Help for Long-time Gerstley Borate Sufferers!

I will show you why people love/hate this material and how I substituted it for Ulexite to make a much easier-to-use glaze that fires just as good or better.

B. Glazes

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Here is how to completely fix Gerstley-Borate glaze recipes

This outlines work I did to create an alternative transparent base glaze recipe in the popular 50:30:20 GB:Kaolin:Silica system (e.g. Worthington clear) that is employed from cone 04 to 6. Hundreds of common recipes add opacifier, variegators and colors to this base.

Gerstley Borate is a remarkable raw material, it melts to a crystal clear glass at a temperature far lower than any other material, it is mother natures super frit. It appears in thousands of glaze recipes for low and medium temperature. But unfortunately, that is Dr. Jekyll. GB also has a Mr. Hyde side: It is very plastic and really holds on to its water. This latter property can make glazes having high-percentages of Gerstley Borate absolutely miserable to use. They have high water contents, their slurries turn to jelly, they take much longer to dry, then they shrink on drying and crack. Then they crawl on firing!

The best solution is to use other materials to source the B2O3. Frits are the obvious choice and if the glaze has less than about 30% GB it is usually possible to reformulate using common borax frits. But if a glaze has 50% GB no common frit will supply enough (there are uncommon ones that can be purchased). However there is another raw material: Ulexite. It has high B2O3 also, Gerstley Borate is actually composed of Ulexite and Colemanite. Ulexite is the superior source of the two. Ulexite is used as a melter in the fiber glaze industry. In this video I will show you how I substituted it and the fantastic results that I got: The glaze is wonderful to use and fires exactly the same.

The key to my ability to do many of these things is that I work within an account at I enter the recipes there, do the chemistry, upload the picture, analyze them side-by-side.

Why would a glaze turn into a jelly like this?

Why would a glaze turn into a jelly like this?

This is one of the things Gerstley Borate does to your glazes. Stir this and you might have 2 seconds to dip something before it turns to jelly again (this was even deflocculated with Darvan and it was OK yesterday). It has a low specific gravity (a high water content) and will dries very slowly on my bisque ware. How can I make this glaze into a fast drying beautiful-to-use slurry? Replace the Gerstley Borate with something else. What? Gerstley Borate sources B2O3, I can supply it using frits or Ulexite (depending on the amount of GB that is in the recipe). I can do that in my account at

A cure for long-time low and medium fire Gerstley Borate sufferers: Ulexite!

A cure for long-time low and medium fire Gerstley Borate sufferers: Ulexite!

These are various different terra cotta clays fired to cone 04 (also a low fire white-buff fritted stoneware) with a recipe I formulated to source the same chemistry as the popular Worthington clear, but sourcing the B2O3 from Ulexite and a frit instead of Gerstley Borate (G2931B). All pieces are fired with a soak-soak-slow cool firing. Fit is good on all except a fritted terra cotta stoneware where it is shivering slightly (all were boil:ice tested). This outlines work I am doing to create an alternative recipe for the popular 50:30:20 GB:EPK:Silica recipe (Worthington clear) that uses Ulexite instead of Gerstley Borate (the later is notorious for turning glaze slurries into jelly!).

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By Tony Hansen

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