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Hardening.  Utilizing heat treating to temper or harden metal, as steel dies in coin and medal making; usually accomplished by careful heating and rapid quenching. A knowledge of steel and heat treating is mandatory. Hardening of dies is required prior to striking, it is a recrystalization accomplished by the heating and cooling, closely controlling both time and temperature. The procedure requires a vat of molten salt in which the die is immersed; it is removed only when it reaches the proper temperature (old timers could tell the temperature by the color of the steel, as to remove it when it is straw color); modern equipment has an indicating pyrometer to indicate temperature.

The die is taken out of the vat and quickly immersed in cold water (or oil for oil hardened steels). normalizing is required to allow the die to return to room temperature for a period of time before it can be placed in use. Also, you don't want to take a freshly quenched die outside in freezing winter temperature (with the normalizing not yet fully completed, such foolhardy act would cause the die to crack with the first blow).

Testing the hardness is done with any of several hardness testers, for readings of several scales. Brinnel is popular, as well as Rockwell. The instrument measures the amount of pressure for a slightly rounded point, called the indenter, to dent the surface. Some indenters have a steel ball at the end, others have a diamond point. The size and angle of the apex is important.

The Rockwell scale is an arbitrary number preceded by a letter indicating the weight of the load and the size of the indenter. Rockwell B, for example, indicates a load of 100 kilogram on a 1/16-inch steel ball. B21 is typical for a soft metal, like magnesium, while B90 is a hard alloy like yellow brass. Tool steels can be hard at C55 to C62, soft at C20 to C32; dead soft is any number under 10.

See heat treating.

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators


Roger W. Burdette, Editor

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