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In ceramics, cristobalite is a crystalline form of silica formed in the matrix of clay bodies as they fire in the kiln. Silica can also exist as quartz (the most common) and tridymite.
Cristobalite is a crystalline form of silica. Silica normally exists in nature as quartz. But cristobalite is available as a powder, it is manufactured by mixing quartz with the right fluxes/catalysts and calcined to the necessary temperature (1100C+), then cooling it quickly. But such a material is not useful in traditional ceramics since it would convert back to quartz on heating. However, quartz particles in a clay body, such as a whiteware that does not fire too vitreous, find themselves in a mix of clays and fluxes that provides an ideal environment for the conversion to cristobalite. The firing temperature, fineness of the quartz particles (finer is better) and soaking time determine the completeness of the conversion.
Cristobalite at Wikipedia
Article about cristobalite in clay bodies
In ceramics, cristobalite is a form (polymorph) of silica. During firing quartz particles in porcelain can convert to cristobalite. This has implications on the thermal expansion of the fired matrix.