The sheet of glass, whose size and working are defined (finish of the edges, holes, notches), is placed inside a furnace at a temperature of 650°C. The sheet is placed horizontally on rollers or supports and, depending on the time it has to stay inside at a given temperature, it can become cylindrical or spherical due to the pull of gravity. From a temperature of 650°C the sheet is cooled brusquely by air blasted from suitably sized nozzles, arranged symmetrically at a given distance from the two surfaces. This sudden cooling imparts a strong, surface compression balanced by a central traction and freezes a great quantity of energy inside the glass. In the case of breakage this energy transforms into surface energy resulting in a multitude of tiny fragments while a crack runs rapidly along the central zone in traction. All tempering process variables (furnace heating temperatures, blown pressure, transfer speed, etc.) are controlled by a computer to optimise the process and make it repeatable. Surface compression imparts greater mechanical resistance to tempered glass compared to annealed glass and we have minute fragmentation thanks to the energy frozen at a high temperature which turns "tempered" glass into safety glass. Compared to annealed glass, tempered glass has greater mechanical resistance (from 45 N/mm2 to 120 N/mm2 according to the latest European standards) and like safety glass, in the case of breakage, the fragments must be very small and therefore harmless.