A note on Gerstley Borate
Gerstley Borate (aka colemanite, calcium borate, borocalcite) is a sodium-calcium-borate compound used as a flux in ceramics. Gerstley Borate is a common ingredient in glazes for all firing temperatures. It is essentially a way to introduce boron into glazes and acts as a melting agent. It also helps to prevent crazing. Gerstley borate begins to melt around 1550 F and becomes a clear and glossy glass by around 1828 F (cone 06).
The mine for Gerstley Borate has been depleted and the available supply is unfortunately now extremely limited. As a result, like the majority of glaze manufacturers, we will need to use a substitute in our glazes. We’ll be using a blend of material in our glazes from this point forward and replacing Gerstley Borate with the blend in our raw materials catalog.
The new blended material is a mixture of several different compounds that together recreate the effect that natural Gerstley Borate has in glazes. As with any change in a dry material, however, this may cause a change in how our glazes (and many other commercial glazes) perform. We highly recommend that each individual potter conducts tests to see how this new substitute will affect their personal glazes and work.
~ The SPS team