Silver coated lead coins: An appraisal of ancient technology

In this paper we report some experimental archaeology research consisting in applying a two-step electrochemical displacement procedure in order to simulate the fabrication route which could have been used by coin counterfeiters of the 3rd century BC for the silvering of a Pb substrate. A material characterisation of a relevant archaeological item [5] has revealed a bi-layer coating, with a Cu interlayer and an Ag top layer. SEM morphology and XRD textures of the Cu and Ag layers cannot be explained with thermal or mechanical procedures and exhibit typical features of electrochemical displacement processes. We replicated a coating process with materials that are documented for the historical period of interest and that lead to the formation of a bi-layer coating exhibiting essentially the same morphological and structural properties of the archaeological object. The direct displacement coating of Pb with Ag has been shown not to be feasible with period chemicals, whilst the duplex coating could be achieved by sequential use of a Cu(II)-acetate solution, displacing Pb, and of a Ag(I) solution obtained by dissolving AgCl in dilute ammonia. The coatings obtained have been characterised by SEM; XRD and colorimetry. The double displacement process yields a cauliflower-like agglomerate structure, showing close similarities with the morphology of residual white-metal coating of the coin. Cu and Ag displacement layers exhibit a typical (111) texture that cannot be reconciled with that obtained by thermal or mechanical plating techniques and that matches the texture found in the counterfeit coin. The spectral reflectivity and colorimetric parameters of the final Ag layer perfectly match those of bulk Ag.