Several things are needed for high silica glazes to crystallize as they cool. First a sufficiently fluid melt in which molecules can be mobile enough to assume their preferred connections. Second, cooling slowly enough to give them time to do this. Third, the slow cooling needs to occur at the temperature at which this best happens. Silica is highly crystallizable, melts of pure silica must be cooled very quickly to prevent crystallization. But Al2O3, and other oxides, disrupt the silicate hexagonal structure, making the glaze more resistant to crystallization.
The Chemistry, Physics and Manufacturing of Glaze Frits
A detailed discussion of the oxides and their purposes, crystallization, phase separation, thermal expansion, solubility, opacity, matteness, batching, melting.
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.
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|