelain or stoneware bodies. Tile #6 kaolin is thus a much better choice for plasticity (although higher in iron and titania).
But EPK shines in glazes. A slurry of pure EPK (bottom), mixed to only 1.15 specific gravity, holds this spatula upright! Glazes employing as little as 20% EPK, without added electrolytes, are naturally thixotropic. And its sticky nature helps with adherence and dry hardness. Consider the engobe employing it (top left). It gels and clings to this rubber spatula in an even layer, with no drips. If leather-hard ware is carefully dipped and extracted, it stays put (even if it takes hours to dry). The performance of EPK suspending glazes for use on bisque is even better.
Are there other sticky kaolins that gel? Yes. Grolleg and New Zealand kaolin. What if you cannot get either? Tile #6 is a good choice because it dries hard. But, it may need the help of Epsom Salts to produce a slurry having thixotropy like EPK.

This picture has its own page with more detail, click here to see it.
Layered dipping glazes usually 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, even with single layering, is a matter of the shrinkage being low enough, the drying being fast enough, the layer being thin enough, the bisque being absorbent enough, the water content being low enough and the bond with the bisque being good enough. Glazes with high clay content (including Gerstley or Gillespie Borate), 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 (if you don't mind long drying times).
| Materials |
Alberta Slip 1900F Calcined
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| Materials |
Alberta Slip 1000F Roasted
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| 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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