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.
What you are seeing here is strange indeed. On the bottom is a porcelain tile that was glazed using the recipe indicated. Multiple ones of these were stacked into a kiln, each on its own round kiln shelf. During the firing the boiling of the glaze melt was so violent that it splashed upward, coating the undersides of the shelves above them. Yet no glaze was splattered around the tiles themselves! This phenomenon was created and reported by a researcher working on leadless ultra-gloss metallic tile glazes. This recipe is part of a project to create a similar chemistry to previous work done with Fusion Frit FZ-16. Glazes made using that frit do not have this boiling problem and produce surfaces comparable to leaded frits. We speculate that the vigorous melt is a product of the interaction between the high zinc oxide and borax in the recipe.
A natural source of boron that melts at a very low temperature.
Subsitute Gerstley Borate in Floating Blue Using Desktop Insight
Use Desktop Insight to explore ways of calculating substitutes for Gerstley Borate in the popular Floating Blue cone 6 glaze recipe while maintaining or improving the other raw and fired properties of the glaze.
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.
Getting Frustrated With a 55% Gerstley Borate Glaze
I show you why people love/hate this material and how I substituted it for Ulexite in this crazy recipe to make a far easier-to-use slurry that fires identical.