What is Meant by Die Radius?

Posted by David W. Lange, NGC Research Director on 7/1/2006

In recent years, Nancy Oliver and Richard Kelly have emerged as two of my favorite numismatic sleuths and authors.

David LangeIn recent years, Nancy Oliver and Richard Kelly have emerged as two of my favorite numismatic sleuths and authors. In addition to producing two books, they also penned a nice feature article for the May 2005 issue of Numismatist ("The Saga of the 1870-S Silver Dollar"). In this article the authors quoted a letter from Carson City Mint Superintendent Abraham Curry to his colleague in San Francisco, Superintendent Oscar H. LaGrange, in which Curry made reference to "silver-dollar radius plates." The authors stated that their research had failed to turn up an explanation of this term. Since I have encountered the word "radius" in reference to die preparation on a few occasions, my first impulse was to contact the authors or draft a letter to the editor, but then it seemed that a more thorough look at the subject might be in order.

Published references to die radius are actually quite rare. The term has never been, to my knowledge, explained in detail. Instead, what I've found are passing references, such as those found by Ms. Oliver and Mr. Kelly. Usually, these turn up in the writings of U. S. Mint Chief Engraver Charles Barber (in office 1879-1917) or, less often, those of his successor, George T. Morgan (1917-25). More must be inferred from their indirect usage of the term than is actually stated. The closest thing I've found in print to an actual explanation of die radius is a brief reference in the Comprehensive Catalog and Encyclopedia of Morgan & Peace Dollars by Leroy C. Van Allen and A. George Mallis. Though not detailed, a person already familiar with the die-preparation process should be able to make a fairly good interpretation.

In short, die radius refers to the curvature of the die face. Early United States coins typically had flat fields as a consequence of the primitive method of die preparation. Starting with the Christian Gobrecht designs of 1836-40, USA coinage began to reveal a slight concavity to the fields, though this isn't always apparent on casual inspection. Concave fields were the result of convex die faces, and this slight curvature had to be applied in a step that was separate from the actual sinking of the die. A working die that was ready for polishing was set into a jig with its face upward. The face of the die was then brought into contact with a polishing disc, or plate, that had a very shallow concavity to it. When spun against the face of the die, the disc imparted the same curvature profile, but the result was convex.

The purpose for giving the die face a slight curvature was to facilitate the movement of metal during the coin striking process. Experimentation with different degrees of curvature would ultimately determine the best standard for filling the dies in a single blow from the press. These experiments were carried out at the Philadelphia Mint's Engraving Department with all new designs starting at least as early as the Morgan dollar coinage of 1878 and possibly earlier. The difficulty that the Mint experienced in producing a satisfactory number of coins from each die pair with this coin type's original, high-relief obverse and its eight-tailfeather reverse demonstrated the need for such trial and error before working dies were shipped to all the branch mints.

Once the optimum die radius was determined in Philadelphia, duplicate radius plates having this ideal curvature were sent along with the working dies to each of the other mints, so that the process could be repeated on-site. It was necessary that the various mints finished their dies locally, because the dies were sent from Philadelphia in an unhardened state (hardened dies, if intercepted in shipment, could more easily be used by counterfeiters).

If all went according to plan, the coins struck by each mint would be identical throughout in their degree of definition. But, as any collector of uncirculated Morgan dollars can attest, the sharpness of these coins varied considerably. This variance occurred in a characteristic manner from one mint to another. For example, New Orleans Mint Morgan dollars typically are soft at the centers and have strong edge reeding, while those coined at Philadelphia have sharp central details and mushy reeding. One can actually feel this difference by handling the coins' edges. Clearly, the movement of metal was being directed in accordance with differences in die radius.

The creation of radius plates went hand in hand with the practice of "basining," which I've described in previous columns. Basining was the process of giving dies their face polish when being used for the first time. In fact, the term "basin" often was used interchangeably by Mint employees to describe the face curvature, or radius. Polishing of a die performed after its initial use to repair flaws or to extend its useful life is not properly called basining, since the work was crude and, it may be assumed, was done without a radius plate.

The new, sculpted designs submitted by outside artists beginning in 1907 gradually rendered both radius plates and basining obsolete. The models as submitted already included the desired curvature, though the Mint's own staff sometimes had to modify this radius in the hub reduction stage.

David W. Lange's column USA Coin Album appears monthly in Numismatist, the official publication of the American Numismatic Association