Devitrification of glass

Devitrification is crystallisation in glass, a material with an initially amorphous structure. Some or all of the material can become crystalline. Store norske leksikon explains that this can reduce transparency. The term describes a structural change, rather than a particular glassworks, period style or form of decoration.

The Corning Museum of Glass explains that crystals may form as glass cools from the melt, and that devitrification can also result from unsuitable heat treatment. The museum distinguishes this from chemical deterioration caused by reactions between glass and its environment.

When assessing older glass, describe the visible evidence before assigning a cause. Cloudiness alone is insufficient to diagnose devitrification. Record whether the change appears at the surface or within the material and document it with photographs. Do not treat the term as automatic evidence of great age, authenticity or a particular manufacturer.

Sources: Corning Museum of Glass: Devitrification; Store norske leksikon: devitrifikasjon.