Glaze peeling on drying
Happens as clay-containing glazes shrink during drying on the bisque ware, first cracking then pulling away.
Details
A common problem with dipping glazes. Glazes contain clay, so they shrink when they dry. There is a mechanical bond between the drying glaze and the bisque body (surface roughness provides a place for it to hang on). But, when drying shrinkage is too great, the bond can be compromised, producing cracks (and subsequent crawling in the firing). But, if shrinkage continues as it dries further, the glaze can actually peel away.
Related Information
This pottery glaze is peeling when multi-layered:
The recipe reveals why.
This picture has its own page with more detail, click here to see it.
Dipping glazes can, in very controlled circumstances, be multi-layered. If you have done it for some time, with success, you may have been just lucky. These pieces demonstrate one of many factors that can produce failure: The top glaze contains 7% bentonite and 5% zinc oxide - that is 12% hyper-fine particles, perfect to create the drying shrinkage to make this happen. The recipe author must have reasoned that it could "pinch hit" for the inadequate clay content. But 7% bentonite in any glaze is highly unusual. And, it is actually not even necessary here. Why? The high percentage of Ferro Frit 3124 is sourcing needless Al2O3 (alumina), that should be coming from kaolin or ball clay instead. Frit 3134 is the perfect stand-in, it contains almost no Al2O3, but otherwise is quite similar. The equivalent recipe we calculated on the right has the same chemistry, but does pass a sanity check. It is not guaranteed to work, but has a better chance than this one. For even more assurance of success, it should be mixed as a base-coat dipping glaze.
Shrinking glaze = peeling, cracking glaze
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).
Links
| 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. |
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