The processing of flat glass or glass products by physical (thermal) or chemical methods to generate permanent stress uniformly distributed in the glass to obtain high strength and high thermal stability, is called glass tempering.
The tempering process of flat glass can be divided into the physical (thermal) tempering process and the chemical tempering process. Usually, tempered glass is made from annealed glass via a thermal tempering process, in which the glass is heated to a certain temperature, and then rapidly cooled at the required rate, producing permanent stress inside the glass, hence improving the mechanical properties of the silicate glass. An alternative chemical toughening process involves forcing a surface layer of glass at least 0.1 mm thick into compression by ion exchange of the sodium ions in the glass surface with potassium ions (which are 30% larger), by immersion of the glass into a bath of molten potassium nitrate. The chemical tempering method has a good performance on thin glass and ultra-thin glass.
