Annealing, in glassblowing and lampworking, is heating a piece of glass until its temperature reaches a stress-relief point, that is, a temperature at which the glass is still too hard to deform, but is soft enough for internal stresses to ease. The piece is then allowed to heat-soak until its temperature is even throughout; the time necessary for this varies depending on the type of glass and thickness of the thickest section. The piece is then slowly cooled at a predetermined rate until its temperature is below a critical point, at which it can no longer generate internal stresses, and then the temperature can safely be dropped to room temperature. This relieves the internal stresses, making the glass much more durable. Glass which has not been annealed will crack or even shatter when subjected to a relatively small temperature change or other shock.

See also

• Annealing (metallurgy)

Glass | Industrial processes