| Description |
A natural source of boron that was mined in southern California for many years. No common natural material comes anywhere close to melting like Gerstley Borate (GB), it is completely fluid, ultraclear and glossy at cone 06. 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 plastic and suspends and hardens glazes, so it is common to find GB based recipes having no clay content. Since GB based 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). High GB glazes often have alot of micro-bubbles in the fired glass and micro-dimples on the fired glaze surface (most visible in transparents). Slurries also tend to gel and flocculate, causing problems with glaze application, drying and adhesion. |