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SIMS21, Poland 2017 - Rana N Sodhi abstract

Rana N Sodhi oral presentation (OA2-Mon4-3-3)

ToFSIMS and other surface spectroscopies applied to the study of ancient artefacts: preliminary investigation of a silver-plated tetradrachm

Rana N Sodhi1, Peter Brodersen1, Sal Boccia1, Amandina Anastassiades2, Cristiana Zaccagnino3

1 University of Toronto - Dept. of Chemical Engineering and Applied Chemistry, 200 College Street, M5S 3E5 Toronto, Canada
2 Queen's University - Department of Art History and Art Conservation, , K7L 3N6 Kingston, Ontario, Canada
3 Queen's University - Classics/Languages, Literatures and Cultures, , K7L 3N6 Kingston, Ontario, Canada

A group of ancient coins are 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 silver 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 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 our 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 ToFSIMS, XPS, and EDX analysis to investigate the viability of utilizing these combined techniques in the study of ancient coins. In order to gain information on the composition and distribution of the various chemical components present at the surface and into the bulk, it is necessary to profile into the sample, which by definition is destructive. Thus, it is important to identify areas of interest for further analysis so that the amount of damage can be minimized while maximizing the information obtained. To accomplish this, large area imaging ToFSIMS has been performed using the stage-raster option in an IonTOF ToFSIMS V. Bi3++ primary ions were used to map the surface and the positive secondary ion distribution images were obtained. Because of the varying topography of the sample, delayed onset of the generated secondary ion had to be used. Four areas were identified for further analysis, namely: (1) a Ag rich (minimal Cu content) area; (2) a Cu rich (minimal Ag content) area; (3) an area showing the presence of both Cu and Ag; and finally (4) an area displaying visibly different surface characteristics. Chemical composition was obtained from these areas using XPS. To obtain further (non-destructive) depth information, EDX images were obtained, which would arise from a deeper sampling depth. In this case Ag was seen to be more dominant except for region (2). This implies a Cu enrichment at the surface as observed by the ToFSIMS.

In this paper the results of depth profiles into the bulk at the chosen areas are presented in order to ascertain the distribution of the various chemical components and investigate how the coins were manufactured.