Gerstley Borate

Gerstley Borate has been a valuable glaze constituent for many years and is a key part of many interesting and successful glazes over a range of firing conditions.As the supply has dwindled due to closing of the supply source, the Hammill & Gillepsie technical staff went to the lab and began experimenting. Beginning with x-ray diffraction to determine the mineral constitution of Gerstley Borate and following up with other physical and chemical testing, we fully characterized the material and identified three key characteristics:  extended particle size distribution including a colloid fraction, a specific and unique distribution of oxides, and a fired property that promotes a variegated mottled texture in many glaze compositions.

To generate a replacement, we arrived at a new material, Gillespie Borate, that is virtually identical to the original Gerstley Borate in terms of physical, mineralogical and chemical composition. Gillespie Borate is comprised principally of ulexite (sodium calcium borate) with small amounts of colemanite (calcium borate) and other minerals -- just like Gerstley Borate.  From a pre-fired perspective the most critical element the new materials is its colloid content.  Containing approximately 10% clay-like materials, Gillepsie Borate disperses well in water and makes a reasonably stable glaze slip in the neat state.  Further clay addition at the point of use is required to further improve the stability of the slip and the prefired properties such as strength and adherence.  In the fired state, Gillespie Borate is virtually identical to Gerstley Borate by virtue of the chemical match of oxide content.