Glaze slurries can gel if they contain soluble materials that flocculate the suspension. Gelling is a real problem since the glaze will not pour properly, pieces cannot be dipped without getting an excessively thick layer and the glaze does not drain off the ware to form an even layer. When water is added to thin the slurry, drying shrinkage increases and the glaze cracks during drying (this leads to other problems like crawling). On subsequent storage the glaze can gel again and further water additions are needed. Gerstley Borate glazes are most well-known for this problem (although other slightly soluble materials like Nepheline Syenite can cause it). Common recipes that employ 50% or more Gerstley Borate will stir up to a nice suspension, but within minutes they can turn to a gel that is so firm the bucket can be turned upside down. Gelling glazes often dewater or dry very slowly, this becomes a production issue.
Can you actually throw a Gerstley Borate glaze? Yes!
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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