Using Negative Muons as a Probe for Depth Profiling Silver Roman Coinage

Bethany Hampshire,Don Paul,Adrian Hillier,Kevin Butcher,Katsu Ishida,George Green

doi:10.3390/heritage2010028

Bethany Hampshire, Don Paul + Show 4 more

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https://doi.org/10.3390/heritage2010028

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Abstract

Debasement of silver Roman coins is a well-known phenomenon and understanding the quality of ancient silver coinages can provide an idea about the underlying fiscal condition of the issuing states. These coins are made from a silver-copper alloy, the surfaces of which were deliberately enhanced at the mints by a process of surface-enrichment to give them the appearance of being made of pure silver. Therefore, any surface analysis would provide a composition of the silver-copper alloy that would not be representative of the original alloy from which the coin blank was made; the result would be too high in silver. However, the bulk of the sample, the interior, should provide a composition that is true to the original alloy. Elemental analysis using negative muons has been used to provide a depth dependent compositional, completely non-destructive analysis of a silver-copper alloy denarius of the empress Julia Domna datable to 211–217 CE. The composition of the coin, beyond the surface enrichment layer, is 51 ± 1.8 % copper and 49 ± 1.9% silver, taken at a muon depth of 402 ± 61 µm. The surface enrichment layer is approximately 190 µm thick.

Highlights

The quality of ancient silver coinage is often seen as a comment on the fiscal health of the issuing states
Most silver Roman coins are made from a silver-copper alloy that undergoes surface-enrichment, making the coin appear to be pure silver
Once the blanks for the coins were cast, they were kept at red heat in order to oxidize the copper at the surface

Summary

Introduction

The quality of ancient silver coinage is often seen as a comment on the fiscal health of the issuing states. This is nowhere more apparent than with Roman coinages, which witnessed heavy debasements during the first three centuries of our era. The blanks were soaked in an organic acid which stripped the copper out of the alloy, leaving a silver-enriched layer [1]. This technique was employed on alloys with a composition as low as 80%

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