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 Gerstley Borate sufferers

A cure for long-time Gerstley Borate sufferers

These are various different terra cotta clays fired to cone 04 with a recipe I formulated to source the same chemistry as the popular Worthington clear (50:30:20 GB:Kaolin:Silica). But instead of getting the B2O3 from Gerstley Borate I sourced it first from Ulexite (G2931B) and then a mix of frits (G2931K). All pieces are fired with a drop-and-hold firing (like C03DRH). Fit is good on all except a fritted terra cotta stoneware where it is shivering slightly (all were boil:ice tested). Frits are so much better for sourcing B2O3 than of Gerstley Borate (the later is notorious for turning glaze slurries into jelly!) and the recipe I ended up with was adjustable for thermal expansion.

Colemanite and what its decrepitation does in glazes

Colemanite and what its decrepitation does in glazes

Decrepitation refers to a decomposition accompanied by scaling, delayering, even disintegration of the glaze layer. Moving rightward these glazes have increasing percentages of colemanite. At its worst (far right) the glaze is spattering off the sample and onto the kiln shelf. The others are crawling, first pulling away from the corners (far left) moving toward pulling away on the flat surfaces (center). Gerstley Borate and Ulexite, similar minerals, are far less likely to do this (but they have other serious issues also). A much better solution is to use frits to source the oxide B2O3 (easy to do in your account at Photos courtesy of Nigel Hicken.

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By Tony Hansen

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