CORE COINING TECHNOLOGY: DIE HEAT TREATING

CORE TECHNOLOGY YOU NEVER HEARD OF

If I asked 100 numismatists to list the most important technologies in the making of coins and medals -- the core knowledge -- I'll wager not one would mention Heat Treating (unless, of course he worked in the metal working field like, say, Craig Sholley).

Yet we would not have any coins and medals were it not for Heat Treating. This holds true for dies. Dies must be made of steel; they need to be softened to cut or impressed an image, then hardened for use in striking. Often, more so for medals, they need to be resoftened -- as to change some lettering, or a date -- then rehardened and placed back into production.

Fortunately, iron and steel can be softened and hardened at our will. Mankind has learned to abide by nature's rules We're thankful for the Iron Age, where we have had 4000 years of experience, and for early blacksmiths who first learned how to use that iron to their advantage.

Early die makers knew how to employ iron technology. They were making dies 600 years BC and making hubs 530 years BC. All by the knowledgeable use of Heat Treating, basically conditioning dies by heating and cooling.

This hardening of dies -- by heating and cooling -- must by closely controlled for time and temperature. The procedure requires a vat of hot molten salt in which the die is immersed. It is removed only when it reaches a certain temperature. Old timers could tell the temperature by the color of the steel, it was ready when it was a straw color. Modern equipment has an indicating pyrometer to reveal tan exact temperature.

Out of the vat, quickly immersed into cold water (or oil for oil hardened steel). It must be allowed to return to room temperature slowly on its own, called "normalizing."

A friend, Paul, tells of a client who had him cut a Columbus medal The client was late getting the medal into production. Paul heat treated the die, but the client demanded custody of the die immediately.. It was winter,. He took the die outside, but was back within days. It broke on first strike because it was not normalized properly.

Terms of annealing, quenching, tempering, drawing are used in heat treating in addition to normalizing. Tempering, for example, is to harden a die deeper into the internal metal mass for greater strength. Time and temperature requires even more tight control. Workers in the heat treating department have to know what they are doing.

Hardness is tested with a hardness tester. Brinnel and Rockwell are the two most popular. They employ an "indenter" a hard steel ball or diamond point to press into the metal being tested. A gauge reveals the pressure required to indent the surface.

Ironically, for large art medals which require multiple striking to bring up a high relief, both bronze and silver, react to a heating and slow cooling -- the opposite of steel -- to soften them for a subsequent blow. These metals become "work hardened" after one or two blows. Further striking would not move any more surface metal. You must know nature's rules.

Numismatists should know this core knowledge.

Wayne Homren, Editor

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