A mixture of colemanite, ulexite and clay-like minerals, Gerstley borate is a naturally occurring source of boric oxide, a glass former which promotes early fusion. It is no longer being mined but significant supplies remain available. Good blended substitutes and borosilicate frits with similar, early melt properties are also available.
For more information on Gerstley borate, see the Digitalfire.com Reference Library of ceramic materials.