In a discussion of the existence of ridges and valleys forming a corrugated type surface on the fields of copper plated zinc cents and the effects of a strike on plating irregularities, I have suggested that the greatest portion of the energy of the strike is dedicated to forcing metal into the recesses on the die to form the devices rather than that energy being directed equally onto the fields, and thus allowing corrugation features to remain on the coin.
Here I want to outline information that feeds into this suggestion.
Even though the experiment I reported was producing a brockage with the cent and not a direct impact from a steel die on the surface of the cent, the strike was 70 tons compared to 30 tons strike for cents, possibly enough to offset the lack of direct contact with a steel die.
The corrugation found on cents is also found on unstruck planchets, suggesting the ridges and valleys appear pre-strike. This "corrugated effect" is extremely common, indicating that it is not simply a die state or some other more occasional expression. Similar variations are found on proof cents.
While there is a credible explanation for the formation of corrugated features pre-strike, no such explanation has been offered to explain how the features might appear after the strike and I have none to offer.
A die face is not flat. If it were, there might be a stronger argument that the ridges and valleys giving a corrugated effect might be eliminated. From the earliest days of the
Lincoln Cent, the mint has designed dies with a radius. For example, "25 Radius" is used by
Charles Barber (Chief Engraver for the Mint) in a letter (February 13, 1909) to
Victor Brenner (Designer of the
Lincoln Cent) concerning the design model preparations for the
Lincoln Wheat cent. He is explaining to Brenner that his initial design with multiple field levels cannot be basined. He mentions that 25 is the radius of the current cent, but that the radius is determined by the disposition of the design and the area of the coin.
Die camber focuses die pressure on the central area of the design during the initial instance of a strike and then on initially filling of device recesses on the die as the pressure radiates outwards towards the rim. Numismatic writer Walter Breen called the creating of die camber "basining" and described it as "imparting variable radius of curvature to the fields."
We know that a die strike does not produce uniform strike pressure. This is acknowledged by the mint in its justification for engaging in research to improve die curvature design. "Specifically, matching planchet upset and die curvatures will reduce coining pressures around the coin edge, producing more uniform normal pressures. This "matched system" would enable coining presses to use less stamping force and still obtain optimal detail in the coins produced using current coinage materials." (From 2020 Biennial Report to the Congress as required by The Coin Modernization, Oversight, and Continuity Act of 2010 (Public Law 111-302), Page 8)
Advanced die curvature experimentation has only been undertaken by the mint over the last 10/15 years. "Stamping trials were conducted where nickel die curvatures were changed from spherical to exponential curvature to "match" existing nickel planchet upset geometry and thus lower stamping tonnage. Increased die life was realized during stamping trials conducted in the coin development room.
However, the 2021 transition to exponential crown for circulating nickel production dies did not yield similar results. Both the Philadelphia and Denver Mints realized a marginal decrease in stamping tonnage, but no comparable increase in fatigue die life. These results suggest that further design optimization is required to increase fatigue die life."
Page 13, 2022 Biennial Report to Congress as Required by the Coin Modernization, Oversight, and Continuity Act of 2010, (Public Law 111-302)
https://www.usmint.gov/wordpress/wp...P5_FINAL.pdf Research into the science and technology of coin minting (Finite element design procedure for correcting the coining die profiles, By Paulo Alexandrino, Paulo J Leitão, Luis M Alves, and Paulo A.F. Martins Manufacturing Review, © P. Alexandrino et al., Published by EDP Sciences 2018) can be divided into three main periods before which convex die formation geometry was largely trial and error.
The first period (1960-1980) focused on the development of a theoretical framework for understanding the mechanics of coin minting.
The second period (1980-2000) enhanced previous knowledge on the deformation mechanics of coin minting by giving special attention to the analysis of material flow and calculation of pressure. During this period, the mint was learning about the required techniques for copper plating at the beginning of the period. Researchers were able to characterize plastic deformation inside a coin and to calculate the energy and the shape of a disk to produce a coin with a central circular design and an outer annular legend. The plating learning curve during this time is apparent in my papers on Proof Cent Plating and on Plating Corrugation.
It is only at the beginning of the third period that the mint is exploring exponential die curvatures.
I think this means that plating irregularities can indeed survive a strike (and by extension planchet striations, more often found on other denomination coins) can also survive a strike.
Posted 04/21/2025 11:23 am
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