Gerstley Borate – Na₂O2CaO5B₂O₃16H₂O (m.p. 1225^o F/660^o C) is a sodium Calcium Borate compound. Its water solubility is minimal and, therefore, it is one of the only quasi-insoluble sources of Boron other than frit. It is widely used as a low-temperature flux and has the advantage of producing low temperature leadless glazes that tend to craze less than other Alkaline varieties. It also tends to advance a milky blue opalescence to a glaze that contains it as flux.