Annealing is a process of slowly cooling hot glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a lehr. Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to its durability. If glass is not annealed, it will retain many of the thermal stresses caused by quenching and significantly decrease the overall strength of the glass.
The glass is heated until the temperature reaches a stress-relief point, that is, the annealing temperature (also called annealing point) at a viscosity, η, of 1013Poise ("Poise" is a measure of absolute viscosity; 1 poise = 1 dyne-second/cm²), at which condition the glass is still too hard to deform, but is soft enough for the stresses to relax. The piece is then allowed to heat-soak until its temperature is even throughout (this annealing temperature is usually in the range of 850–900 °F). The time necessary for this step varies depending on the type of glass and its maximum thickness. The glass is then slowly cooled at a predetermined rate until its temperature is below the strain point (η = 1014.5 Poise). Following this, the temperature can safely be dropped to room temperature at a rate limited by the heat capacity, thickness, thermal conductivity, and thermal expansion coefficient of the glass. After the annealing process the material can be cut to size, drilled or polished.
At the annealing point (η = 1013 Poise) stresses relax within several minutes, while at the strain point (η = 1014.5 Poise) stresses relax within several hours. Stresses acquired during and remaining from temperatures above the strain point are permanent unless annealed and may lead to short-term or much delayed failures. Stresses acquired during cooling below the strain point are considered temporary, although they may be adequate to cause short-term failure.