Alternate Names: Colemanite, Calcium Borate, Borocalcite, Gillespie Borate
A sodium-calcium-borate compound used as a low temperature flux which helps prevent crazing. Can act somewhat as an opacifier.
No common natural material comes anywhere close to melting like Gerstley Borate (GB). It begins to melt between 840°C and 870°C and is a clear amber glass by 950°C 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.
GB natural source of boron that was mined in southern California for many years. Mineralogically it is a combination of colemanite, ulexite and high plasticity clay (likely hectorite). The melting behaviour of ulexite and colemanite is quite different, the unusual early melting behaviour GB exhibits this, it suddenly implodes to a brown opaque melt (because of the earlier fluxing of ulexite) which later turns transparent (when the colemanite joins in).
Since GB glazes melt well and are so easy to make, most people have overlooked issues surrounding its use. Glazes with high GB content that host potentially toxic metallic colorants or other materials are often assumed to be non-leachable because they melt well (where as, in fact, they may have an unbalanced chemistry). Gerstley Borate has almost no Al2O3, this is a problem because glazes need it and Al2O3 is normally sourced from clays, especially kaolin. But since GB is so plastic, adding more plastic materials to a glaze causes excessive drying shrinkage (producing cracks and ultimately crawling). One solution is to use calcined kaolin. Another option is to source Al2O3 from feldspar, however to get enough to create a stable glass oversupplies KNaO and causes crazing.
High GB glazes often have a lot of micro-bubbles in the fired glass and micro-dimples on the fired glaze surface (most visible in transparents). Slurries also tend to flocculate and gel causing problems with glaze application, drying and adhesion. Because this material melts so well, potters who use it have been willing to endure a lot of these issues. One common low to middle fire transparent, for example, has 50% GB and adds 30% kaolin to that, producing a slurry the dries even more slowly, gels quite badly and shrinks considerably as it dries.