Gillespie Borate - Gerstley Borate Substitute
A very close substitute for Gerstley Borate that is close to extinction. Gillespie Borate is a naturally occurring source of boric oxide, promoting early fusion in glazes. It's a blend of the same minerals proportioned to preserve the chemistry and working properties of Gerstley Borate. Use it as a direct 1:1 substitute. Testing in your glazes when substituting for Gerstley Borate is recommended as there are some subtle differences and impurities that could cause color shifting.
For more information on Gillespie Borate borate, see the Digitalfire.com Reference Library of ceramic materials.
Used as a substitute for: Gerstley Borate, Colemanite, and Calcium Borate