Calcium borate

Alternate Names: Colemanite, Calcium Borate, Borocalcite

Description: Plastic Calcium Borate

Calcium borate, otherwise known as Gerstley borate, supplies B2O3, CaO, other fluxes, SiO2, and Al2O3. Gerstley borate is a calcium borate ore containing the mineral Ulexite ( 3CaO Na2O 5B2O3 16H2O ), along with accessory minerals such as Bentonite, Colemanite, and Probertite. Gerstley borate was formed in marshes and drying lake basins during the Miocene (10 million years ago).
Gerstley borate is named after James Gerstley, former managing director of Borax Consolidated Ltd. It is manufactured by U.S. Borax and Chemical Corp.; the mine is located near Death Valley, CA.
G.B. is an inexpensive source of B2O3 and CaO in glazes. It is often used in place of colemanite. The primary differences are:

1. Colemanite contains a higher concentration of B2O3,
2. G.B. contains Na2O, which may affect the color and surface of some glazes, and
3. the Bentonite component of G.B. – up to 18% – significantly affects the properties of the raw glaze. (Some other sources indicate that this is not bentonite, but rather shale. GB glazes do tend to gel in either case.)

Other points from IMC:

1. GB is slightly soluble and tends to gel the glaze, necessitating excessive water and resulting in greater shrinkage of the raw glaze, which leads to crawling and other problems, especially when there is also a lot of clay in the glaze. Epsom salts (4g per 100g GB) can help. 2) The chemical and physical properties of the material (composition, mesh size, etc.) change over time. 3) GB can contain gypsum and lead to lime popping.

Linda Arbuckle’s Ceramic Materials Guide:

Substitute for colemanite. Named after a Calif. man who operated a colemanite mine. Boron is both flux and viscosity agent. Thixotropic. Mining ceased in 1999.

Analyses:
Axner: (web site, 2001)
% 4.5 Na2O 0.25 K2O 3.5 MgO 22.0 CaO 1.0 Al2O3 29 B2O3 0.2 Fe2O3 10.0 SiO2 29.0 LOI
Obstler: (Hammill & Gillespie)(also includes 6.2 LOD whatever that is)
% 3.6-5.3 Na2O 2.9 MgO 17.1-20.6 CaO 1.3 Al2O3 25-28.9 B2O3 0.35 Fe2O3 10.6 SO2 30.5 LOI
Hopper:
2 CaO Na2O 3 B2O3 7 H2O
Cushing:
CaO B2O3 5 H2O
IMC:
% 9.31 Na2O 16.85 CaO 52.25 B2O3 21.59 LOI

ClayArt from Axner:

This product is composed of a mixture of the minerals Colemanite and Ulexite. This material contains approximately 28.0% Boric Oxide (B2O3), 5.3% Sodium Oxide (Na2O), 20.6 Calcium oxide (CaO) and 25% water of crystallization.