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URL: Ovo Perfect Matte glaze recipe at Glazy.org
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This glaze is not working with Gillespie Borate. What to do?
These two mugs employ the same cone 6 pottery glaze recipe, the high-feldspar calcium matte (Ovo Perfect Matte). Like other mattes, it is high in calcium carbonate/wollastonite and kaolin but has no silica. But the one on the left has 13% Gerstley Borate while the one on the right uses Gillespie Borate. Gerstley Borate is a complex material, one that Mother Nature has uniquely endowed. It is a brown powder, a mix of two calcium borate minerals, ulexite and colemanite. And it is plastic, very plastic, from a hyper-fine particled clay (likely hectorite). And trace minerals. Gillespie Borate, by contrast, is a white powder, a synthetic blend attempting to replicate the obvious melting and physical properties of Gerstley Borate. It has, what some call, "a cleaner chemistry", enabling it to enhance rather than muddy whatever colorants are present. Any borate can melt well and foster crystallization, but Gerstley Borate is a mix of two borates that have different melting temperatures and patterns, this encourages phase separation and thus variegation in the aesthetic (its sub-micron clay particles may also act as catalysts).
What could be done? Add some iron to dirty-up the material, if well dispersed in the slurry, 0.25% added to the recipe, might be sufficient. While Gillespie also has MgO, it might not be in the same form. A 1-2% addition of magnesium carbonate could help. And a small percentage of hectorite would provide some super-fine particles (with MgO).
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