Surface topology investigation for ancient coinage assessment using optical interferometry

Abstract

In order to demonstrate the capabilities of white-light interferometry depth profiling (WLI-DP) for ancient coinage assessment, we investigated a series of notorious 1786 gold coins, bearing Louis XVI’s ‘horned’ effigy, and allegedly minted in Strasbourg. Scanning electron microscopy as well as WLI-DP observations unambiguously indicate that both previously differentiated ‘single’- and ‘double’-horned varieties originated from a unique minting tool. Moreover, from topological measurements, we infer that ‘single-horned’ coins, rather than wearing out into ‘double-horned’ coins, proceeded from the latter variety during minting by progressive failure of an already altered die. Whereas present observations do not exclude initial forgery, they suggest that protrusions resulted from progressive incidental in-service die deterioration.