NNP

Work Harden.  A tempering of a metallic piece caused by any form of stress work. This condition of metalworking must be taken into consideration with every step in the manufacture of a coin or medal, from blanking, through upsetting, striking, coining and edgelettering. Press work, in particular, severely work hardens the struck piece. In striking high relief medals this is so critical, a work-hardened piece must be softened before any other metalworking step – including another blow – can be attempted. The softening is done by annealing, heating and rapid cooling of the work-hardened piece.

The goal of press work is to move the maximum amount of surface metal to fill as

much of the die cavities as possible. The greatest stress occurs adjacent to the areas of steep-pitched relief or highest relief where surface metal had to travel the greatest distance to fill the cavities. Molecules in these areas, where the stress is the greatest, become set in their molecular arrangement after each blow. This precludes any further molecular movement until the struck piece is annealed; the annealing realigns the molecular arrangement. The piece could then be struck again.

Work hardening is a benefit in coining. With one blow the blank is formed into the fully struck piece, filling every die cavity and forming the edge; it also work hardens the completed piece so that the resulting coin is harder after being struck than it was as a blank. For a coin this work hardened state is excellent to help reduce wear during circulation.

On the other hand, art medals or any struck piece with high relief requiring multiple striking, work hardening demands that after every blow the work-hardened partially-struck piece must be annealed. In actual practice a high relief medal which requrires, say, eight blows would require annealing seven times, after each blow except for the last.

Ironically the effect of work hardened stress may be seen with the naked eye on certain white metal objects. Because of the physical properties of tin certain medals made of white metal exhibit a reverse shadow which does not tone; it remains bright while the remainder of the surface darkens. This is work hardened stress and exhibits on a struck piece exactly where the stress was the greatest and where work hardening occurs.

See also ANNEALING, HEAT TREATING.

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators

COMPILED AND WRITTEN BY D. WAYNE JOHNSON

Roger W. Burdette, Editor