Tempered glassThere are two tempering methods:
- Thermal tempering: the glass is heated almost until it softens (approx. 600°), and its surface is then rapidly cooled (by air blast or cold bath). This creates tension stresses at the bottom and compression stresses at the surface. Tempering gives the glass excellent shock resistance properties (impact or thermal shock). Tempered glass is very resistant but not unbreakable; it is particularly fragile at the edges. If it breaks, it shatters into a multitude of small, sharp pieces: the risk of injury is then very low. The glass must be cut and shaped before tempering.
- Chemical tempering: This is an ion exchange between the glass and a hot molten salt bath (potassium ions replace sodium ions); tensions are created on the surface of the glass as it cools. The mechanical resistance of the glass is thus improved. The process is more expensive than thermal tempering.