Glass tempering, also known as glass toughening is a thermal process whereby the ordinary float glass is heated to extremely high temperatures before being rapidly cooled. This heating and rapid cooling process causes the centre and surface of the glass to cool at different rates. The surface of the glass thus goes into compression, whist the centre of the glass is under tension.
This process of thermal toughening causes the glass to be around 4 times stronger than ordinary glass. Another benefit is that the glass also becomes safer if it does break. This is because if the toughened glass were to break, it would break in many small, relatively harmless pieces. Ordinary glass, when broken, can cause injury as it can break into jagged shards of glass.