Tempering is a process of thermal hardening performed on glass after all other processes that pierce the surface or substantially modify the edges have been carried out, such as grinding and drilling. The glass is placed in a furnace at 640/670 °C until it reaches its softening point. It is then quenched rapidly with air. This rapid lowering of the temperature, which starts externally, creates compression force on the interior, which is however balanced by the internal tension. Tempered glass has a tensile strength that is approximately four times greater than that of untreated float glass.
The thermally toughened glass undergoes a process which increases its resistance to thermal and mechanical stress: the glass is heated to the core temperature of more than 600°C and then gradually cooled.
The bending resistance is almost half of the tempered one, but the superficial distortion is lower; for this reason, this type of glass is suitable for applications in which it is necessary to avoid thermal shock effects. It does not belong to the category of safety glasses: in case of break, the glass shatters into large fragments.