Residual Elastic Stress in Historical Coins as a Criterion for Authentication

Abstract

Authentication of coins is frequently presenting complicated problem in numismatics practice. Metal composition only may not be enough to identify forgeries, notably when one deals with noble metals – silver, gold or electron. Isotopic ratios, providing in certain cases information regarding metallic ore source (geographical provenance), require a sample to be extracted for MS-analysis while sampling is not permissible for numismatic material in principle. To solve the problem it becomes crucial, in addition to metal’s composition, to establish the method used for a coin’s manufacturing, as forgers are rarely realizing or following the method that was used in the original historical production. Conventional visual studying under microscope is not always being enough to understand how the coin was produced while metallographic examination of polished and etched metal samples, which might provide necessary information, is not applicable here for its destructive character. Necessary information regarding manufacturing could have been extracted from residual stress analysis, but, in general, there were very few published works regarding this kind of analysis for coins, and those few did not consider any connection between manufacturing process and residual stress in metal. The expected types of residual elastic stress arisen under historically known methods of coins production (casting in hot or cold mold, striking hot or cold metal) are considered in this work. On this base, non-destructive X-rays diffraction method (Sin²Ψ-method) is offered to distinguish between various methods of manufacturing. The results may be applied in museum’s laboratory as useful criterion for authentication of coins and medals.