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The GLFL test ball of pure Gerstley Borate has shrunk and vitrified to a porcelain state here at 1550F (the ball has shrunk to half the original size and gets even smaller by 1600F). Gerstley Borate has a significant LOI, it finishes off-gassing at about 1400F, which enables this high shrinkage that happens between 1350 and 1600F. Gillespie Borate, on the other hand, is obviously experiencing an overlap between the gassing and melting phases. What does that mean? It is already melting while gasses of decomposition are being expelled. Glazes having a high percentage of it are going to do this as they are heated through this range in a firing. It was not clear at first how this might affect glazes but it became evident later: Crawling.
This is a variation on the 50:30:20 cone 6 pottery glaze recipe, it contains 22% Gillespie Borate (GB) and 12% calcined kaolin. Our objective was to reduce its melt fluidity. But the crawling is so bad that it is almost unusable. The reason was not obvious until we fired a sample to 1550F and 1650F. At the former the integrity of the glaze layer is great, but by 1650F it does this (many of the edges of these are curling upward). Ulexite, which GB contains, is known for the behaviour of suddenly shrinking and then suddenly melting over a narrow range of temperatures. Since GB is plastic and suspends slurries well we thought calcined kaolin would be better than raw kaolin (to minimize drying shrinkage). However, the improvement is minimal.
Materials |
Gillespie Borate
A Gerstley Borate substitute that became available during the early 2000s and is still available in 2023. |
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