Part VI. Striking Errors:

Impact-Induced Warping

Definition: A well-accepted principal holds that any part of a coin or planchet that remains between the dies throughout the strike should be flat on both faces.  That makes abundant sense since both die faces are essentially flat.  There is a notable exception, though.  Sometimes, when a coin is greatly thinned by the strike, it develops a warped surface as soon as the hammer die retracts.  The face struck by the hammer die bows upward while the opposite face develops a complementary pucker.

     The buckling of the coin may be due to the heat that is generated by the strike.  Another possibility is that the coin experiences “radial springback”.  In this phenomenon, the coin metal is radially stretched by the strike, but retains some resilience.  When the pressure is lifted, the coin contracts radially, causing the thin metal to buckle.

     Impact-induced warping is most commonly seen and most severe in uniface strikes (coins struck against another planchet).  The aggregate double thickness increases the effective striking pressure and the coin’s final thickness will be half the minimum die clearance.  These two factors produce a thinner coin.

     This 1991 cent takes the form of a cupped, uniface broadstrike.  The planchet represented by this coin was struck against an underlying planchet.  Neither planchet was confined by the collar.  The center of the obverse face bulges upward while the featureless reverse face is strongly concave