The capability of transforming molten glassy matter into a stony mass by slowly cooling: The result of this devitrifiable process is the formation of crystallites, microbites, etc, in the glassy base, which are then called devitrification products.

The procedure involves devitrifiable glass becoming partly crystallized as it cools very slowly from the molten state.

Devitrification may also occur on the surface as a result of unsuccessful "annealing"* or accidental heating to a high temperature. It is not caused by chemical reaction between glass and its environment, which is known as "weathering".

*Annealing refers to the process of slowly cooling a completed object in an auxiliary part of a glass furnace, or in a separate furnace.

This is an integral part of glassmaking because if a hot glass object is allowed to cool too quickly, it will be highly strained by the time it reaches room temperature and indeed, it may break as it cools. Highly strained glasses break easily if subjected to mechanical or thermal shock.