This is the G2826A 50:30:20 GB:kaolin:silica base clear recipe. It is been used for decades as a base for all kinds of glazes. It starts melting early enough for use on low-temperature earthenware and is widely used in the raku process. Yet it is also common at middle temperatures (obviously care must be taken or it will run off ware onto kiln shelves when fired to cone 5-6). These tests were fired to cone 6 using the PLC6DS schedule.
The samples on the left use Gerstley Borate, on the right Gillespie Borate. The GBMF test tiles (lower left and right) reveal how much off-gassing is still happening on both when melting starts (they are full of bubbles). The GLFL test (centre) shows the melt flow of the two glazes, it is very similar (normal glazes do not run off the end of the runway like this). The two porcelain test tiles show it to fire crystal clear (there is some pooling since these were applied too thick). There is thus good reason to believe that Gillespie Borate will work well in this class of recipes.
Gerstley Borate in matte base glazes at cone 6
Gerstley Borate in transparent base glazes
Gerstley Borate Substitutes
Many development efforts to create Gerstley Borate substitutes took place during the early 2000s (the initial period when the demise of Gerstley Borate appeared imminent). A number of companies, including Laguna Clays itself, produced and sold these for many years. When Laguna secured another stockpile at the mine and began producing the original material again, interest in substitutes gradually waned. However, the sudden dramatic price increase in 2023 appears to have initiated the process again. Gillespie Borate appears to be the only viable and visible substitute now. Thus, the substitutes listed here are mostly no longer made. Other high-boron materials shown are also no longer available. We continue to recommend sourcing B2O3 from frits instead. Please contact us if you have a specific recipe and we can work with you in your Insight-live account to develop a new recipe that both eliminates the GB and improves overall working and firing properties.
Gerstley Borate Glaze Calculation examples
Examples of how we use glaze calculations (in an Insight-Live.com account) to replace Gerstley Borate with other materials, especially frits, in various glaze recipes. In doing so we take the opportunity to improve the recipe in other ways (e.g. reduce thermal expansion, improve slurry properties, reduce bubbling and crawling).