|Monthly Tech-Tip |
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 (to a lesser extent of course) 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.
A Gerstley Borate substitute that became available during the early 2000s and is still available in 2023.