Gillespie Borate

This substitute became available during the early 2000s and won the attention of many Gerstley Borate (GB) users (it is still available in 2023 and they state readiness to take the market from GB). With Gillespie Borate they claim to achieve chemistry, mineralogy and physical properties very similar to GB. They went well beyond what other companies did in explaining their research, development and testing (however their QuickFacts web page is now gone). Of course, it is assumed this material does not turn glaze slurries into a bucket of jelly the way GB did.

Here is a summary of some of the claims they were making:

"Gillespie Borate is a blended borate mineral for use in glaze formulas replacing Gerstley Borate on a pound-for-pound basis. It has better consistency, gives increased glaze surface smoothness, reduces crawling and pinholes and has fewer impurities so glazes and colors are brighter."

By virtue of having the same mineral profile, their product variegated glaze surfaces the same way. They did x-ray diffraction and physical tests to fully characterize GB and identified three key characteristics: extended particle size distribution including a colloid fraction, the oxide analysis, and firing properties that promote variegation. They identified immiscibility and solubility as key characteristics, the former generating the characteristic variegated texture and the latter the mottled glaze surface (via migration of soluble oxides). They emphasized that theirs was a consistent, reliable raw material with fewer impurities to throw colors off.

Their recommendations for using it were:
- Do not use excess water in mixing the glaze.
- Store the wet glaze in an airtight container.
- Always test first it in a small batch.
- Test on the same clay body you will use in production.
- Test on vertical tiles placed throughout the kiln.

As to the recipe, they hinted it was "comprised principally of ulexite (sodium calcium borate), with small amounts of colemanite (calcium borate) and approximately 10% colloidal clay-like materials". They claimed to add "various clay minerals" (vs Boraq that only used hectorite). In addition, they said the product contained "alkaline earth carbonates and silicates" (like talc and calcium carbonate). We got an exact analysis early but they later said the product was "approximately 25% B₂O₃, 23% CaO, 4% total alkali and 31% loss on ignition". For some reason they did not state the amount of SiO₂, it would have to be at least 10%.

It should be noted that the melting properties of Gerstley Borate were a product of its being a mix of ulexite and colemanite. They melt at distincly separate temperatures, this is an important factor in the material's contribution to variegation. It is uncertain how well Gillespie Borate duplicates this aspect.