Gerstley Borate, also known as calcium borate, is a sodium-calcium-borate compound that is used as a low- and mid-range glaze flux.

This is a conglomerate mineral composed primarily of Colemanite, Ulexite and Hectorite.

This material is partially soluble and can flocculate glaze slops.

In small amounts, it can prevent cracking in the glaze, known as crazing, and in large amounts, it gives the glaze a milky appearance.