These cone 04 glazes both have 50% Gerstley Borate. The other 50% in the one on the left is PV Clay, a very low melting plastic feldspar. On the right, the other 50% is silica and kaolin, both very refractory materials. Yet the glaze on the right is melting far better. How is that possible? Likely because the silica and kaolin are supplying Al2O3 and SiO2, exactly the oxides that Gerstley Borate needs to form a good glass.
Glass vs. Crystalline
In ceramics, understanding the difference between what a glass and crystal are provides the basis for understanding the physical presence of glazes and clay bodies.
|Oxides||SiO2 - Silicon Dioxide, Silica|
|Oxides||Al2O3 - Aluminum Oxide, Alumina|