URL: Replacing the Gerstley Borate in recipes containing 50% or more of it

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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.

Related Information

Can you actually throw a Gerstley Borate glaze? Yes!

A thrown vase made of a Gerstley Borate glaze

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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.

Click here for case-studies of Insight-Live fixing problems

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You will see examples of replacing unavailable materials (especially frits), fixing various issues (e.g. running, crazing, settling), making them melt more, adjusting matteness, etc. Insight-Live has an extensive help system (the round blue icon on the left) that also deals with fixing real-world problems and understanding glazes and clay bodies.

Somehow the Gerstley Borate 50:30:20 glaze worked.

But does it work using Gillespie Borate?

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This recipe, G2826A, a base transparent recipe having 50% Gerstley Borate plus 20% kaolin, is "jelly city". Although a low temperature base, this was much more commonly used at cone 5-6. This recipe, G2826A, was at the limit of how melt fluid a glaze could be. And at the limit of the slurry properties that could be tolerated with this material. In this test, even with 2.5g of Darvan deflocculant in this jar, it was still thick enough to require pushing this tile down into it! It still needed 5 seconds to build up enough thickness. And did not cover the recesses properly. Yet people have used this popular fluid-melt recipe for 50+ years to get the surface variegation it produces (because of boron blue) and the fluid melt (because it is so high in boron). They added all manner of colorants and opacifiers and it generally performed without blistering. The melt fluidity required careful control of thickness (to avoid it running onto shelves). This was a "Dr. Jekyll and Mr. Hyde" of ceramic materials!

Potters are using Gillespie Borate in this recipe (with issues), see the G2826A2 recipe. Other approaches are to source the boron (B₂O₃) from a frit (or mix of frits). An example is G2826A1, it does not variegate as much but added titanium or rutile can emulate that. Another hybrid option is the G2826A3 that employs both Gillespie Borate, nepheline and talc.

Links

Media	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.
Recipes	G2931L - Low Expansion Low-Fire Clear A cone 03-02 low expansion clear ceramic glaze developed from Worthington Clear.
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.
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?
Typecodes	Gerstley Borate Glaze Calculation examples Examples of how we use glaze calculations (in an Insight-Live.com account) to replace Gerstley Borate with other materials, especially frits, in various glaze recipes. In doing so we take the opportunity to improve the recipe in other ways (e.g. reduce thermal expansion, improve slurry properties, reduce bubbling and crawling).

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