Devitrification
Devitrification is the formation of crystals on or in glass when it is held in a particular temperature range in the kiln. It shows as a rough, cloudy, whitish or scummy surface and cannot be removed by polishing, only by grinding or re-firing with a coating of clear glass.
It is more likely when the glass surface is dirty or dusty, when it is held for a long time between roughly 650 and 800°C, or when certain colours, especially some opal and bright reds and oranges, are used. Clean glass, a short time in the risk range, and a coating of devitrification-resistant spray all reduce it.
It is one of the commonest faults in fused glass.