Tempering is a process of hardening glass and metals, especially steel. First, the steel is heated to a high temperature. Next, it is quenched (cooled rapidly) by plunging it into water, oil, or other liquid. Then, it is heated again to a temperature lower than that used before quenching it, and is allowed to cool slowly.
Tempering changes the internal structure of the steel. Different uses of steel require different properties, such as varying degrees of hardness, strength, and toughness. To obtain those properties, the structure of steel is changed by tempering it in different gases at various temperatures and for various lengths of time.
Thin films of iron oxide form on steel that is being heated in the tempering process. Those films have different colors, known as temper colors, which vary with the tempering temperature.
Glass is tempered in a somewhat similar way. It is heated until it becomes almost soft, then chilled by blasts of air or by plunging it into oil or other liquids. Glass which has been tempered may be up to five times as hard as ordinary glass. It may be used to hammer nails into wood.
