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.