ToF-SIMS and other surface spectroscopies applied to the study of ancient artifacts: Preliminary investigation of a tetradrachm of Claudius

A group of ancient coins is among the Diniacopoulos collection of Central and Eastern Mediterranean antiquities housed at Queen's University. At first glance, nine coins in the collection appear to be billon tetradrachms minted in Alexandria, Egypt, dating to the period of the Emperor Claudius (41–54 AD). On the obverse, there is a portrait of the Emperor Claudius, on the reverse his wife Messalina is holding their two children. A closer examination reveals, however, that each of these coins weighs substantially less than the majority of specimens belonging to the same issues. All of the coins appear to have silver-rich plated surfaces with copper-rich cores. Questions are raised, therefore, about their authenticity and methods of manufacture. The surfaces display a variety of corrosion products, some of which may also indicate past restoration treatments. A single coin was selected for time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), and energy dispersive...