Material failure often affects hard and brittle planchets
- Published: Dec 7, 2012, 7 PM
The planchets used for coining must balance hardness and a capacity for plastic deformation.
A planchet that is too hard will not easily conform to the recesses of the die face, leaving parts of the design inadequately defined. This principle is demonstrated in the illustrated 1983-P Jefferson 5-cent coin that was struck on a planchet that was too hard. The center of Jefferson’s face is sunken and featureless, while the center of Monticello (not shown) is also devoid of details.
A planchet that is too soft will wear quickly, rapidly accumulating scratches, dents and other contact marks. This is why small amounts of tin and zinc were added to the copper used for cents from 1864 to 1982. These additional metals made the alloy harder than pure copper.
Hard planchets tend to suffer from the curse of brittleness. A brittle coin can crack, split, crumble or break into pieces. Not all hard materials are brittle, of course. Ordinary glass shatters easily when dropped, but tempered glass is tough (resistant to failure).
By the same token, not all brittle materials are hard. A brittle fortune cookie scratches easily, demonstrating that the baked dough is stiff, but not particularly hard.
Most planchets are heat-softened (annealed) before being introduced into the striking chamber. This allows the metal to flow more easily into the recesses of the die face. It also extends die life.
The impact of the dies hardens the coin once again. This effect is called work-hardening and it extends the life of a coin. Copper-plated zinc cents provide an exception to these general rules. Copper-plated zinc planchets are not annealed and they do not experience work-hardening.
For other coins, though, if a planchet is inadequately annealed, or skips the annealing oven entirely, it will be abnormally hard and brittle. Brittleness in the absence of increased hardness can be caused by a heavy load of contaminants, trapped gas bubbles or the presence of micro-cracks.
One sign of brittleness is the formation of radial splits when a coin is struck out of collar (broadstruck). The illustrated 1994-P Roosevelt dime displays seven radial splits. As this is a minor broadstrike, it’s unlikely that tensile stresses alone are responsible for the splits. The planchet most likely bypassed the annealing oven.
Another sign of brittleness is a tendency to crumble at the surface or along the edges. The illustrated 1980-D Lincoln cent shows an obverse face that has crumbled in many areas. The reverse face (not shown) shows no evidence of brittle failure. In this case the likely culprit is the presence of contaminants in the alloy.
A brittle coin or planchet can split along the horizontal plane, respectively producing a split-after-strike error and split-before-strike error. Such errors are usually attributed to the presence of contaminants in the alloy.
Brittle coins and planchets can also crack, producing “cracked planchet” errors. The illustrated Winged Liberty Head dime displays a long crack that extends through the right side of the coin. Weakness in adjacent lettering indicates that the planchet crack was present before the strike.
A crack that propagates from one point on the rim to another will produce a broken planchet or broken coin error. The accompanying 1967 Lincoln cent lost a significant portion of its mass after the strike. It would therefore be classified as a broken coin.
Our final illustrated coin combines several forms of brittle failure. This 1962 Jefferson 5-cent coin was struck out of collar. The left side broke away immediately after the strike. On the right side, the edge is crumbling. Numerous radial splits are also visible.
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