Toughened glass can be made from annealed glass via a thermal tempering process. Toughened or tempered glass is glass that has undergone processes of controlled thermal treatment to increase its strength
In this process annealed glass is heated above 720°C, which is the highest annealing point and after the extreme heating procedure, rapid cooling is conducted with air. This causes the surface to become hard and contracts while the core still flows for a while. Finally, the core cools into numerous layers under tensile stress, balanced by compressive surface stresses. Because of these stresses, the toughened glass is strong and tough. However even a scratch on the surface can weaken the layer and it may lead to breaking into small pieces.
Due to the increased heat treatment and rapid cooling of the glass, especially between the surface and the inside of the glass, the manufactured glass produces different physical properties.
Before toughening, the glass must be cut to size or pressed to shape. This is because once it is toughened, it cannot be re-worked on.