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Oil-spot effects depend on being able to layer glazes. Normally, a black underneath and matte white over top. Using dipping glazes is obviously advantageous for this type of piece; the second dip covers the other half and creates the double layer needed. But dipping glazes contain clay in the recipe (almost always kaolin or ball clay), it satisfies multiple needs: It suspends the slurry, hardens the drying glaze, and supplies critical Al2O3 to the chemistry. The upper glaze here is a matte; so it needs extra Al2O3, which means it likely has extra clay. 20% kaolin (non-plastic like EPK, Grolleg, NZ) is about the maximum or the glaze will shrink too much when drying. If extra Gerstley Borate is added, then it will be worse. Drying cracks are the result when the fragile body-bond of the lower one fails to withstand the stresses of being rewetted and tugged upon by the upper layer being applied and drying. When the cracked double-layer begins to melt, it pulls itself into islands, leaving bare body between. That is called "crawling".
How to prevent this. Be smart about glaze layering and use recipe logic. The lower black can likely be adjusted to be a first coat dipping glaze. The clay content in the upper white one can be adjusted to reduce drying shrinkage (by splitting it between raw and roasted/calcined powder, or by using a clay having lower plasticity).
| Glossary |
Glaze Layering
In hobby ceramics and pottery it is common to layer glazes for visual effects. Using brush-on glazes it is easy. But how to do it with dipping glazes? Or apply brush-ons on to dipped base coats? |
| 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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