High Quality Content by WIKIPEDIA articles! Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. It is also a technique used to increase the toughness of glass. For metals, tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to a much lower temperature than was used for hardening. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered to much higher temperatures. In glass, tempering is performed by heating the glass and then quickly cooling the surface, increasing the toughness.