Worthington Clear is a popular low fire transparent glaze recipe. It has 55% Gerstley Borate plus 30% kaolin (Gerstley Borate melts at a very low temperature because it sources lots of boron). GB is also very plastic, like a clay. I have thrown a pot from this recipe! This explains why high Gerstley Borate glazes often dry so slowly and shrink and crack during drying. When recipes also contain a plastic clay the shirinkage is even worse. GB is also slightly soluble, over time it gels glaze slurries. Countless potters struggle with Gerstley Borate recipes. How could we fix this one? First, substitute all or part of the raw kaolin for calcined kaolin (using 10% less because it has zero LOI). Second: It is possible to calculate a recipe having the same chemistry but sourcing the magic melting oxide, boron, from a frit instead.
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