Unusual Coin from the Parabita Hoard: Combined Use of Surface and Micro

Journal of Cultural Heritage 11 (2010) 233–238

Case study Unusual coin from the Parabita hoard: combined use of surface and micro-analytical techniques for its characterisation a, a a b b Antonio Serra ∗, Daniela Manno , Emanuela Filippo , Rosa Vitale , Giuseppe Sarcinelli , Lucio Calcagnile c, Gianluca Quarta c, Giuseppe Giovannelli d, Benedetto Bozzini e, Aldo Siciliano b a Dipartimento di Scienza dei Materiali, Università del Salento, 73100 Lecce, Italy b Dipartimento di Beni Culturali, Università del Salento, Lecce, Italy c CEDAD-Centro di Datazione e Diagnostica, Dipartimento di Ingegneria dell’Innovazione, Università del Salento, Lecce, Italy d Dipartimento ICMMPM, Università di Roma “La Sapienza”, Roma, Italy e Dipartimento di Ingegneria dell’Innovazione, Università del Salento, Lecce, Italy Received 7 January 2009; accepted 16 July 2009 Available online 13 November 2009

Abstract Out of the staters collection of the National Archaeological Museum of Taranto, during the full examination of about one hundred coins minted by the Greek colony of Taras between the V century BC and the III century BC, our attention has been devoted to a lead coin, which has been regarded for many years as a genuine silver coin. This artifact, entry number 13 in the inventory list for the Parabita hoard, has been studied with the combined use of surface and micro-analytical techniques (SEM, EDX, PIXE, XRD). The joint use of different analytical techniques allowed us to obtain information about the morphology, the structure and the chemical composition of the analysed coin, that revealed a lead core coated with a bi-layer of copper and silver. © 2009 Elsevier Masson SAS. All rights reserved.

Keywords: Great-Greece; South-Italy; V-III century BC; Coin; Micro-analytical techniques

1. Introduction attracted by a lead coin which has been regarded for many years as a genuine silver coin. This coin has been classified as entry The analysis of archaeological objects requires simultane- number 13 in the inventory list for the Parabita (Apulia, South ously non-destructive (the objects are unique and precious), fast Italy) hoard. Rather surprisingly, the elemental composition of (large number of pieces to be analyzed), versatile (samples with this coin went undetected through several examinations, until the different geometry), sensitive (trace elements are often impor- last few years. It was now that in the course of a comprehensive tant) and multi-elemental methods [1]. In this context, scanning research, performed by the authors, on the silver content of coins electron microscopy (SEM) linked to X-ray diffraction (XRD) minted by the Greek colony of Taras (actually Taranto–South to obtain information about the morphology and the structure Italy) allowed the identification of lead as the minting metal for of coins. Proton Induced X-ray emission (PIXE) and energy this specimen covered by a thin copper/silver patina. The coin dispersive X-ray (EDX) microanalysis linked to SEM investiga- is identified by Fischer-Bossert [2] as an unicum because of its tion have been used to determine the chemical composition of being minted with mismatched sides that combine obverse and the coin. reverse designs, both pertaining to the Western Greek coinage In this work, the authors, involved in the full examination of [3], but not usually paired in regular issues. about one hundred silver coins coming from collection of the It is right to emphasize that all the analysis were performed, National Museum of Taranto, minted by the Greek colony of maintained in great consideration the elevated historical and Taras between the the V century BC and the III century BC, was artistic value of piece, and, in agreement with the staff in charge of the “Soprintendenza della Regione Puglia”, the coin has been analysed any hard cleaning procedure and, for morphological

∗ Corresponding author. and EDX characterization making use of “low vacuum mode” E-mail address: [email protected] (A. Serra). to avoid charge effect due to pieces more contaminated.

observation was performed in low-vacuum mode. SEM images have been obtained by backscattered electrons (BSE) with an accelerating voltage of 20 kV and a beam current of 80 ␮A. To analyse the coin composition, about 20 area (of about 2000 ␮m2) were randomly selected on the two side of coin, X-ray spectra were recorded both at 20 and 30 KV acceleration voltage, the X-ray intensities were converted in wt% atomic concentration by ZAF4/FLS quantitative analyses software support of Oxford-Link Analytical (UK). Fig. 1. Images of the silver- coated “Parabita 13” coin. The coating is still present The X-Ray spectra have been collected for 200s for every on the edges. SEM image; this allowed to recognise both the main elements and the trace elements. SEM-EDX analysis is a fast method for 1.1. The numismatic problem element identification and quantization. It allows to obtain the concentration of the element by the relationship: The Parabita hoard, discovered in the 1948, represent a very Isp interesting numismatic case, among the constituting coins, there C Cst Fsp = I is a very rare specimen, the entry number 13 in the inven- ! st " tory list. This coin (showed in Fig. 1) was marked from W. where Isp and Ist are the intensities of the element in the sam- Fischer-Bossert as very interesting for the peculiarity due to the ple and in the standard respectively, Cst is the concentration unexpected coupling reverse D/- and obverse R/types never finds of the standard and Fsp is a factor obtained by the ZAF cor- up to now in other coins [2]. The coin typology is the following: rection procedure in which ideal flat samples are assumed. In fact, due to electron-sample interactions, there occur processes D/Rider crowing horse walking right, dolphin below; which influence the production and collection of X-Rays. The • R/young man on dolphin right holding phrigian helmet, stars ZAF procedure performs a correction for the atomic number • on either side, waves below. effect (Z), the absorption effect (A) and the fluorescence effect (F). Z represents the difference in electron scattering and retar- The coin is identified as an unicum because of its being dation between the sample and the standard. Loss of X-Rays due minted with mismatched sides that combine obverse and reverse to absorption in the sample is represented by (A) and the artifi- designs. The typology of D/ is known for different series nom- cial increase of X-Ray intensity of an element, due to ionization inally defined “campano-tarantine”. Indeed, the reverse of the by X-Rays originated by another element, is corrected by (F). coin is linked (typologically, but not technically) to a series of Without correction, errors in excess of 10% could result. Tarentine didrachms minted in the years 333-331/0 BC, the stan- Afterwards, an X-Ray dotted map has been acquired for one dard weight of which is 7.8 g. The obverse design of the coin hour for every chemical element recognized by the X-Ray spec- belongs to a series of so-called Campano-Tarentine didrachms trum. The X-Ray maps are images in which the contrast depends [4], minted in the middle of the 3rd century BC, not in Taranto, on the punctual presence of the element selected in the X-Ray but presumably in Campania or Lucania. The standard weight spectra: the brighter areas correspond to a higher concentration of this latter series is about 0.8 g lighter than the Tarentine one of the element; darker ones correspond to a lower concentration. of the reverse. The specimen Parabita 13 is 8.05 g and conse- Several acquisitions were performed on the coin surfaces; in this quently, as weight considerations dictate, it was attributed [2] to way, the distribution of elements on the analysed region has been the heavier (i.e. older) series. identified and corrosion phenomena were identified. One must The typology of D/ reminds the woman, head in the emissions mention that the compositions we obtained is relevant only for of Neapolis. On the contrary, the typology of R/ with the youth a thin layer at the surface (tens of micrometers in depth). onto a dolphin, it is typical of coins minted in Taras. The problem of surface enrichments has been considered by collect X-ray spectra at 30 kV acceleration voltage. So, AgK␣ 2. Experimental and AgL␣ signals referred to CuK␣ and CuL␣ signals at 30 kV acceleration voltage have been considered: whereas AgL␣ radi- A detailed morphological and chemical composition charac- ation (energy 3.0 keV) originates dominantly from a depth of up terisation as well as the microchemical structure of the corrosion to 2 ␮m, the information depth of the AgK␣ radiation (energy products grown on the silver coins was obtained by SEM meth- 22.2 keV) is up to 20 ␮m in a silver–copper matrix owing to its ods. A scanning electron microscope Jeol JSM 5410-LVcoupled higher energy [5]. Ag-Cu alloys (being Ag w% 98, 95, 90, 80 and to an Oxford Link ISIS 300 Series energy-dispersive spectrom- 70) have been used in order to determine the calibration curves eter having a Si(Li) windowsless detector has been used to and information on silver concentration at different depth were perform EDX microanalysis. The apparatus allows a resolution obtained. However, the spatial distribution of elements, as we of 156 eV. Because the analysed coins are precious historical shall see later, excludes the possibility of a process of surface artefacts and they cannot endure some type of cleaning, SEM enrichment, as described by Beck et al. [1]. observation were performed without any kind of polishing and XRD patterns were recorded directly on the silver coin, by in the case of elevated contamination due to corrosion, the multiple scanning using a Rigaku miniflex diffractometer. The A. Serra et al. / Journal of Cultural Heritage 11 (2010) 233–238 235 scan rate adopted was 1 deg/s. The employed radiation was below in as-found conditions. As illustrated in thumbnail images unmonochromated CuK␣. shown in Fig. 1, it is almost completely covered with a layer of PIXE analyses were performed at CEntro di DAtazione e corrosion products; on both sides of the coin, the peripheral area Diagnostica (CEDAD), University of Salento, Lecce, Italy by is sparsely covered with the remnants of what could have been extracting in air a 3.5 MeV, 1 mm large proton beam through a an applied white metal coating. The image processing allowed 16 ␮m thick Al extraction window. Characteristic X-rays were the detection of some subtle features. On the uncovered area of detected by a Si(Li) detector with 80 mm2 active area and energy the coin, some surface flaws recalling the solidification patterns resolution (FWHM) of 150 eV at 5.9 keV. The analysis of the of a cast metal core [6] are observed; these flaws are partially PIXE spectra was carried out by using the GUPIX code. obliterated in the relief portions by intentional brushing (two nearly orthogonal bands of roughly parallel carved microglyph 3. Results and discussion can be detected on the device of the coin. Intentional brushing is also observed on the rim of the artefact seemingly to eliminate 3.1. Morphological and compositional analysis possible edge seams. Additional surface scratches (may be pur- posely inflicted in order to provide evidence of forgery) on the This coin did not receive any conservation treatment and con- remnant coating strongly suggest that this could be constituted sequently, it has been subjected to the investigation described by a two-metals layer.

Fig. 2. Morphological features of the edge still covered (picture a) obtained by secondary electrons signal. Analysed edge of the coin (picture b). Compositional X-ray maps showing the spatial distribution of silver (picture c), copper (picture d), lead (picture e), and antimony (picture f). Magniﬁcation of silvered region (picture g). 236 A. Serra et al. / Journal of Cultural Heritage 11 (2010) 233–238

Fig. 3. PIXE analyses of the covered and uncovered areas.

The morphological and compositional analysis evidences 3.2. Structural analyses widths regions to elevated concentration of Pb, with moderated concentration of As and Sb and traces of Ag and Cu. The mor- The topics of the alteration products on lead metallic archae- phologic analysis carried out on the edge of the coin (Fig. 2) puts ological findings have been covered on several works [7] in evidence the detailed structure of the optically visible traces and the measurement of the corrosion extent of archaeologi- of film that, probably at the beginning, covered it all. Compo- cal lead artefacts has been performed [8]; according to these sitional maps (Fig. 2c–f) reveal that the film is made by two studies, cerussite (PbCO3), the orthorombic (massicot) and layers: under, to contact with the lead heart, there is a thin layer the tetragonal variety (litharge) of PbO and hydrocerussite of copper and so an other thin silver layer. The SEM images of (Pb3)(CO3)2(OH)2 are expected. Fig. 3 shows an X-rays diffrac- the residual white-metal coating exhibit a cauliflower granular tion spectrum of the uncoated portion of the coin. Indexing of this morphology, crushed by coinage, with average diameter of ca. XRD pattern reveals that the surface area is composed of Pb cor- 15 20 ␮m; such morphologies are typical of metals grown by rosion products (mainly hydrocerussite, massicot and litharge) electrochemical÷ processes. The tips of the granules are worn and with metallic Pb as minor component. No Ag-related reflections exhibit a strong compositional contrast with respect to the rest could be detected. The XRD pattern shown in Fig. 4, pertaining of the crystallite. EDX mapping shows that the material of the worn tip of the grains is Cu while rest of the coated surface is Ag. These facts support the interpretation of the coating as a bilayer with a Cu underlayer and an Ag top-layer. The EDX spectra of the uncovered areas showed clear Pb, Sb, O and C peaks, traces of As and Si, thus hinting at an heavily corroded inner core of a Pb rich (90–95% Pb) Pb-Sb alloy. PIXE analyses confirm the different compositions of the covered and uncovered areas (Fig. 3). Infact, covered areas show intense signals corresponding to Cu and Ag, which are not seen in the spectrum acquired for the uncovered area where only intense signals from Pb and Sb are evidenced. In order to obtain a rough estimation of the thickness of the patina, PIXE spectra were also interpreted by using the GUPIX iterative routine for the analysis of layered targets. For this purpose, the sample was (roughly) schematised as formed by a bulk Pb + Sb alloy and an uppermost Cu + Ag layer. Under these hypotheses, a thickness of the patina ranging between 2 and 15 ␮m was estimated over Fig. 4. X-ray diffraction spectrum of the uncovered (a) and coated portion (b), the sample. respectively, of the “Parabita 13” coin. A. Serra et al. / Journal of Cultural Heritage 11 (2010) 233–238 237 to the coated area, indicates the presence of metallic Ag and On the basis of the following pieces of evidence: Cu. The texture analysis of the reflections pertaining to both Ag and Cu indicate a (111) preferred orientation, which can impossibility of producing these layers of high melting-point hardly be reconciled with a pyrometallurgical coating process, • metals onto a low melting-point substrate by a pyrometallur- as it would be the case for a subærati-type counterfeit coin [9]. gical approach; In fact, the diffractograms of hammered-and-annealed metals SEM morphology exhibiting a bumpy structure of the [10] would show (220) and (311) as the most intense reflec- • cauliflower structure, crushed by coinage, which can be tions, this ensuing from the very complex mixed textures that obtained only by an electrochemical phenomenon; FCC metals develop upon thermomechanical treatments [11,12]. XRD texture which allows to rule out hammering-and- The observed (111) prevailing orientation is quite surprisingly • annealing processes. typical of an electrochemical deposition process [13]. We can conclude that the achievement of the observed struc- 4. Conclusions ture by an electrochemical displacement process is extremely likely. As long as nobody proves that this coin is a modern intrusion There is no written record showing positive use of electro- into the Parabita hoard, the collected data reveal a contempo- chemical displacement reactions in classical antiquity, either rary counterfeiting procedure (most probably dating back to the rd in hydrometallurgical processes or in base metal coating treat- 3 century BC when combing types of two different series was ments. While it is claimed [18] that the use of metal iron to in use [2] based a duplex Cu-Ag patination process onto a lead recover Cu from sulphate solutions was current practice in substrate. This technique does not belong to any established ancient China and Arabic Spain, the first citation in the liter- method for the growth of an artificially silvered coating in classi- ature to this reaction is found in the works of Paracelsus [19] cal antiquity. Indeed, the gathered evidence clearly indicates that who mentioned the process to support his alchemic belief on the original appearance of this artefact neither can be explained metal transmutation. in terms of the pyrometallurgical process utilised to obtain the This investigation opens up several problems that are relevant subærati-type counterfeit coins [14], nor can it be reconciled to both the technologies of heritage conservation and restoration. with the various colouring treatments [15] for silver imitation, of Among these, the following can be pinpointed for future studies: which we are now aware of having been adopted in ancient times. the archive search for similar examples of metal plated archae- In the pyrometallurgical process, a production method [9] ological items, which might have escaped identification owing based on diffusion annealing treatments of copper flans wrapped to the fact that very often, the scholars in charge of the study of with a thin (250 ␮m thick) silver foil was in use in order to these objects have limited surface-science skills. obtain “stuffed” flans; striking of these “stuffed flans” at ambi- ent temperature ensued. 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