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Glaze Gelling

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!

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.

Out Bound Links

  • (Glossary) Crawling

    A condition where glaze separates into clumps or islands (during firing) leaving bare clay patches showing in-between. This problem is almost always caused by glazes shrinking too much during drying and then cracking. Those cracks become the crawl-points during firing. Excessive shrinkage is norm...

  • (Materials) Gerstley Borate - Plastic Calcium Borate

    Colemanite, Calcium Borate, Borocalcite

  • (Glossary) Flocculation

    The opposite of deflocculation. Flocculation in a slurry can be a desired or undesired property. For the latter, a ceramic glaze or clay slurry that would otherwise be thin and runny can be made into a gel by the simple addition of a flocculant. This is typically done to improve suspension prope...

In Bound Links

  • (Glossary) Mocha glazes

    Mocha diffusion is a technique of layering slips onto ware so that the edges of the upper one bleed outward into the lower. An alkali/acid mechanism is employed. The lower layer is a typical water based slip (usually white or cream) that is gelled enough to stay stable on the ware and wet long enoug...

By Tony Hansen

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