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PV Clay:Gerstley Borate Cone 6 Glazes

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.

The Plastic Vitrox : Gerstley Borate combination was very popular and used for many years as a base glossy glaze for cone 6. This recipe doubtless originated in California since PV Clay is mined there and was in common use in the 1970s and earlier (i.e. the California Artware casting body contained PV Clay). Since this recipe is comprised entirely of clay-containing minerals it shrinks and cracks off ware when applied too thick (actually, it is hard to understand how it is even possible to apply it without cracking). On the other hand it makes a good slip to apply to leather hard ware since it will shrink with the clay as it dries.

To create a white from this most people add 10-20% zirconium silicate.

Using Boraq opens the option of leaving out the Hectorite to reduce the drying shrinkage.

INSIGHT 5.3 Comparison Report

By Tony Hansen Follow me on