Gerstley Borate is no longer being mined, the amount that we have in stock is the last of what is avaliable to us. Gillespie Borate is the substitute for this Gerstley Borate, when we run out we will NO LONGER HAVE ANY TO SELL.
No common natural material comes anywhere close to melting like Gerstley Borate (GB). It begins to melt between 1550℉ and 1600℉ and is a clear amber glass by 1750℉ and ultraclear and glossy by cone 06 (Ulexite melts better but it is not commonly in use in ceramics). It has thus been a staple among potters for many years. 50% can be found in many cone 06-02 glazes, up to 30% in cone 6 glazes. Gerstley Borate is also very plastic and thus suspends and hardens glazes as they dry. In fact, few clays have the plasticity and the ability to retain water that GB has. A GB slurry can take many hours to dewater on a plaster batt, even in a very thick layer. Thus it is common to find Gerstley-Borate-based recipes having no clay content.