What is "devitrification"

Devitrification \De*vit`ri*fi*ca"tion\, n. The act or process of devitrifying, or the state of being devitrified. Specifically, the conversion of molten glassy matter into a stony mass by slow cooling, the result being the formation of crystallites, microbites, etc., in the glassy base, which are then called devitrification products.

n. The formation of small crystals in a glass as a result of slow cooling from the molten state

Devitrification is the process of crystallization in a formerly crystal-free ( amorphous) glass. The term is derived from the Latin vitreus, meaning glassy and transparent.