This recipe is part of a project to reproduce chemistry previously achieved using Fusion Frit FZ-16. Glazes made with that frit do not exhibit this violent boiling and produce ultra-gloss surfaces comparable to those formerly obtained using lead frits. The likely culprit here is not simply the oxide chemistry, but the way it is being sourced: borax pentahydrate releases large amounts of water during heating and calcium carbonate releases CO2, while the borax is already helping to form an early borate-rich melt. The high ZnO likely influences the viscosity and surface tension of that melt, possibly making gas escape more violent. In the fritted version, much of this chemistry has already been reacted during frit manufacture, dramatically reducing gas evolution during the glaze firing.
This illustrates something broader than a weird glaze defect: you can duplicate an oxide formula without duplicating the way the glaze behaves on the way to becoming glass.

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Decrepitation refers to a decomposition accompanied by scaling, delayering, even disintegration of the glaze layer. Moving rightward these glazes have increasing percentages of colemanite. At its worst (far right) the glaze is spattering off the sample and onto the kiln shelf. The others are crawling, first pulling away from the corners (far left) moving toward pulling away on the flat surfaces (center). Gerstley Borate and Ulexite, similar minerals, are far less likely to do this (but they have other serious issues also). A much better solution is to use frits to source the oxide B2O3 (easy to do in your account at Insight-live.com). Photos courtesy of Nigel Hicken.
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Use Insight-live to substitute materials in a recipe
We will substitute wollastonite for whiting and a frit for Gerstley borate in the G2571A cone 10 matte while maintaining the chemistry of the original recipe. |
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Glaze Chemistry
Case studies where glaze chemistry was used to solve a problem. |
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