Focused Ion Beams and Complementary Techniques for Study of the Silvering of Ancient Roman Coins

Two ancient Roman silver coins dating back to the 3rd–4th century AD were studied. A set of modern micro- and non-invasive analytical techniques was used: a focused ion beam, scanning electron microscopy, energy dispersive X-ray microanalysis, micro-X-ray fluorescence analysis, neutron radiation analysis, and other methods. The research showed that Ag–Cu and Ag–Cu–Pb–Sn alloys were used, common in the prosperous years of the Roman Empire, when the Romans produced alloys with a relatively high silver content for both outer layers and the inner core of the coins. Surface silvering processes were used during different periods of crisis under the reign of Antoninii. It was established that even during the crisis, the Romans produced high quality “Antoninianus”, attempting to improve the silvering procedure using silver amalgam (Hg–Ag) – mercury was detected in the near-surface silver layer of the coins. A porous morphology of the coin surface was also revealed, which might be the result of an uncontrolled heating process and the removal of mercury through boiling.