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G2931 Worthington Clear is a popular low to medium-fire transparent glaze recipe. It contains 55% Gerstley Borate (GB) plus 30% kaolin (GB melts at a very low temperature). GB is also very plastic, like a clay. I have thrown a pot from this glaze 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 like this one the shrinkage is even worse. GB is also slightly soluble, over time it gels glaze slurries even in smaller percentages. Countless potters struggle with Gerstley Borate recipes.
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Here the melt fluidity of Gerstley Borate (GB) is being compared to Ferro Frit 3134 (using a GLFL test). Clearly, these are two very different materials. GB is a clay, Frit 3134 is a man made powdered glass. Notice the GB shrinks to about half its original size by 1600F and then suddenly by 1650 it has exploded out of the starting gate and crossed the finish line! The frit, conversely, slowly softens through the entire 1350-1650 range and then starts down the runway between 1650 and 1700F. While it is clear that frit 3134 is not a direct substitute for the Gerstley Borate (GB) it's more gradual melting make it a better source as a source of B2O3 (boron).
Glossary |
Glaze Gelling
Glaze slurries can gel if they contain soluble materials that flocculate the suspension. Gelling is a real problem since it requires water additions that increase shrinkage. |
Glossary |
Plasticity
Plasticity (in ceramics) is a property exhibited by soft clay. Force exerted effects a change in shape and the clay exhibits no tendency to return to the old shape. Elasticity is the opposite. |
URLs |
https://insight-live.com/insight/share.php?z=HzyNzj9ELs
Replacing the Gerstley Borate in recipes containing 50% or more of it Many recipes were built on bases employing exceptionally high percentages of Gerstley Borate. At medium temperatures these melt fluid transparents hosted additions of colorants, variegators and opacifiers. At low fire they were used as transparents over underglaze decoration. |
Materials |
Gillespie Borate
A Gerstley Borate substitute that became available during the early 2000s and is still available in 2023. |
Materials |
Gerstley Borate
Gerstley Borate was a natural source of boron for ceramic glazes. It was plastic and melted clear at 1750F. Now we need to replace it. How? |
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