Colemanite
43.9% B2O3
26% CaO
4.5% SiO2

A naturally occurring source of Boron. A powerful primary or secondary flux of 4.5% SiO2 in glazes. It intensifies the effect of colouring oxides and increases the craze resistance of glazes. P2954 (Calcium Borate Frit) may be used as an alternative.