Under natural conditions, the surface of molten glass will cool more rapidly than the centre. This results in internal stresses which may cause the glass sheet or object to crack, shatter or even explode some time later.
The annealing process is designed to eliminate or limit such stresses by submitting the glass to strictly controlled cooling in a special oven known as a 'lehr'. Inside the lehr, the glass is allowed to cool to a temperature known as the 'annealing point'. When the glass reaches this point, the lehr temperature is stabilized for a specific length of time (depending on the glass type, its thickness, its coefficient of expansion and the amount of residual stress required) to allow stresses present in the glass to relax. This phase is followed by a period of cooling with a pre-defined temperature gradient.