From copper-based alloys to coins—Part I: Metallurgical characterization from complex manufacturing processes of imitation coins dated to the third century CE

Abstract

The evidence for imitation coinage found on the Gallo-Roman site of Châteaubleau (Seine-et-Marne, France) has been attested for several years. Successive excavations and findings related to the production of imitation coins, both struck and cast, have revealed major minting activity during the second half of the third century CE. This paper presents the study of 11 archaeological samples issued from this counterfeiting workshop site, which is today considered the most important in the north-western provinces of the Roman Empire. These samples, which correspond to different stages in the production chains leading to coins, mainly radiates (antoniniani, double sestertii), were characterized by metallography, energy-dispersive X-ray fluorescence (EDXRF), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) and the Vickers hardness test, in order to understand the different manufacturing processes, especially the ‘rod to coin’ one. This project is divided into two parts. The first, which is the subject of this paper, focuses on the chemical and metallurgical study of the archaeological artefacts. This has enabled us to identify and characterize the manufacturing processes thanks to the highly characteristic microstructure of the samples from the rod-to-coins process. Some of the cylinders derived from this process were also silvered by plating a foil of silver or silver–copper on a flan of pure copper in order to obtain irregular radiates. As regards fake double sestertii, several manufacturing techniques were identified: direct moulding of coins in stacked terracotta moulds, the casting of flans to strike double sestertius and the cutting of flans from metal plate is also a technique used in Châteaubleau. Characteristic composition and microstructures were also observed in these samples. This paper will be followed by a second focused on two experimental archaeology sessions carried out in order to reproduce the process ‘from rod to coins’ and then to compare the reproduced samples with the archaeological samples.