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

-Two ancient Roman silver coins dating back to the 3rd-4th century AD are studied. A set of modern micro- and nondestructive analytical techniques, namely, a focused ion beam, scanning electron microscopy, energy-dispersive X-ray microanalysis, micro-X-ray fluorescence analysis, neutron-radiation analysis, and other methods is used. Research shows that Ag-Cu and Ag-Cu-Pb-Sn alloys were used, which were 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 coins. Surface silvering processes were used in different crisis periods during the reign of Antonine. It is established that even during the crisis, the Romans produced high quality Antoniniani, attempting to improve the silvering procedure using silver amalgam (Hg-Ag). Mercury is detected in the surface silver layer of the coins. A porous morphology of the coin surface is also revealed, which might be the result of an uncontrolled heating process and the removal of mercury through boiling.