Temper, Tempering. Strengthening metal for hardness and toughness. Heat treating changes the hardness of metal, softening steel, for example, to cut or recut a die, or hardening and toughing – tempering – steel so dies can be used in a press for striking. Tempering is accomplished by heating the steel dies to a high degree of temperature, then rapid cooling. Modern heat treating furnaces have temperature control; set a gauge and the steel can be heated to a known temperature. Previously temperature was determined in open furnaces by the color of the heated steel, from straw yellow at 450° to very dark blue at 600°. The cooling is accomplished by quenching – plunging the die into water or oil (thus the terms water quenched steel or oil quenched steel). Drawing the temper or simply drawing, is a repeated process of heating and quenching again. Such repeated tempering can achieve a high degree of hardness (this is how sword blades are hardened for example). Testing the hardness is done with a hardness tester (as a Brinnel gauge, among others), by applying a probe to the metal surface to be tested. See heat treating.