Silver surface enrichment in ancient coins studied by micro-PIXE

Nuclear Instruments and Methods in Physics Research B 306 (2013) 241–244

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Silver surface enrichment in ancient coins studied by micro-PIXE F.J. Ager a,b,⇑, A.I. Moreno-Suárez a,b, S. Scrivano a, I. Ortega-Feliu a, B. Gómez-Tubío a,c, M.A. Respaldiza a,d a

Centro Nacional de Aceleradores (Universidad de Sevilla-CSIC-J. Andalucía), C/ Thomas Alva Edison 7, E-41092 Seville, Spain Departamento de Física Aplicada I, Universidad de Sevilla, Seville, Spain c Departamento de Física Aplicada III, Universidad de Sevilla, Seville, Spain d Departamento de Física Atómica, Molecular y Nuclear, Universidad de Sevilla, Seville, Spain b

a r t i c l e

i n f o

Article history: Received 24 July 2012 Received in revised form 21 December 2012 Available online 2 January 2013 Keywords: PIXE Nuclear microprobe Roman silver coins Silver enrichment Silver–copper alloys

a b s t r a c t The surface enrichment of archeological silver–copper alloys, either intentional or due to corrosion processes, has been known for many years. The most used non-destructive techniques, such as particleinduced X-ray emission (PIXE) and X-ray fluorescence (XRF) are surface techniques, with penetration depths typically ranging from a few microns to a few tens of microns. Therefore, these techniques could produce results which are not representative of the bulk composition of the alloy. In order to gain insight into the silver enrichment process and the effects on the data obtained with these techniques, a set of silver roman denarii were cross sectioned and analyzed at the Centro Nacional de Aceleradores micro-PIXE facility. Elemental maps show silver surface enriched layers up to 250 lm thick. Besides, silver-enriched surface layers are not found for alloys with 96–98 wt.% Ag. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction

2. Materials and methods

The determination of the bulk composition of ancient silver– copper coins by means of non-destructive surface techniques like PIXE, XRF, SEM/EDX, etc., is a challenging issue because of the silver enrichment of the near surface layers of this alloy [1,2]. The extension in depth of this silver enrichment can reach some hundreds of microns, far beyond the useful penetration depth of those techniques. The formation of a silver-enriched layer prevents these non-destructive surface analysis techniques from determining the fineness of ancient silver–copper coins [3–7]. The aim of this work is to study the thickness and composition of silver-enriched layers by means micro-PIXE and micro-XRF to expand the scarce collection of data on ancient silver–copper coins which have been cut to access the bulk and thus provide more information about this problem. The characterization of these coins would also help to provide a method for the correction of the surface composition obtained by means of regular surface analysis (PIXE, XRF) by combining these techniques with other nondestructive techniques such as gamma-ray transmission (GRT) or specific gravity measurements (SG), in order to get more precise results about the composition of the original alloy.

2.1. Coins

⇑ Corresponding author at: Universidad de Sevilla-CSIC-J. Andalucia, Centro Nacional de Aceleradores, C/ Thomas Alva Edison 7, 41092 Seville, Spain. Tel.: +34 954460553; fax: +34 954460145. E-mail addresses: [email protected], [email protected] (F.J. Ager). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.12.037

A small set of Roman Republican coins from 211 BC to 86 BC was analyzed: four denarii and two victoriati (Table 1). The classification of the coins was done according to Crawford [8]. The surface elemental composition of these coins was determined by means of XRF. In order to determine the bulk composition directly, avoiding the surface corrosion and/or enrichment, all of the coins except N1 were cut in half. A cross-section of each coin was polished with diamond solutions up to 1 lm and then analyzed by micro-PIXE and micro-XRF. 2.2. Experimental

l-PIXE measurements were performed with 3.0 MeV protons focused to a 3  3 lm2 beam normal to the sample using the CNA scanning nuclear microprobe [9]. The beam currents used were in the range of 100–200 pA. A Si (Li) detector (area 80 mm2, resolution 145 eV) was mounted at 135° for X-ray detection. For all measurements, a 50 lm thick Mylar absorber was used. Quantification was performed with the GUPIXWIN V2.1 software package [10]. The l-XRF experimental setup [11] is based on a low power (30 W) air-cooled micro-focus portable X-ray source (iMOXS– MFR) [12,13] with a Rh anode (50 kV, 800 A), combined with a polycapillary mini-lens in the excitation channel. This lens has a 50 m FWHM spot size at 7.5 keV. The X-ray tube output is filtered

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Table 1 List of Roman coins analyzed. Coin

Type/reference

Date (BC)

N1 N2 N8 N9 N10 N11

Denarius/Cr. 218 Denarius/Cr. 350 Denarius/Cr. 113 Denarius/Cr. 114 Victoriatus/Cr. 53 Victoriatus/Cr. 83

147 86 206–195 206–194 211 211–210

with 1 mm thick aluminum foil. In the detection channel, a super silicon drift detector (super SDD) from Amptek was used. This detector has a crystal of 25 mm2 and a thickness of 500 m, with a resolution of about 127 eV for the Mn Ka-line (5.9 keV). The experiments were performed at a voltage of 50 kV, using a current of 600 lA. A computer-controlled X–Y–Z stage from STANDA Ltd. (Vilnius, Lithuania) is used for positioning the samples with a resolution of 0.156 lm. Scans through the cross sections in 50 lm steps were performed. The XRF spectrometer used for surface analysis is based on a portable X-ray tube with a W anode (12.7 lm Be window). Analyses were performed at 30 kV and 590 lA, normal incidence. An aluminum filter (1 mm) was interposed in the excitation channel. The characteristic X-rays emitted from the sample were detected with a SDD detector (Ketek AXAS, with 140 keV FWHM at Mn-Ka, 450 lm Si crystal thickness, 7 mm2 active area, 8 lm Be window and a Zr internal collimator). The acquired XRF spectra were analyzed using the WinQxas software [14]. For the calibration of the system, certified standards with compositions similar to those of the analyzed samples have been used.

Fig. 2. Micro-PIXE elemental maps of Ag and Cu of the cut cross-section of coin N2 (analyzed area: 2.5  2.5 mm2). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Micro-PIXE elemental maps of the cross-section of coin N8 (analyzed area: 300  300 lm2). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3. Results and discussion 3.1. Micro-PIXE measurements With the help of the ion microprobe, the cross sections of coins N2, N8, N10 and N11 were analyzed Fig. 1. Elemental maps obtained by micro-PIXE are useful to find evidences of surface enrichment and also to estimate the thickness of the enriched layers.

Once the microstructure is known, those maps are complemented with analyses at selected points or regions of interest, allowing the quantification of major and trace element present in the coins. In this case, micro-PIXE technique allowed detection and

Fig. 1. Coin N2, denarius/Cr. 350: (a) intact and sectioned (b) obverse and (c) reverse. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Table 2 Mean concentrations and standard deviations obtained by micro-PIXE of the cut cross-sections of the coins. ‘‘Surface’’ stands for the surface silver-enriched layers. Concentrations (wt.%) Coin N2 N8 N10 N10 N10 N11 N11

Bulk Bulk Surface Bulk-1 Bulk-2 Surface Bulk

Ag

Au

Cu

97.8 ± 0.8 97.4 ± 0.4 98.8 ± 0.4 79.4 ± 0.5 63.7 ± 0.4 89.5 ± 0.3 56.7 ± 0.2

0.184 ± 0.002 0.270 ± 0.002 0.059 ± 0.002 0.212 ± 0.003 0.174 ± 0.002 0.371 ± 0.003 0.202 ± 0.002

1.35 ± 0.02 1.957 ± 0.007 0.872 ± 0.004 19.71 ± 0.12 35.6 ± 0.2 9.78 ± 0.05 42.50 ± 0.14

Fe

Pb

0.0211 ± 0.0004 0.0434 ± 0.0002 0.0637 ± 0.0007 0.1079 ± 0.0007 0.0479 ± 0.0002 0.1550 ± 0.0005

0.590 ± 0.005 0.372 ± 0.002 0.193 ± 0.002 0.420 ± 0.003 0.385 ± 0.002 0.328 ± 0.002 0.347 ± 0.002

Bi

Hg

Br

0.080 ± 0.002 0.0426 ± 0.0003 0.125 ± 0.003

0.062 ± 0.003

0.0182 ± 0.0002

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Fig. 4. Micro-PIXE elemental maps of the cross-section of coin N10 (analyzed area: 250  250 lm2). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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ius N8 is also found to be silver-rich (97.3% Ag) and homogeneous (Fig. 3). On the contrary, both the victoriati N10 and N11 present a surface silver-enriched layer. In particular, coin N10 (Fig. 4) is characterized by different phases: a copper-rich phase (63.7% Ag/35.6% Cu) and at least two distinct silver-rich phases: 98.8% Ag/0.9% Cu and 79.4% Ag/19.7% Cu. The near surface silver-enriched layer extends to about 150 lm. In the case of coin N11 (Fig. 5), the enriched layer extends to about 250 lm with an average composition of 89.5% Ag/9.8% Cu, presenting a bulk composition of 56.7% Ag/ 42.5% Cu. The victoriati are known to be debased coins with highly variable fineness below 93% [15]. According to Beck et al. [3], for alloys with a silver content between 20% and 92% in the bulk, silver-rich phases segregate towards the external part of the samples in such a way that silver content is between 71% and 99% on the surface, in agreement with these results. 3.2. Micro-XRF measurements

Fig. 5. Micro-PIXE elemental maps of the cross-section of coin N11 (analyzed area: 2.5  2.5 mm2). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

quantification of Ag, Cu, Au, Pb, Fe, Zn, Bi, Hg and Br. Table 2 shows mean values and standard deviations for the analyzed coins in different regions of analysis. Fig. 2 shows Ag and Cu maps (2.5  2.5 mm2) of denarius N2. This coin presents a highly homogeneous composition, 97.8% Ag/ 1.4% Cu, and no signs of surface enrichment are found, only an apparent thin layer due to the effect of the geometry of the experiment. It is known that silver/copper alloys with a silver content higher than 92% solidify as a single homogeneous phase [3]. Denar-

The micro-XRF system was used with the aim of obtaining the elemental profiles across the cross-section of the coins. The same elements detected with micro-PIXE were quantified in this case. Fig. 6 shows the Ag and Cu profiles of coin N8 (homogeneous) and N11 (silver-enriched surface). Those profiles confirm the thickness of the enriched layers of coins N10 (150 lm) and N11 (250 lm), as well as the homogeneity of the denarii. Cross-section of coin N9 was also analyzed, showing a homogeneous composition like the rest of the denarii at least at this lateral resolution (50 lm). Table 3 shows mean values and standard deviations for the analyzed coins. In this case, the composition of the enriched layers has been averaged across the layer (left and right sides of Fig. 6(b)), and the bulk composition is also the average of the concentrations found across the bulk area. The concentrations found in this case are in a very good agreement with the concentrations found by micro-PIXE. 3.3. Surface analysis Due to the thickness of the enriched layers, the accessible analytical depth of XRF (30–40 lm) or PIXE (5–10 lm) [16] is insufficient to reach the bulk. Therefore the compositions found by surface analysis of the coins is unreliable in order to assess the fineness of the coins if silver enrichment or corrosion exists. In effect, surface analysis of this set of coins confirms this fact (Table 4). The surface compositions of homogeneous coins N2 and N8 are compatible with the bulk compositions found before, although

Fig. 6. Micro-XRF elemental profiles of Ag and Cu across the cross-section of coins (a) N8 and (b) N11.

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Table 3 Mean concentrations and standard deviations obtained by micro-XRF of the cut cross-sections of the coins. Concentrations (wt.%) Coin N2 N8 N9 N10 N10 N11 N11

Bulk Bulk Bulk Surface Bulk Surface Bulk

Ag

Au

Cu

Fe

Zn

Pb

Bi

97.1 ± 0.3 97.2 ± 0.4 96.2 ± 0.2 87 ± 3 61.3 ± 2.4 82 ± 4 58.3 ± 1.9

0.33 ± 0.02 0.39 ± 0.07 0.61 ± 0.08 0.06 ± 0.14

1.7 ± 0.2 1.8 ± 0.3 2.4 ± 0.2 13 ± 3 38.6 ± 2.4 18 ± 3 41.6 ± 1.9

0.024 ± 0.002 0.024 ± 0.002 0.023 ± 0.005

0.086 ± 0.005 0.092 ± 0.008 0.090 ± 0.011 0.09 ± 0.05 0.066 ± 0.007 0.088 ± 0.014 0.064 ± 0.007

0.68 ± 0.10 0.35 ± 0.07 0.47 ± 0.06

0.091 ± 0.012 0.029 ± 0.008 0.074 ± 0.011

Hg 0.028 ± 0.005

Mn 0.027 ± 0.003 0.09 ± 0.16 0.026 ± 0.003 0.06 ± 0.03 0.048 ± 0.008 0.066 ± 0.015 0.057 ± 0.009

Br

0.1 ± 0.3 0.020 ± 0.012 0.001 ± 0.003 0.02 ± 0.06

Table 4 Mean concentrations and standard deviations obtained by XRF of the surface of the coins. Concentrations (wt.%) Coin

Ag

Au

Cu

Fe

Zn

Pb

Bi

N1 N2 N8 N9 N10 N11

97.8 ± 0.9 97.9 ± 0.5 97.5 ± 1.6 98.3 ± 0.4 95.5 ± 1.5 94.2 ± 0.6

0.25 ± 0.25 0.28 ± 0.02 0.6 ± 0.4 0.9 ± 0.3 0.28 ± 0.03 0.49 ± 0.04

0.13 ± 0.05 1.1 ± 0.3 1.0 ± 0.6 0.56 ± 0.03 3.8 ± 1.5 4.8 ± 0.6

0.7 ± 0.4 0.08 ± 0.03 0.048 ± 0.010 0.067 ± 0.014 0.046 ± 0.008 0.11 ± 0.07

0.033 ± 0.010 0.017 ± 0.011 0.037 ± 0.019 0.024 ± 0.005 0.024 ± 0.005 0.031 ± 0.008

0.05 ± 0.03 0.51 ± 0.14 0.5 ± 0.5 0.16 ± 0.02 0.34 ± 0.06 0.29 ± 0.06

0.02 ± 0.02 0.08 ± 0.02 0.04 ± 0.04 0.03 ± 0.02 0.069 ± 0.012 0.026 ± 0.005

some effects due to the exposition of the samples to the environment and the cleaning procedures might be expected. Coin N9 seemed also to be homogeneous when inspected by micro-XRF (96.2% Ag), but it could present an enriched layer thinner than the lateral resolution of the system, since surface analysis shows an enriched surface (98.3% Ag). Surface concentrations of coins N10 and N11, which presented an enriched surface, are obviously altered by the near surface silver-enriched layers.

4. Conclusions Silver-enriched layers of silver–copper coins have been characterized by means of micro-PIXE, measuring the thickness and composition of these layers. The use of micro-XRF helps to further study the enriched layers as well as the bulk and to confirm the results found by micro-PIXE. Regular surface XRF analyses evidence the lack of reliability of surface analysis (XRF, PIXE, RBS, etc.) if the fineness of ancient-silver coins is to be determined, since the usual penetration depth is lower than the thickness of the enriched layers. To circumvent this problem, we are currently studying the possibility of correcting the surface concentrations by means of GRT, a technique successfully used for the analysis of archeological bronzes [17,18], although the porosity of the surface of the coins makes the necessary determination of the density difficult.

Acknowledgments We thank Pierluigi Debernardi for supplying most of the samples. Work partially supported by the project HAR2009-07449 from the Spanish Ministry of Science and Innovation and Criolab Lda. Company (Portugal).

Hg 0.011 ± 0.007 0.003 ± 0.006 0.005 ± 0.006 0.010 ± 0.007

Mn

Br

0.02 ± 0.05

1.0 ± 0.5

0.23 ± 0.30 0.005 ± 0.006 0.003 ± 0.005 0.009 ± 0.006

0.04 ± 0.05 0.02 ± 0.02 0.02 ± 0.02

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