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Devitrification

The term “super-cooled liquid” refers to the fact that, at least from a thermodynamic equilibrium point of view, quartz glass should actually be a crystalline solid rather than a liquid. This fact is the key to understanding why quartz glass devitrifies. Although the thermodynamically preferred state of quartz glass is crystalline, the high viscosity prevents the structural rearrangement necessary to achieve it. In other words the molecules cannot arrange themselves quickly enough compared to the relatively fast rate of cooling that quartz glass normally experiences. However, under certain conditions this constraint can be removed resulting in the glass reverting to a crystalline state. This usually happens at elevated temperature at the presence of a contaminant that drops the viscosity by breaking up the highly connected silicon-oxygen network as well as acting as a nucleating source. Alkalis like sodium or potassium are the most common contaminants that cause devitrification.
Atmospheres high in water vapor or chlorine also exacerbate this process significantly. The growth of the devitrified layer usually starts in the surface and progresses into the material at a rate that depends exponentially on temperature. The crystalline material formed is a high temperature form of silica known as high cristobalite.
Devitrification
High cristobalite has nearly the same density as glassy silica and thus cannot be seen on the surface. However, upon cooling, high cristobalite undergoes a structural change from a cubic to a tetragonal crystal structure at about 275 °C. this is accompanied by a large decrease in density that can result in some cracking and spalling. Refractive index differences resulting from the birefringent tetragonal crystal structure also cause the devitrified spots to turn white.