Analysis of corrosion layers in ancient Roman silver coins with high resolution surface spectroscopic techniques

摘要

Determination of the microchemistry of surface corrosion layers on ancient silver alloy coins is important both in terms of understanding the nature of archaeological environmental conditions to which these ancient coins were exposed and also to help in their conservation. In this present study, five ancient silver alloy coins (225 BCE-244 CE) were used as test vehicles to measure their immediate surface microchemistry and evaluate the appropriateness and limitations of High Sensitivity-Low Energy Ion Scattering Spectroscopy (HS-LEIS, 0.3 nm depth analysis), High Resolution-X-ray Photoelectron Spectroscopy (HR-XPS, 1-3 nm depth analysis) and High Resolution-Raman Spectroscopy (HR-Raman, similar to 1000 nm depth analysis). Additional information about the deeper corrosion layers, up to similar to 300-1000 nm, was provided by dynamic HS-LEIS and HR-Raman spectroscopy. While not archeologically significant, the use of these coins of small commercial value provides data that is more representative of the weaker signals typically obtained from ancient corroded objects, which can be in stark contrast to pristine data often obtained from carefully prepared alloys of known composition. The oldest coins, from 225 to 214 BCE, possessed an outermost surface layer containing Cu2O, Na, Al, Pb, and adsorbed hydrocarbons, while the more recent coins, from 98 to 244 CE, contained Cu2O, Ag, N, F, Na, Al, S, Cl, and adsorbed hydrocarbons in similar corresponding surface layers. It thus appears that alloying with copper, even in small amounts, leads to the formation of an outer Cu2O layer. Depth profiling revealed the presence of K, Na, Cl, and S as key corrosion components for both sets of coins with S, most likely as Ag2S, concentrated towards the surface while the Cl, most likely as AgCl, penetrated deeper. Schema to understand the overall chemistry of the corrosion layers present on these silver alloy coins were developed from the equipment limitations encountered and are presented. (C) 2016 Elsevier B.V. All rights reserved.