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Raku Gerstley Borate Glaze

These pages were a study of how we substituted Boraq 1, Boraq 2 and Boraq 3 into common recipes. Boraq was developed by Plainsman Clays as a substitute for Gerstley Borate (under code number L3127E). The development effort took place during the early 2000s, the initial period when the demise of Gerstley Borate appeared imminent. Other companies, including Laguna Clays, introduced similar products at the time. Later Laguna Clays began processing a last stockpile of the material they found at the mine and interest in substitutes waned. In 2023 the cycle appears set to repeat so these pages are pertinent again.

Since Gerstley Borate has a high LOI and it seems logical that it would tend to create bubbles and imperfections in low fire glazes. However in practice it is amazing how transparent and crystal clear some of the glazes can be. It is even common to see Gerstley Borate in low fire glazes that also contain significant amounts of boron frit. Boraq 1 is highly likely to work well in these. In addition it will lower the thermal expansion of glazes slightly making crazing less likely or less severe.

INSIGHT 5x Comparison for 2826B

This recipe has a 60% frit mixture, 20% GB, along with a little wollastonite and 15% kaolin to suspend the slurry. It has some suspended bubbles in the glass and crazing is not too severe (so as not to overly weaken the pieces).

2826B1 06-05 Raku Base with GB		2826B2 06-05 Raku Base with Boraq 1
Unity Formula Analysis Mole% CaO 0.72 14.4 18.0 MgO 0.05 0.7 1.2 K2O 0.01 0.5 0.3 Na2O 0.22 4.8 5.4 Al2O3 0.33 11.9 8.1 B2O3 0.61 15.2 15.2 SiO2 2.07 44.5 51.6 LOI 7.9	This closeup shows crazing lines. Glaze is transparent and shows body color through.	Unity Formula Analysis Mole% CaO 0.73 13.6 16.9 MgO 0.03 0.4 0.8 K2O 0.02 0.5 0.4 Na2O 0.22 4.5 5.1 Al2O3 0.36 12.2 8.3 B2O3 0.70 16.3 16.3 SiO2 2.25 44.9 52.1 LOI 7.2	Glaze is transparent and shows body color through.

Nepheline Syenite:GB Raku Base

While there are many variations on this base, the 50:50 GB:Nepheline mix is typical. We have seen versions all the way to 80% GB and 20% Nepheline. This range of blends melts very well at cone 06 and most are remarkably stable and bubble free. The higher the nepheline syenite the greater the likelihood of crazing (a desired effect in many raku applications). For mixes toward the 80:20 end we suggest adding some silica, perhaps 5-10% to reduce the crazing if excessive. This base is a good demonstration of the effect of the lower LOI and slightly better melting of Boraq 1. The Boraq version is clearer with fewer suspended bubbles and surface dimples and pinholes. The craze pattern is also slightly less severe.

Unity Formula Analysis Mole% CaO 0.51 11.2 15.2 MgO 0.11 1.8 3.3 K2O 0.07 2.5 2.1 Na2O 0.31 7.6 9.3 Al2O3 0.31 12.3 9.2 B2O3 0.53 14.5 15.9 SiO2 1.50 35.2 44.8 LOI 14.8

Unity Formula Analysis Mole% CaO 0.49 9.1 12.3 MgO 0.08 1.1 2.1 K2O 0.09 2.7 2.2 Na2O 0.34 7.0 8.6 Al2O3 0.38 13.1 9.7 B2O3 0.74 17.2 18.8 SiO2 1.82 36.5 46.1 LOI 13.0

A similar version of the glaze employs 80% GB and 20% Nepheline. It melts and flows much better and that above 50:50 mix and fires to a crystal clear glass. We have compared Boraq and GB and found that the Boraq 1 version crazes a little less, goes on thicker and melts to a very similar finish.