XPS and XRF depth patina profiles of ancient silver coins

Applied Surface Science 272 (2013) 82–87

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XPS and XRF depth patina profiles of ancient silver coins F. Caridi a,b,∗ , L. Torrisi c,d , M. Cutroneo c , F. Barreca e , C. Gentile c , T. Serafino f , D. Castrizio g a

Facoltà di Scienze MM. FF. NN., Università di Messina, V.le F. Stagno D’Alcontres 31, Messina, Italy INFN-Sez. CT, Gr. coll. di Messina, V.le F. Stagno D’Alcontres 31, Messina, Italy c Dip.to di Fisica, Università di Messina, V.le F. Stagno D’Alcontres 31, Messina, Italy d INFN-Laboratori Nazionali del Sud, V. S. Sofia 62, 95123 Catania, Italy e Advanced and Nanomaterials Research S.r.l., V.le F. Stagno D’Alcontres 31, Messina, Italy f Dip.to di Fisica della materia e ingegneria elettronica, V.le F. Stagno D’Alcontres 31, Messina, Italy g Dip.to di Scienze dell’Antichità, Università di Messina, Italy b

a r t i c l e

i n f o

Article history: Available online 28 February 2012 Keywords: X-ray photoelectron spectroscopy X-ray fluorescence Silver coins

a b s t r a c t Ancient silver coins of different historical periods going from IV cent. B.C. up to recent XIX century, coming from different Mediterranean countries have been investigated with different surface physical analyses. X-ray photoelectron spectroscopy (XPS) analysis has been performed by using electron emission induced by 1.4 keV X-rays. X-ray fluorescence (XRF) analysis has been devoted by using 30 keV electron beam. Scanning electron microscopy (SEM) has been employed to analyze the surface morphology and the X-ray map distribution by using a 30 keV microbeam. Techniques were used to investigate about the patina composition and trace elements as a function of the sample depth obtained coupling XPS to 3 keV argon ion sputtering technique. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The compositional analysis of ancient coins, based on bulk and patina impurity patterns, represents an important aspect of the archeological research. In particular, the patina layer, representing an important aspect of the coin surface, endures strong changes, as a consequence of the coin conservation environment, impacts with other materials, absorbent contaminants promoted by thermal and chemical processes, etc. Its composition may be changed as a result of the interaction between its constituting metal alloy and the environmental agents or contaminants. On ancient coins, which have been usually buried in the ground for thousands of years, the patina, from an archeometrist’s point of view, is essential for identifying the nature of the environment in which the coins remained so long. Compositional data constitute a fundamental tool in the study of ancient coins and in the knowledge of extractive sites and fabrication technologies [1]. Many physical techniques can be used to investigate the patina elemental composition. X-ray photoelectron spectroscopy (XPS), energy dispersed X-ray fluorescence (EDXRF), electron microscopy (SEM), surface profilometry (SP), mass spectrometric techniques (MST), inductively

∗ Corresponding author at: Facoltà di Scienze MM. FF. NN., Università di Messina, V.le F. Stagno D’Alcontres 31, Messina, Italy. Tel.: +39 0906765120. E-mail address: [email protected] (F. Caridi). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.071

coupled plasma-mass spectrometry (ICP-MS), and laser ablationmass quadrupole spectrometry (LAMQS), represent only some of the many analysis techniques useful for these investigations [2–5]. The main techniques adopted for the present study were X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF) coupled to SEM analysis, both possessing easy sample preparation together with a good attainable degree of experimental information. Furthermore, they can be considered non-invasive and can be safely used according to the integrity requirements of the analyzed pieces. In the present investigation, in order to analyze the composition and the depth profile of peculiar elements, these two techniques are employed to study a set of ancient silver coins coming from the Mediterranean. 2. Materials and methods The silver coins submitted to XPS and XRF analyses were a 500 Lire coin, 1965, Italian Republic, a Denario Romano, II B.C. and a Dracma, Alessandro Magno, Macedonia, IV B.C. Each coin, after been cleaned in an ultrasounds bath, was directly analyzed without any other special preparation of its surface. A photo of the three coins is reported in the inset of Figs. 1–3. The patina chemical composition was investigated by means of non-invasive X-ray photoelectron spectroscopy (XPS) acquiring spectra by means of a K␣ system from Thermo Scientific equipped

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Fig. 2. XPS results for the Denario Romano, II B.C., for an etching time of 0 s (a) and 250 s (b), respectively.

Fig. 1. X-ray photoelectron spectroscopy (XPS) results for the 500 Lire coin, 1965, Italian Republic, for an etching time of 0 s (a) and XPS results for the 500 Lire coin, 1965, Italian Republic, for an etching time of 351 s (b).

3. Results

with a monochromatic Al K␣ source (1486.6 eV) and operating with an analyzer in constant analyzer energy (CAE) mode with a pass energy from 50 to 200 eV for survey and high resolution spectra, respectively. Inside the analyzer are maintained electrostatic fields to allow electrons of a specific kinetic energy (pass energy) by setting voltages, to arrive at the detector slit and onto the detector itself. A spot size diameter on the sample of about 400 ␮m was adopted. Surface charging effects occurring in an isolant samples were avoided using an electron flood gun. Ar+ ions with an energy of 3 keV were used for the sputtering procedure in order to perform the depth profile measurements. XPS analysis permitted to investigate on about 5 nm thickness; the coupling with the ion sputtering gave depth profiles of peculiar elements. The sputtering rate was of the order of 0.3 nm/s and sputtering depths of the order of 100 nm were investigated using a time sputtering of about 350 s. The non-invasive XRF spectroscopy permitted to evaluate the elemental composition by using 30 keV electron beam of a SEM microprobe (scanning electron microscopy). Elemental composition was monitored for thicknesses of the order of few microns. A Si(Li) detector with a thin Be windows was employed to investigated on the elements with atomic number higher than 6.

Results of the X-ray photoelectron spectroscopy (XPS) are reported in Fig. 1 for the 500 Lire coin, 1965, Italian Republic, for an etching time of 0 s (a) and 351 s (b), respectively. In the first case a great amount of carbon, silver and oxygen and trace elements as Cu, Cl and Na was detected on the coin surface, while only a high quantity of silver (carbon, oxygen, chlorine, copper and sodium as trace elements) was detected in the patina at a depth of the order of 100 nm, after a sputtering procedure with Ar+ ions at an energy of 3 keV, for an etching time of 351 s. Results obtained for an older silver coin, Denario Romano, II B.C., are reported in Fig. 2 for an etching time of 0 s (a) and 250 s (b), respectively. In particular a greater amount of trace elements (Si, Au, Ca, Cu, Cl and Na) was detected without the sputtering procedure (etching time = 0 s) while, also in this case, a high quantity of silver was detected in the patina at a depth of the order of 75 nm (etching time = 250 s). Similar results were obtained for the oldest coin, Dracma, Alessandro Magno, Macedonia, IV B.C., as reported in Fig. 3 for an etching time of 0 s (a) and 351 s (b), respectively. A great amount of trace elements (Cu, Cl, Na, Si) was detected without etching time and, also in this case, a high peak of silver was detected in the patina at a depth of the order of 100 nm (etching time = 351 s). The XPS permitted to evaluate the elemental composition of coins as a function of the etching time, as reported in Fig. 4. In

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Fig. 3. XPS results for the Dracma, Alessandro Magno, Macedonia, IV B.C., for an etching time of 0 s (a) and 351 s (b), respectively.

particular for the 500 Lire coin the atomic percent profile shows a fast increase of the silver content until a percentage of about 45% at the sputtering depth of the order of 100 nm. It also shows a decrease of other elements as carbon, oxygen, chlorine and sodium and a slow increase of copper, as reported in Fig. 4a. A special attention of the XPS analyses was given to the Ag/O ratio measured by detecting characteristic peaks of relative spectra. This ratio increases with the sputtering depth until a value of 11.41 at about 100 nm in the surface patina, confirming that the oxygen and the silver oxide decrease with the depth, and, on the contrary, the silver content increases. Results obtained for the Denario Romano coin are reported in Fig. 4b. The atomic content profile shows a fast increase of the silver amount until a percentage of about 70% at the maximum sputtering depth, while all other element content decreases. The Ag/O ratio increases with the sputtering depth until a value of 17.23 at about 75 nm in the surface patina. Similar results were obtained for the Dracma coin, as reported in Fig. 4c. The atomic content profile shows a fast increase of the silver amount until a percentage of about 65% at the maximum sputtering depth, while all other element content decreases. The Ag/O ratio increases with the sputtering depth until a value of 16.36 at about 100 nm in the surface patina. These results must be linked to the extremely thin region (about 100 nm) analyzed in the surface patina. In fact the patina full thickness evaluation in the 500 Lire is of about 30 ␮m and

in the Dracma is of about 200 ␮m, as reported in literature [6]. Fig. 5 shows a typical XRF spectrum on a wide region of the 500 Lire coin, where characteristic peaks of carbon, chlorine, copper, silver and oxygen were identified. The weight percentage is of 7.54%, 0.35%, 1.40%, 65.43% and 25.28%, respectively, as reported. In this case the Ag/O ratio is 2.59, at a depth of few microns in the surface patina, typical of XRF analyses. Fig. 6 shows a typical XRF spectrum of the Denario Romano, where characteristic peaks of carbon, chlorine, silver and oxygen were identified. The weight percentage is of 5.38%, 2.04%, 72.83% and 19.75%, respectively, as reported. In this case the Ag/O ratio is 3.68, at a depth of few microns. Fig. 7 shows a typical XRF spectrum of the Dracma coin, where characteristic peaks of carbon, chlorine, silver and oxygen were identified. The weight percentage is of 5.59%, 1.61%, 72.51% and 20.28%, respectively, as reported. In this case the Ag/O ratio is 3.57 for few micron depth, and its trend with the coin age is in good agreement with XPS results. Non-invasive X-ray fluorescence (XRF) analyses demonstrated that coins are composed by a matrix with a high silver content. Trace elements are present in surface and their content generally increases with the age of the coin. The high oxygen content of the patina is consistent with a possible presence of high concentrations of AgOH, Ag2 O, AgNO3 and Ag2 CO3 compounds.

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Fig. 4. XPS elemental composition of coins as a function of the etching time.

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Fig. 5. A typical XRF spectrum of the 500 Lire coin.

Fig. 6. A typical XRF spectrum of the Denario Romano coin.

Fig. 7. A typical XRF spectrum of the Dracma coin.

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The content of other elements is consistent with the presence of chemical compounds such as Ag2 S, AgCl, Ag2 CrO4 , AgBrO3 and AgIO3 . 4. Discussion and conclusions XPS and XRF represent useful non-invasive techniques to analyze the surface layers of metallic samples such as coins. Obtained results for the characterization of the silver oxide patina thickness of recent and old coins are very interesting for numismatic classification of coins, for historical and archeological investigations, for museum coin conservation, for a comparison between coin composition and mineral composition and to distinguish true coins from false ones [7–10]. In this work investigations concerning the presence of trace impurity in the patina and the study of the Ag/O ratio were performed. Results reported in this article can be compared with the evaluation of the silver oxide patina thickness for the same coins obtained with laser ablation-mass quadrupole spectrometry (LAMQS) and surface profile analysis [6]. LAMQS is a micro-invasive technique and it was employed to evaluate the Ag/O ratio as a function of the irradiation time at 1 Hz repetition rate, i.e. by measuring the ratio vs. depth with a resolution given by the ablation yield of 1 ␮m/pulse.

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This technique has demonstrated that analyzing old silver coins and measuring the patina thickness, the Ag/O ratio is lower in old coins and it increases in recent ones, demonstrating that the coin patina is generally rich in silver oxide proportionally to the age of the coin. The bulk is rich in silver, while the surface is rich in silver oxide. References [1] D. Castrizio, in: I. Antinoupolis (Ed.), Preliminary Relation, vol. I, Istituto Papirologico G. Vitelli Scavi e Materiali, Firenze, 2008, pp. 217–227. [2] M. Resano, E. Garcia-Ruiz, F. Vanhaecke, Mass Spectrom. Rev. 29 (2010) 55–78. [3] S. Kyuseok, C. Hyungki, K. Dukhyeon, M. Kihyung, Bull. Korean Chem. Soc. 25 (1) (2004) 101–105. [4] B. Giussani, D. Monticelli, L. Rampazzi, Anal. Chim. Acta 635 (2009) 6–21. [5] L. Torrisi, F. Caridi, L. Giuffrida, A. Torrisi, G. Mondio, T. Serafino, M. Caltabiano, E.D. Castrizio, E. Paniz, A. Salici, NIM B 268 (2010) 1657–1664. [6] L. Torrisi, G. Mondio, A.M. Mezzasalma, D. Margarone, F. Caridi, T. Serafino, A. Torrisi, Eur. Phys. J. D 54 (2009) 225–232. [7] L. Torrisi, F. Caridi, A. Borrielli, L. Giuffrida, A. Torrisi, G. Mondio, A. Mezzasalma, T. Serafino, M. Caltabiano, E.D. Castrizio, E. Paniz, M. Romeo, A. Salici, Radiat. Eff. Defects Solids 165 (6–10) (2010) 626–636. [8] F. Caridi, L. Torrisi, A. Borrielli, G. Mondio, Radiat. Eff. Defects Solids 165 (6) (2010) 668–680. [9] D. Aiello, A. Buccolieri, G. Buccolieri, A. Castellano, M. Di Giulio, L. Sandra Leo, A. Lorusso, G. Nassisi, V. Nassisi, L. Torrisi, in: XVI International Symp. on Gas Flow and Chemical Laser & High Power Laser Conference, Gmunden, Proc. SPIE 6346 (2006). [10] A. Picciotto, L. Torrisi, D. Margarone, P. Bellutti, Radiat. Eff. Defects Solids 165 (6–10) (2010) 706–712.