Thermal Tempering
In the toughening process, the surfaces of the glass are heated in a furnace. Recommended temperatures vary but the glass reaches temperatures about 600°C - 700°C. The hot glass is then cooled rapidly by a blast of air over a period of between 10 and 30 seconds. As a result, the surfaces shrink, and at first tensile stresses develop on the sides and goes to the center of the glass.