SKH-1c-1991-02 Spiked Head

Description

The metallurgy of die steel plays a central role in spiked head formation, and SKH-1c-1991-02 of the 1991 Lincoln Memorial cent — reported on June 18, 2016 — exemplifies how material science intersects with numismatic variety creation. U.S. Mint coining dies are forged from tool steel alloys engineered to withstand millions of high-pressure impacts, yet the very hardness that gives these dies their longevity also makes them brittle at stress concentration points. At the boundary where Lincoln's deeply sculpted portrait meets the flat field, the abrupt change in die surface geometry creates a stress riser — a zone where mechanical forces concentrate rather than distributing evenly. Thermal cycling compounds the problem: each strike generates localized heat, and the repeated expansion and contraction of the die steel at this geometrically abrupt transition eventually nucleates micro-cracks that grow with continued use. The 1991 Philadelphia cent's 19 documented spiked heads suggest that the die steel used that year, or the specific hubbing parameters employed, may have been particularly susceptible to this boundary-zone cracking. Struck at Philadelphia on copper-plated zinc planchets composed of 97.5% zinc and 2.5% copper, weighing 2.5 grams and measuring 19mm.

Attribution History

• Discovered by Earl M
• Cross reference: none. 06/18/2016.
• Expert attribution by Cuds on Coins

External References