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GALLO-ROMAN METALLURGY

Ancient counterfeiting

Aurélie Deraisme, a young researcher working on her PhD thesis at The Orléans Institute of Research into Archaeomaterials,1 recently figured out what techniques were used to make forgeries of coins in Roman Gaul.  The scission between the Roman and Gallic Empires made the third century a time of serious political and economic upheaval. A consequence was that in addition to the official production of coins at the mint in Trèves, parallel workshops began large-scale production of forgeries bearing the image of the usurper Postumus, then head of the Gallic Empire. Since money was in short supply at the time, circulation of these counterfeit pieces was eventually tolerated.


 Examples of the coins have been found on the site of the village of Châteaubleau, near Provins, 85 km east of Paris. The copper cylinders and silver-plated blanks found at the site seem to indicate that a parallel workshop existed here. To find out more about these illegal coins and how they were made, Aurélie Deraisme, a physico-chemist specializing in the metallurgy of copper alloys, and her fellow researchers began by examining the coins' composition.2 They found that while official coins contained 15% silver and 85% copper, the others contained only 2% silver to 98% copper–considerable savings for the counterfeiters! Using electron microscopy, Deraisme found that while the silver was uniformly distributed throughout the mass of the official coins, it was only found in the surface layer (about 200 mm thick) of the forgeries.


 To understand how craftsmen of the time made these coins, Deraisme tried to reproduce them in the laboratory. She heated copper cylinders which had been coated with a thin layer of silver at different temperatures and for varying lengths of time, in conditions that prevented any possible surface oxidation. Heating them for four minutes at 950°C, she obtained the composition and structure of silver-plating closest to that of the counterfeit coins. This study gives us a better idea of the silver-plating techniques used by Gallo-Roman metallurgists, in particular with regard to the manufacture of counterfeit coins.


Stéphanie Belaud

Notes :


1. Institut de recherche sur les archéomatériaux, Orléans. Joint lab: CNRS/ Université Bordeaux-III.
2. Collaboration with researchers from the Atomic Energy Commission (CEA, Commissariat à l'énergie atomique) and the National Institute for Nuclear Sciences and Techniques (INSTN, Institut national des sciences et techniques nucléaires).

Contacts :

Aurélie Deraisme
Institut de recherche sur les archéomatériaux, Orléans
Deraisme@cnrs-orleans.fr


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