Devitrification

Also known as "devit." A deterioration process in which crystals form in the glass, or appear as a fog or scum on the surface and give a dull appearance. A fault of the manufacturing process when glass is heated incompletely or unevenly. Also caused by chemical impurities on the surface, and "cold working". Crystallization occurs when glass in held somewhat below its liquidus temperature, about 1400 degrees F. for most glass.