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 The mineral Gerstley Borate will soon be history because U.S. Borax Corp. has stopped mining the small deposit in California. And it is unlikely that Colemanite from Turkey or Argentina will replace it. However, a mineral deposit in California will offer insoluble calcium borate ("Cadycal") for use in low-fire and mid-fire glazes. It will, however, be almost as costly as high borate frits (eg, Ferro 3134, Pemco 2201, Fusion 309).

Raku potters, wanting Gerstley Borate's peculiar behaviour, will undoubtedly discover that neither Cadycal nor frits as such will give their pots the zip provided by Gerstley Borate. Is there a possible substitute? Perhaps. By using glaze calculation methods, I have come up with a first approximation.

The way I proceeded was to use analytical data from Mary Simmons, a geologist at University of New Mexico. On Jan 15, 2000, she posted a detailed analysis of Gerstley Borate, bagged in 1999, and she reported a lower level of B2O3 and higher amount of SiO2 than previous analyses.

Ms. Simmons's data was more inclusive than previously reported analyses for Gerstley Borate. She wrote to me that her values omitted just the last 1% of unknowns, and she hoped to track them down soon.

Ms. Simmons cited this analysis for Gerstley Borate (she labeled it GB2) and the following is rounded from four figures to three, percent by weight:

CaO 25.3; MgO 4.85; Na2O 4.00; K2O 0.56; Al2O3 1.68; SiO2 15.0; Fe2O3 0.50; MnO 0.03; TiO2 0.08; P2O5 0.08; LOI 25.9; B2O3 22.0 (by difference).

The values will change slightly when Ms. Simmons is able to analyze for B2O3 directly.

Using these numbers I calculated the Seger Formula for Gerstley Borate and then proceeded to match these molar amounts using available raw materials. There are two frits that could lead to synthetic Gerstley Borate, namely, Pemco Frit 2201, and Fusion Ceramics Frit 309.

P2201's analysis is reported as:

  Na2O 6.8 wt%; CaO 22.2%; MgO 4.8%; Al2O3 1.8%; B2O3 51.5%; and SiO2 13.2%.

F309's analysis is:

  K2O 6.20 wt%; CaO 18.00; MgO 4.30; Al2O3 2.50; B2O3 45.00; and SiO2 24.00. 

Very low amounts of Al2O3 and SiO2 are essential so P2201 would likely work better than F309. My recipes below are based on P2201 but F309 would serve with minor adjustments.

After several tries, this is what I determined:

  Synthetic Gerstley Borate #1, 1000-gram batch

  490 Pemco frit 2201 (or F309)
  280 Whiting
  125 Talc new york (or equal)
  65 Feldspar (G-200, Custer, Kona F-4, etc.)
  40 Unwashed woodash (well screened).
  5 Iron oxide red (1 teaspoon)
  1 Titanium dioxide (one-fifth teaspoon)

and a second version using Magnesium Carbonate:

  Synthetic Gerstley Borate #2, 1000-gram batch
  520 Pemco frit 2201 (or F309)
  300 Whiting
  70 Magnesium carbonate
  70 Feldspar (G-200, Custer, Kona F-4, etc.)
  40 Unwashed woodash (well screened)
  5 Iron oxide red (1 tsp)
  1 Titanium dioxide (one-fifth teaspoon)

Notes:

Dry mix the above ingredients, and use the mixture in place of Gerstley Borate, on a one for one basis. Also, since the woodash contains solubles, this mixture is best used in a fresh mixed glaze for immediate application to a pot (crystals may form on standing, making the glaze non-uniform).

The unwashed woodash supplies a small amount of alkalis (Na/K) and Phosphorus (as oxide). The ash also will contain minute amounts of unknown/trace elemental oxides.

When this mix is used to make a glaze, it may be necessary to use a flotation agent, namely, a member of the Darvan family - a sodium salt of an organic polymeric material). Do not use a suspension agent composed mainly of clay - ie, bentonite (montmorillonite) or V-Gum (smectite).

This mixture is yet to be tested by me but I am working to prepare it and test it soon, once our Ontario Spring occurs and an outdoor firing becomes manageable.

Copyright January 2000 by Tom Buck, B.Eng.(Chem).