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A number of rumors have circulated over the years about the variation in color of certain rare United States gold patterns. Mint facility records have not been clear on the subject, but it has been surmised that these coins have been treated using a chemical process to reduce the copper content of their outer layer, thereby enhancing their appearance and value.
Six coins from the National Numismatics Collection were examined non-invasively in situ by a team from the Smithsonian Center for Materials Research and Education using recently developed equipment-a handheld X-ray fluorescence spectrometer. This equipment is portable and provides fast multi-elemental analysis of a wide variety of materials.
The coins were analyzed to determine whether there was any variation in the surface composition and differences were recognized. The variation could be interpreted as due to factors such as the thickness of the outer layer of pure gold. If we accept the hypothesis of the chemical treatment of the outer layers the differences in composition may relate to minor variation of the efficiency of the chemical leaching process. Similar testing procedures have built up the case for this type of enrichment of coins in other collections. These findings will contribute to a study by Q. David Bowers in his forthcoming book on the history of United States gold coins.