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Identification of silver surface enrichment in ancient silver coins: a method improvement and a study of trace element behaviour

Mahir Hrnjic1, Graham Adrian Hager-Peterm1,2, Thomas Birch1, Gry Barfod1,2, Søren Michael Sindbæk1, Charles Lesher1,2

mahir.hrnjic@cas.au.dk1Centre for Urban Network Evolutions (UrbNet), Aarhus University

2Aarhus Geochemistry and Isotope Research Platform (AGiR), Aarhus University

Introduction• Surface treatments and corrosion can alter the surface composition of ancient

silver coins. Difference in electrode potential between Ag and Cu in a two-phasealloy may cause Cu to precipitate to the surface of a coin and to corrode onceexposed to environmental influences (Beck et al. 2004, 2008). In such cases, thesurface analysis of coins can give inaccurate elemental concentration of majorconstituents Ag and Cu, and potentially other minor and trace elements (Borgeset al. 2017).

• The first part of the study evaluates the role of standards in a previously reportednon-destructive XRF method, applied for identifying silver surface enrichment oncoins. The method is based on measuring and comparing peak intensity ratios ofAg Kα and Ag Lα lines between coins and a silver alloy standard with a similarcomposition (Linke et al. 2000, 2003). The secont part investigates the behaviourof minor and trace elements in surface enriched coins by LA-ICP-MS analysis, bycomparing the concentration of elements between the coins’ core and rim.

BibliographyBECK, L., BOSONNET, S., RÉVEILLON, S., ELIOT, D. & PILON, F. (2004) Silver surface enrichment of silver-copper alloys: a limitation for the analyses of ancient silver coins by surface techniques. Nuclear Instruments and Methods in Physics Research B. 226. pp. 135-162BECK, L., ALLOIN, E., BERTHIER, C., RÉVEILLON, S., & COSTA, V. (2008) Silver surface enrichment controlled by simultaneous RBs for reliable PIXE analysis of ancient coins. Nuclear Instruments and Methods in Physics Research B. 266. pp. 2320-2324BORGES, R., ALVES, L., SILVA, R. J. C., ARAUJO, M. F., CANDEIAS, A., CORREGIDOR, V., VALÉRIO, BARRULAS, P. (2017) Investigation of surface silver enrichment in ancient high silver alloys by PIXE, EDXRF, LA-ICP-MS and SEM-EDS. Microchemical Journal. 131. pp. 103-111LINKE, R. & SCHREINER, M. (2000) Energy dispersive X-Ray Fluorescence Analysis and X-Ray Microanalysis of Medieval Silver Coins. Microchimica Acta. 133. pp. 165-170LINKE, R. SCHREINER, M. & ALRAM, M. (2003) Determination of the provenance of medieval silver coins: potential and limitations of x-ray analysis using photons, electrons and protons. X-Ray Spectrometry. 32. pp. 373-380

MethodsBruker M4 Tornado μXRF• Coins’ surface and non-corroded Ag-Cu standards were analysed with μXRF, after which Ag Kα/Ag Lα values of

coins and samples were compared (Fig. 1). Surface enrichment should be present if the ratio in a coin is lowerthan the ratio of a standard, while a much higher ratio value should indicate the presence of corrosion.

• Coins were then sampled on edges to expose the core. Samples were mounted in epoxy and analysed with μXRFelemental mapping with the aim of confirming the presence or absence of silver surface enrichment (Fig. 2). In this way, it was possible to validate on which coins was surface enrichment succesfully identified with Ag Kα/Ag Lα method.

Fig. 2 An example of a μXRF elemental map that reveals the presence of surface enrichment in a Roman denarius.

Agilent 7900 Quad LA-ICP-MS coupled with Resonetics 193 nm Laser• Coin samples mounted in epoxy were futher analysed with LA-ICP-MS in order to gain the elemental

concentration of minor and trace elements. A number of ablations were done in the core and on the rim area.

Materials• Twelve ancient high silver content coins (87-98 wt. % Ag) (eight Roman Imperial denarii, three Sassanid coins

and two early Islamic coins) with different chronologies and compositions were selected for the study.

• For all measurements, a number of Ag-Cu standards with different composition were used. The certifiedreference materials were acquired from MBH Analytical Ltd. — 133X AGA1 (77 wt. % Ag), 133X AGA2 (87 wt.% Ag), 133X AGA3 (91 wt. % Ag), 131X AGP1 (99 wt. % Ag). Besides Ag standards, LA-ICP-MS instrumentalcalibration and tuning were done with the certified reference material NIST 612.

100 μm

Primary X-rays Secondary X-rays

X-ray source Detector

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KαLα

Ag-Cu matrix

ResultsμXRF• Elemental mapping shows that seven coins exhibit silver surface enrichment. In the

case of Roman Imperial denarii, the Ag content between the core and the enrichedsurface may differ by 30 %.

• Ag Kα/Ag Lα values confirmed surface enrichment for five coins with AGA1and AGA2 standards, two coins with AGA3, and none with AGP1.

• Coin Sas2 with enrichment has a high Ag Kα/Ag Lα value due to corrosion (Fig. 3a),while Rom 141 has enrichment area deeper than the information deth of Ag Kα. It waspossible to mitigate the effect of the corrosion for Sas2 once Cu Lα/Ag Lα was plotted incorrelation with Cu Kα/Ag Kα (Fig. 3b).

LA-ICP-MS• The results from LA-ICP-MS show that besides Ag and Cu, the surface enrichment

has influence on the concentration of minor and trace elements. The content of Au was significantly higher in surface areas comparing to the core of coins; while the concentration of Co, Ni, As, Rh, Pt for the same coins was reduced (Fig. 4). Other important elements (Pb, Bi, Zn) did not change significantly.

• The amount of Au in enriched layers can differ by the factor of two between the core and the rim of a sample.

• Trace elements such as Co, Ni, and As are associated with Cu and were likely leached together with Cu from the surface.

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Fig. 1 The XRF information depth of Ag Kα and Ag Lα lines in 80 % – 20 % Ag-Cu matrix.

Conclusion• The study concludes that the choice of standards play a key role in identifying the silver enriched surface in

ancient Ag-Cu coins by XRF surface analysis. The multi-standard Ag Kα/Ag Lα method in combination with CuKα and Cu Lα lines offers a more reliable approach in identifying surface enrichment.

• Besides Ag and Cu, the difference in the composition between the surface and the core is also confirmed for anumber of minor and trace elements (Au, Co, Ni, As, Rh, Pt). An overestimate of Au concentration may influencethe provenance intepretation of Ag objects.

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AcknowledgmentThis work was supported by the Danish National Research Foundation under the grant DNRF119 – Centre

of Excellence for Urban Network Evolutions (UrbNet).

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Roman denarius, Traianus (98-117 AD)

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Fig. 3 (a) The Ag Kα/Ag Lα coin values (points) in respect to values obtained from standards (dashed lines). (b) Six out of seven coins with enriched surfaces (exception is Rom 141) plotted below the regression line.

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Fig. 1 Results of LA-ICP-MS analysis showing the difference in elemental concentration between the core and the rim of surface enriched coin samples.

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R2 = 0.9998