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URL: Ovo Perfect Matte glaze recipe at Glazy.org
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This glaze not working with Gillespie Borate
These two mugs employ the same cone 6 pottery glaze recipe, the high-feldspar calcium matte (Ovo Perfect Matte). Significant percentages of both wollastonite and calcium carbonate are present. And like other mattes, high in kaolin and no silica. But the one on the left has 13% Gerstley Borate while the one on the right uses Gillespie Borate.
This reactive glaze, like others, depends on the idiosyncrasies of Gerstley Borate. These can be hard to define because it 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 hectorite clay. 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. Gillespie Borate 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 act as catalysts).
What could be done? Add some iron to dirty-up the material, if well dispersed in the slurry a very tiny percentage 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.
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