Electrochemical reconstruction of a heavily corroded Tarentum hemiobolus silver coin

In this paper we report on the electrochemical reconstruction of a Tarentum hemiobolus Ag coin, severely corroded in marine environment. As assessed by conventional analytical tools, most of the initially metallic Ag coin had been converted to AgCl by exposure to the aggressive coastal burial conditions. X-ray computed microtomography proved that only small portions of the artefact had preserved their metallic nature. Since the engraving was preserved partly in the corrosion product bulk and partly in the metallic rests, electrodeposition of Ag from the AgCl layer, under controlled conditions ensuring shape preservation, resulted in the reconstruction of the coin surface with full recovery of the original engraving. Such optimal electrodeposition conditions were identified by a combination of electrochemical and quasi-in situ X-ray microtomography experiments, carried out with artificially corrored engraved Ag wires. Microtomography of the reconstructed coin confirmed the compaction of the external Ag layer and disclosed that the central core of the coin still contains unconverted AgCl. The presence of such a mineralised core does not however impact the numismatic use of the coin and the safeguard of the original engraving.