Gerstley Borate is a sodium-calcium-borate compound used in ceramics as a low and mid-range temperature flux. It is essentially a source of boron in glazes, and acts as a melting agent. It also can act somewhat as an opacifier and prevent crazing. It is a conglomerate mineral made up mainly of Colemanite, Ulexite and Hectorite. Gerstley borate is the substitute for Colemanite which has been gone for many years.
US Borax Corp, the only source of Gerstley Borate in the world closed the mine in southern California in 2000. After several years of without a good substitute, Laguna Clay Company negotiated with US Borax to purchase the remaining unprocessed 'boulders' of Gerstley Borate from the closed mine. Laguna Clay Co. was able to process the Gerstley Borate for consumption by the pottery industry and continues to be the only source for Gerstley Borate.
Synonyms: Colemanite, Calcium Borate