Annealing is a process by which glass is cooled to relieve internal stresses after it was formed. The process is carried out in a temperature-controlled kiln called a Lehr. Annealing of glass is critical to its durability and overall strength. Glass which has not been annealed is liable to crack or shatter when subjected to a slight change in temperature or mechanical shock.
Glass is heated until the temperature reaches a stress-relief point or annealing point. This is the temperature at which glass is still too hard to deform but soft enough to relax the internal stresses. The piece is then allowed to heat-soak until its temperature is even throughout. The time needed for this step varies depending on the type of glass and its maximum thickness. The glass is then cooled slowly. After the annealing process, the material can be cut to size, drilled or polished.