Gerstley Borate - Natural
Gerstley Borate is not a mineral, but an aggregate of minerals and therefore it has qualities that cannot be duplicated by Frits. GB form in evaporite basins where water and sediments collect. It’s used in glazes as a secondary flux at mid-range and often as primary flux for low fire and Raku glazes. GB is popular in middle-temperature glazes because its high melt fluidity produced 'hare's fur', (textured and varied colours). These visual effects are remnants of the interactions between the phases of the glaze melt.
On a material level, Gerstley Borate is a light tan colour and quite plastic. On an oxide level, Gerstley Borate is an inexpensive and generous contributor of boron (boron is the magic behind almost all low fire glazes).
A calcium borate containing the minerals colemanite and ulexite.