Tempered glass starts life as float glass. Float glass, when shattered breaks into large pieces and can be harmful; it is therefore unsuitable for applications. In the tempering process, the surfaces of the glass are heated in a furnace nearly over 670°C.
The hot glass is then cooled rapidly by a blast of air over a period of between 3 and 10 seconds. As a result, the surfaces shrink, and tensile stresses develop on the surfaces.