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Analysis of ancient Indian silver punch-marked coins by external PIXE
Applied Radiation and Isotopes 69 (2011) 1385–1389
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Analysis of ancient Indian silver punch-marked coins by external PIXE Tapash R. Rautray a,b,n, Suman S. Nayak c, Bipin B. Tripathy d, Saubhagyalaxmi Das b, Manas R. Das e, Satya R. Das f, Pranab K. Chattopadhyay g a
Department of Dental Biomaterials, School of Dentistry, Kyungpook National University, Daegu, Republic of Korea Institute of Physics, Bhubaneswar 751005, Orissa, India c Department of Computer Science, College of Engineering, University of Illinois at Urbana Champaign, 201 North Goodwin Avenue, Urbana, IL 61801-2302, USA d Department of Physics, Silicon Institute of Technology, Patia, Bhubaneswar 751024, India e Department of Civil Engineering, Institute of Technical education and Research, SOA University, Jagmohan Nagar, Bhubaneswar 751030, India f Department of Computer Science, Institute of Technical education and Research, SOA University, Jagmohan Nagar, Bhubaneswar 751030, India g Centre for Archaeological Studies and Training, Eastern India, 4 Camac Street (1st Floor), Kolkata 700016, India b
a r t i c l e i n f o
abstract
Article history: Received 21 January 2011 Received in revised form 14 June 2011 Accepted 14 June 2011 Available online 30 June 2011
Seven silver punch-marked coins were analysed using external particle induced X-ray emission technique. The main group of elements like Ag, Cu, Au, Pb and Fe were estimated along with a number of trace/minor elements such as K, Ca, Ti, V, Cr, Mn, Co, Ni and Rb in the analysed silver coins. Gold was found in all the coins and varied between 0.7% and 6.2% indicating the better economic condition of that civilisation. & 2011 Elsevier Ltd. All rights reserved.
Keywords: External PIXE Punch-marked coins Elemental analysis Silver
1. Introduction Punch-marked coins are the oldest known numismatics used in ancient India. Hence its compositional analysis gives valuable information regarding coin minting methodology, politics and economics of the time. Coins are important objects of our cultural heritage and are usually of high artistic and cultural value. Interest in the elemental analysis of ancient coins has increased in recent years due to their direct relation with monetary theory, economic changes and metallurgy of the minting time. Moreover the analysis of coins gives some other important characteristics like minting period, provenance and political aspects. Nondestructive techniques are preferred for analysis of ancient coins as they are precious and unique. Among the various non-destructive techniques, external proton induced X-ray emission (external PIXE) is a unique technique for analysis of ancient coins because the technique is not only non-destructive but also has other features like multi-elemental capabilities, small sample mass requirements, high sensitivity, large dynamic range, and simple or virtually no need for sample preparation (Johansson and n Corresponding author at: Kyungpook National University, Department of Dental Biomaterials, School of Dentistry, Daegu, Republic of Korea. Tel.: þ91 94375 29311. E-mail address: [email protected] (T.R. Rautray).
0969-8043/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2011.06.008
Campbell, 1988; Fleming and Swann, 1993; Flament and Marchetti, 2004). PIXE is based on the measurements of characteristic X-rays induced by the energetic proton beam (MeV energy scale) directed onto the surface of a specimen (Rautray et al., 2010a,b; Kurosawa et al., 1998). This technique has been successfully used for almost three decades for analysis of various types of samples (Mando, 1995; Del Carmine et al., 1993; Vijayan et al., 1995; Hajivaliei et al., 1999; Kumar et al., 2002). This technique employs a proton beam from the 3 MV Tandem pelletron accelerator and samples are irradiated while kept in vacuum. But, while dealing with specific samples of archaeological (sword, axe, etc.), biological (bone, teeth, etc.) and geological (big rocks, etc.) materials, the pre-requisite of pellet-making makes the conventional PIXE technique unusable because either the sample or a part of it needs to be destroyed at the sample preparation stage itself. To overcome this difficulty, there is a need to bring the proton beam outside the accelerator vacuum to carry out the analysis by the external PIXE method. This external PIXE technique is not only non-destructive but also can be used to analyse samples of almost any size and type (Vijayan et al., 2003; Rautray et al., 2010a,b). Apart from easy sample handling and positioning in air, this technique has several advantages. Objects of large size and complex structure and shape can be analysed in-situ without the need of sampling, a fact particularly valuable for art objects.
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T.R. Rautray et al. / Applied Radiation and Isotopes 69 (2011) 1385–1389
Also, it is possible to study materials containing volatile compounds, which could otherwise not withstand a vacuum. The risk of damage due to heating is considerably reduced because of cooling arrangements in air outside the PIXE chamber. Another positive aspect is that the charging of insulating materials can be totally avoided without the need of a thin conducting coating, which is required when operating under vacuum. Indeed, charging effects can produce potentials up to several tens of keV and accelerate secondary electrons with a subsequent extension of the bremsstrahlung background to higher energy. The detection limit would then be markedly increased. These characteristics of the external PIXE technique have successfully been utilised in the compositional analysis of archaeological materials (Menu, 1993; Raisanen, 1992). Punch-marked coins are the oldest known numismatics used in ancient India. In ancient times punch-marked coins were produced in the form of silver punch-marked coins and copper punch-marked coins. Silver punch-marked coins are classified into many series and more than 600 varieties are found almost all over the Indian subcontinent. The importance of these ancient punch-marked coins as a tracer in archaeological excavations and their major role in historical studies is unanimously acknowledged. Presently this analysis investigates seven Indian punchmarked silver coins in order to find the compositional analysis of these coins, possible correlation between the composition of these coins, minting methodology, minting time and political aspects prevalent in ancient India by using the external PIXE technique (Vijayan et al., 2004).
2. Experimental The punch-marked coins were suitably cleaned before analysis. The coins were immersed in a 10% formic acid solution and left to soak for about two days. Afterwards the coins were washed with distilled water, dried in fresh air and cleaned with a glass bristle brush (Tripathy et al., 2010). The proton beam of 3 MeV energy obtained from the 3 MV tandem type pelletron accelerator was collimated by a graphite collimator to a beam size of 2 mm diameter and the beam was extracted into air using a KaptonTM foil at the exit point of a vacuum scattering chamber (8 mm thickness). The scattering chamber has an inner diameter of 80 cm and was designed to cater to the requirements of the external proton beam. The beam is first focused and centred at the target location inside the scattering chamber and then passed through the thin KaptonTM foil placed at the exit port. The chamber is pumped by a high throughput diffstack pump to maintain a vacuum in the range of 10 6 Torr in the chamber and the beam line. The KaptonTM foil is used as an exit window due to its special characteristics like low beam-induced background emission, minimal energy loss and resistance to radiation damage. The beam is allowed to travel 3 cm in air at which point the energy gets reduced to about 2.4 MeV, and the proton beam then irradiates the samples. Beam charge measurements were carried out using a rotating vane chopper as described by Mando (1995). For the measurements to be described below, the samples were kept in air over a sample stand making an angle of 451 to the beam direction. The samples were irradiated with a maximum beam current of 10 nA passing through the 8 mm thick KaptonTM window. A Si(Li) detector, having energy resolution full-width at half maximum (FWHM) of 170 eV at 5.9 keV placed at 901 with respect to the beam direction, was used to detect characteristic X-rays emitted from the target. The detector has an active area of 30 mm2 and an entrance beryllium window of 8 mm thickness. A 25 mm thick aluminised Mylar absorber (with 6% hole) was kept
in front of the detector to attenuate the bremsstrahlung background and the dominant low energy X-ray peaks. Spectra were recorded by using a Canberra S-100 multi channel analyser, which was energy calibrated with 241Am X-ray source. The PIXE analysis was performed using GUPIX-2000 software, which provides nonlinear least square fitting of the spectrum together with subsequent conversion of the X-ray peak intensities to elemental concentrations via a user-defined instrument constant (H), relative charge and X-ray energy dependant H-value. The instrumental constant (H) is the product of the geometric solid angle of the X-ray detector and any systematic normalisation factor present in the charge integration system (Rautray et al., 2007).
3. Punch marked (silver) coins The punch-marked coins were in use in ancient India between 6th Century BC to 6th century AD. They are found in different shapes and sizes as shown in Fig. 1. On one side, several symbols are punched. The process of making punch-marked coins can be summarised in four steps i.e. (i) the metals were first melted in crucibles and purified with alkalies, (ii) they were then beaten into sheets on an anvil with a hammer, (iii) the sheets were later cut into pieces with clippers to a particular weight and then finally (iv) the pieces were stamped with dies or punches having symbols. The present hoard was recovered from the Suktel river valley in Sakma area of Bolangir District, Orissa, India identified to be the ancient site of ‘Suktimati-pura’, the ancient capital city of the Chedi dynasty to which the ancestors of Kharavela belonged. The site is datable to 2nd century BC. The Suktel river has mingled with the Tel river near the confluence of the Mahanadi and the Tel river at Sonepur and is referred in the Seravanijia Jatak of Buddhist literature as ‘‘Telavahanadi’’. It is believed that extensive trade activities were conducted through the Tel river. The finding of ancient silver punch-marked coins in an ancient river valley with developed metal technology, as evident from the current study, is of immense academic importance. Punchmarked silver coins do not reveal any unilateral line of development so far as the manufacturing techniques are concerned and the employment of one technique or the other has very little significance in chronological terms. In fact, the practise of punching small symbols on one side of the coins, leaving the other side completely blank, persisted in ancient time. The first step of
Fig. 1. Both sides of silver punch-marked coins.
T.R. Rautray et al. / Applied Radiation and Isotopes 69 (2011) 1385–1389
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Table 1 Concentration (in weight %) of elements in punch-marked coins. Elements
Coin 1
Coin 2
Coin 3
Coin 4
Coin 5
Coin 6
Coin 7
K Ca Ti V Cr Mn Fe Co Ni Cu Rb Ag Pb Au
2.6 0.87 1.97 2.6 1.5 1.3 3.4 0.45 1.9 14.3 0.46 61.4 1.9 1.2
4.1 1.3 2.2 2.4 1.6 1.3 3.6 0.63 2.1 11.4 0.51 63.5 2.4 0.9
1.9 1.2 2.0 2.0 1.6 0.73 2.9 0.49 0.46 7.9 0.33 67.9 0.89 2.7
1.0 0.54 1.1 1.9 1.4 0.42 4.6 0.86 0.84 6.5 0.27 74.2 1.1 4.3
1.2 0.31 1.4 1.2 1.1 0.71 6.2 0.71 0.56 4.4 0.30 81.7 2.1 6.2
0.95 0.46 0.89 1.8 1.3 0.93 7.3 0.38 1.3 9.7 0.42 65.0 0.7 5.3
1.11 3.8 0.73 0.7 1.3 1.2 6.8 0.66 0.41 18.5 0.44 56.3 0.5 0.7
Fig. 3. Ag and Cu percentage in the analysed punch-marked silver coins.
Fig. 2. Negative correlation between Ag and Cu.
a numismatist’s work, the visual examination, may not always be sufficient to classify the coins, which requires elemental analysis. The present work was mostly aimed at ‘how effectively’ external PIXE can be used for characterising ancient punch-marked coins. The results in the current study show large variations in concentrations of elements from sample to sample and the results are shown in Table 1. 4. Results and discussion From the results it can be inferred that coins were primarily silver coins with copper, iron, gold and lead as the other major constituents. The content of silver in the studied punch-marked coins was found between 56.3% and 81.7% and that of copper was found between 4.4% and 18.5%. The strong negative correlation between Ag and Cu (Fig. 2) is an indication that the substitution of silver by copper was made and copper was deliberately added with silver for hardening purposes and economic reasons, as it is less expensive than silver. Also since small pure silver coins were physically inconvenient and prone to loss, copper was perhaps used as a carrier or matrix to give the coins their desired size and, convenient weight for the purpose of circulation. It is well known that in ancient India there was a paucity of silver and hence silver might have been obtained from foreign countries of western Asia like Persia and Afghanistan for the manufacture of punch-marked coins. The percentage of Ag and Cu in the analysed coins is shown in Fig. 3. Moreover there is a strong positive correlation between Ag and Au (shown in Fig. 4). Hence it can be concluded that as the percentage of Ag in the coin increases, then the concentration of
Fig. 4. Positive co-relation between Ag and Au.
Au increases with a decrease in Cu amount. The variation of Ag, Cu and Au percentage in the analysed coins is shown in Fig. 5 in a 3D diagram. The relative variation of Ag, Cu, Au and Pb in each coin is shown in the bar diagram (Fig. 6; the elements having less than 1% are not shown in the figure). The analysis shows that the coin having the highest percentage of silver has the highest percentage of gold but has the lowest percentage of copper (Coin 5). Similarly the coin having the lowest percentage of silver has the highest percentage of copper, and the lowest percentage of gold (Coin 7). The percentage of gold in the analysed coins varied between 0.7% and 6.2%, which shows that this metal may have been deliberately added to the coin alloys and may be used as a provenance indicator. Also gold is usually taken as an indicator of the origin of the silver used, as the metal does not oxidise during the melting of ore to obtain silver. The absence of Zn and Sn in the analysed coins indicates that Cu was added as pure metal not as a Cu alloy. The content of lead varies from 0.5% to 2.4% in the studied coins, which indicates that the silver refining process was not so good during ancient time in India, as silver is originally obtained in the lead ore ‘argentiferrous galena’. There was a range of variation regarding the weight of the punch-marked coins, which was due to the rise and fall in the price of silver. The weight of each coin was between 2.5 and 3.5 g, which indicates that the coins were dated before 50 AD (Radhakrishnan, 1998). Besides the main group of elements such as Ag, Cu, Au, and Pb, which are related to silver ores and their
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Fig. 5. 3D diagram of Ag–Cu–Au concentration.
Fig. 6. Variation of Ag, Cu, Au and Pb in each coin.
metallurgy, we have also detected other elements, like K, Ca, Ti, V, Cr, Mn, Co, Ni and Rb as minor/trace elements. These minor/trace components originate mainly from the different ores or from the (geographically and chronologically) varying manufacturing processes used for silver production (Linke et al., 2004). Civici et al. (2007) analysed a large number of silver coins (about 120) of the 3rd century BC from Kreshpan hoard (Albania) of different groups by energy dispersive X-ray florescence (EDXRF). The analysis showed that Ag, Cu, Pb, Au and Bi are the main group of elements with Ag concentration in the range of 88.3–99.3%. In addition to the main group of elements they had also detected trace elements like Cl, Ca, Fe, Br and Zn. Kallithrakas-Kanotos et al. (2000) analysed Alexander silver coins of 4th century BC by EDXRF technique, where the concentration of Ag varied between 95.3% and 98.3%. They have also detected Cl, Fe, Au, Pb, Ti, Cr, Mn, Ni, Cu, Zn and Bi as minor/trace elements. In the current study there is a wide variation of Ag percentage in the Indian punch-marked silver coins, which indicates that the numismatics policy was not uniform, further indicating the fragmentation of power of the rulers giving rise to various production processes. The symbols and the weight of these analysed coins show that the coins were produced during the Mouryan period (except Coin 5, which might have been
made before the Mouryan period). According to Jaina tradition, Koutilya issued eight billion debased silver coins called karshpan to fill the treasury. The manufacturing and use of coins in such a large scale led to the deterioration in the quality of coins (wide variation of Ag percentage). In our recent studies on the analyses of punch-marked coins of Bengal (Chattopadhyay et al., 2009), we found that the analysed silver coins were of high purity (Ag 78.78–92.08%, Cu 1.78–14.12%). Moreover, if we compare the punch-marked silver coins with the Kreshpan hoard silver coins and Alexander silver coins, some interesting conclusions can be drawn. Trace elements like K, V, Co and Rb are present in the punch-marked coins but are absent in the other categories of coins. Bismuth, which is present as a minor element and trace/minor element in Kreshpan hoard silver coins and Alexander silver coins, respectively, is absent in punchmarked silver coins. Again Zn is absent in the studied Indian punch-marked coins, but is present as a trace element in the Kreshpan hoard and Alexander silver coins.
5. Conclusion Seven silver punch-marked coins were analysed for their elemental compositions. The external PIXE set-up was very useful for the elemental analysis of our punch-marked coins. Ag, Cu, Au, Pb and Fe were found to be the main constituents of the silver punch-marked coins. The presence of trace/minor elements like K, Ca, Ti, V, Cr, Mn, Co, Ni and Rb have also been detected. Gold was found in all the coins and varied between 0.7% and 6.2% indicating the better economic condition of that civilisation. The present elemental analysis results were compared with the Kreshpan hoard silver coins and Alexander silver coins, which were minted at the same time period as the punch-marked coins and some important conclusions were drawn. The compositional analysis indicates clearly that external PIXE can be used effectively for the non-destructive quantitative analysis of ancient coins.
Acknowledgements The authors are indebted to the staff members of Ion beam laboratory for their help during the course of the present work.
T.R. Rautray et al. / Applied Radiation and Isotopes 69 (2011) 1385–1389
References Chattopadhyay, P.K., Datta, P.K., Vijayan, V., Rautray, T.R., 2009. Punch-marked coins of Bengal: archaeometallurgical studies. In: Hoque, M.M. (Ed.), Selected Essay on History and Archaeology. CAHR, Dhaka. Civici, N., Gjongecaj, Sh., Stamati, F., Dilo, T., Pavlidou, E., Polychroniadis, E.K., Smit, Z., 2007. Compositional study of IIIrd century BC silver coins from Kreshpan hoard (Albania) using EDXRF spectrometry. Nucl. Instrum. Methods B 258, 414–420. Del Carmine, P., Lucarelli, F., Mando, P.A., Pecchioli, A., 1993. The external PIXE setup for the analysis of manuscripts at the Florence University. Nucl. Instrum. Methods B 75, 480–484. Flament, C., Marchetti, P., 2004. Analysis of ancient silver coins. Nucl. Instrum. Methods B 226, 179–184. Fleming, S.J., Swann, C.P., 1993. Recent applications of PIXE spectrometry in archaeology. Part I: observations on the early development of copper metallurgy in the Old World. Nucl. Instrum Methods B 75, 440–444. Hajivaliei, M., Garg, M.L., Handa, D.K., Govil, K.L., Kakavand, T., Vijayan, V., Singh, K.P., Govil, I.M., 1999. PIXE analysis of ancient Indian coins. Nucl. Instrum. Methods B 150, 645–650. Johansson, S.A.E., Campbell, J.L., 1988. PIXE: A Novel Technique for Elemental Analysis. John Wiley and Sons, New York. Kallithrakas-Kanotos, N., Katsanos, A.A., Touratsoglou, J., 2000. Trace element analysis of Alexander the Great’s silver tetradrachms minted in Macedonia. Nucl. Instrum. Methods B 171, 342–349. Kumar, R.A., Kennedy, V.J., Sasikala, K., Jude, A.L.C., Ashok, M., Moretto, Ph., 2002. Trace element analysis of blood samples from mentally challenged children by PIXE. Nucl. Instrum. Methods B 190, 449–452. Kurosawa, M., Sueno, S., Shima, K., Ohsima, H., Ishii, S., Kamiya, H., Kimoto, S., Ohyi, H., Hayashi, K., 1998. Development of a new sample chamber for proton
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microprobe analysis of mineral samples. Nucl. Instrum. Methods B 142, 599–605. Linke, R., Schreiner, M., Demortier, G., 2004. The application of photon, electron and proton induced X-ray analysis for the identification and characterisation of medieval silver coins. Nucl. Instrum. Methods B 226, 172–178. Mando, P.A., 1995. Advantages and limitations of external beams in applications to arts and archeology, geology and environmental problems. Nucl. Instrum. Methods B 85, 815–823. Menu, M., 1993. External beam applications to painting materials. Nucl. Instrum. Methods B 75, 469–475. Radhakrishnan, P.V., 1998. Chronology of silver punch-marked coins in south India. Studies in South Indian Coins 8, 27–34. Raisanen, J., 1992. Experimental arrangements for non vacuum bio-PIXE. Int. J. PIXE 2, 339–350. Rautray, T.R., Vijayan, V., Panigrahi, S., 2007. Analysis of Indian pigment gallstones. Nucl. Instrum. Methods B 255, 409–415. Rautray, T.R., Das, S.L., Rautray, A.C., 2010a. In situ analysis of human teeth by external PIXE. Nucl. Instrm. Methods B 268, 2371–2374. Rautray, T.R., Dutta, K., Das, S.L., Rautray, A.C., 2010b. In situ analysis of gallstone inner layers by external PIXE. Nucl. Instrum. Methods B 268, 2773–2776. Tripathy, B.B., Rautray, T.R., Rautray, A.C., Vijayan, V., 2010. Elemental analysis of silver coins by PIXE technique. Appl. Radiat. Isot. 68, 454–458. Vijayan, V., Ramamurthy, V.S., Behera, S.N., 1995. Particle induced X-ray emission analysis of coal fly ash. Int. J. PIXE 5, 211–219. Vijayan, V., Choudhury, R.K., Mallick, B., Sahu, S., Choudhury, S.K., Lenka, H.P., Rautray, T.R., Nayak, P.K., 2003. External proton induced X-ray emission. Curr. Sci. 85, 772–777. Vijayan, V., Rautray, T.R., Basa, D.K., 2004. EDXRF study of Indian punch-marked silver coins. Nucl. Instrum. Methods B 225, 353–356.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Analysis of ancient Indian silver punch-marked coins by external PIXE Tapash R. Rautray a,b,n, Suman S. Nayak c, Bipin B. Tripathy d, Saubhagyalaxmi Das b, Manas R. Das e, Satya R. Das f, Pranab K. Chattopadhyay g a
Department of Dental Biomaterials, School of Dentistry, Kyungpook National University, Daegu, Republic of Korea Institute of Physics, Bhubaneswar 751005, Orissa, India c Department of Computer Science, College of Engineering, University of Illinois at Urbana Champaign, 201 North Goodwin Avenue, Urbana, IL 61801-2302, USA d Department of Physics, Silicon Institute of Technology, Patia, Bhubaneswar 751024, India e Department of Civil Engineering, Institute of Technical education and Research, SOA University, Jagmohan Nagar, Bhubaneswar 751030, India f Department of Computer Science, Institute of Technical education and Research, SOA University, Jagmohan Nagar, Bhubaneswar 751030, India g Centre for Archaeological Studies and Training, Eastern India, 4 Camac Street (1st Floor), Kolkata 700016, India b
a r t i c l e i n f o
abstract
Article history: Received 21 January 2011 Received in revised form 14 June 2011 Accepted 14 June 2011 Available online 30 June 2011
Seven silver punch-marked coins were analysed using external particle induced X-ray emission technique. The main group of elements like Ag, Cu, Au, Pb and Fe were estimated along with a number of trace/minor elements such as K, Ca, Ti, V, Cr, Mn, Co, Ni and Rb in the analysed silver coins. Gold was found in all the coins and varied between 0.7% and 6.2% indicating the better economic condition of that civilisation. & 2011 Elsevier Ltd. All rights reserved.
Keywords: External PIXE Punch-marked coins Elemental analysis Silver
1. Introduction Punch-marked coins are the oldest known numismatics used in ancient India. Hence its compositional analysis gives valuable information regarding coin minting methodology, politics and economics of the time. Coins are important objects of our cultural heritage and are usually of high artistic and cultural value. Interest in the elemental analysis of ancient coins has increased in recent years due to their direct relation with monetary theory, economic changes and metallurgy of the minting time. Moreover the analysis of coins gives some other important characteristics like minting period, provenance and political aspects. Nondestructive techniques are preferred for analysis of ancient coins as they are precious and unique. Among the various non-destructive techniques, external proton induced X-ray emission (external PIXE) is a unique technique for analysis of ancient coins because the technique is not only non-destructive but also has other features like multi-elemental capabilities, small sample mass requirements, high sensitivity, large dynamic range, and simple or virtually no need for sample preparation (Johansson and n Corresponding author at: Kyungpook National University, Department of Dental Biomaterials, School of Dentistry, Daegu, Republic of Korea. Tel.: þ91 94375 29311. E-mail address: [email protected] (T.R. Rautray).
0969-8043/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2011.06.008
Campbell, 1988; Fleming and Swann, 1993; Flament and Marchetti, 2004). PIXE is based on the measurements of characteristic X-rays induced by the energetic proton beam (MeV energy scale) directed onto the surface of a specimen (Rautray et al., 2010a,b; Kurosawa et al., 1998). This technique has been successfully used for almost three decades for analysis of various types of samples (Mando, 1995; Del Carmine et al., 1993; Vijayan et al., 1995; Hajivaliei et al., 1999; Kumar et al., 2002). This technique employs a proton beam from the 3 MV Tandem pelletron accelerator and samples are irradiated while kept in vacuum. But, while dealing with specific samples of archaeological (sword, axe, etc.), biological (bone, teeth, etc.) and geological (big rocks, etc.) materials, the pre-requisite of pellet-making makes the conventional PIXE technique unusable because either the sample or a part of it needs to be destroyed at the sample preparation stage itself. To overcome this difficulty, there is a need to bring the proton beam outside the accelerator vacuum to carry out the analysis by the external PIXE method. This external PIXE technique is not only non-destructive but also can be used to analyse samples of almost any size and type (Vijayan et al., 2003; Rautray et al., 2010a,b). Apart from easy sample handling and positioning in air, this technique has several advantages. Objects of large size and complex structure and shape can be analysed in-situ without the need of sampling, a fact particularly valuable for art objects.
1386
T.R. Rautray et al. / Applied Radiation and Isotopes 69 (2011) 1385–1389
Also, it is possible to study materials containing volatile compounds, which could otherwise not withstand a vacuum. The risk of damage due to heating is considerably reduced because of cooling arrangements in air outside the PIXE chamber. Another positive aspect is that the charging of insulating materials can be totally avoided without the need of a thin conducting coating, which is required when operating under vacuum. Indeed, charging effects can produce potentials up to several tens of keV and accelerate secondary electrons with a subsequent extension of the bremsstrahlung background to higher energy. The detection limit would then be markedly increased. These characteristics of the external PIXE technique have successfully been utilised in the compositional analysis of archaeological materials (Menu, 1993; Raisanen, 1992). Punch-marked coins are the oldest known numismatics used in ancient India. In ancient times punch-marked coins were produced in the form of silver punch-marked coins and copper punch-marked coins. Silver punch-marked coins are classified into many series and more than 600 varieties are found almost all over the Indian subcontinent. The importance of these ancient punch-marked coins as a tracer in archaeological excavations and their major role in historical studies is unanimously acknowledged. Presently this analysis investigates seven Indian punchmarked silver coins in order to find the compositional analysis of these coins, possible correlation between the composition of these coins, minting methodology, minting time and political aspects prevalent in ancient India by using the external PIXE technique (Vijayan et al., 2004).
2. Experimental The punch-marked coins were suitably cleaned before analysis. The coins were immersed in a 10% formic acid solution and left to soak for about two days. Afterwards the coins were washed with distilled water, dried in fresh air and cleaned with a glass bristle brush (Tripathy et al., 2010). The proton beam of 3 MeV energy obtained from the 3 MV tandem type pelletron accelerator was collimated by a graphite collimator to a beam size of 2 mm diameter and the beam was extracted into air using a KaptonTM foil at the exit point of a vacuum scattering chamber (8 mm thickness). The scattering chamber has an inner diameter of 80 cm and was designed to cater to the requirements of the external proton beam. The beam is first focused and centred at the target location inside the scattering chamber and then passed through the thin KaptonTM foil placed at the exit port. The chamber is pumped by a high throughput diffstack pump to maintain a vacuum in the range of 10 6 Torr in the chamber and the beam line. The KaptonTM foil is used as an exit window due to its special characteristics like low beam-induced background emission, minimal energy loss and resistance to radiation damage. The beam is allowed to travel 3 cm in air at which point the energy gets reduced to about 2.4 MeV, and the proton beam then irradiates the samples. Beam charge measurements were carried out using a rotating vane chopper as described by Mando (1995). For the measurements to be described below, the samples were kept in air over a sample stand making an angle of 451 to the beam direction. The samples were irradiated with a maximum beam current of 10 nA passing through the 8 mm thick KaptonTM window. A Si(Li) detector, having energy resolution full-width at half maximum (FWHM) of 170 eV at 5.9 keV placed at 901 with respect to the beam direction, was used to detect characteristic X-rays emitted from the target. The detector has an active area of 30 mm2 and an entrance beryllium window of 8 mm thickness. A 25 mm thick aluminised Mylar absorber (with 6% hole) was kept
in front of the detector to attenuate the bremsstrahlung background and the dominant low energy X-ray peaks. Spectra were recorded by using a Canberra S-100 multi channel analyser, which was energy calibrated with 241Am X-ray source. The PIXE analysis was performed using GUPIX-2000 software, which provides nonlinear least square fitting of the spectrum together with subsequent conversion of the X-ray peak intensities to elemental concentrations via a user-defined instrument constant (H), relative charge and X-ray energy dependant H-value. The instrumental constant (H) is the product of the geometric solid angle of the X-ray detector and any systematic normalisation factor present in the charge integration system (Rautray et al., 2007).
3. Punch marked (silver) coins The punch-marked coins were in use in ancient India between 6th Century BC to 6th century AD. They are found in different shapes and sizes as shown in Fig. 1. On one side, several symbols are punched. The process of making punch-marked coins can be summarised in four steps i.e. (i) the metals were first melted in crucibles and purified with alkalies, (ii) they were then beaten into sheets on an anvil with a hammer, (iii) the sheets were later cut into pieces with clippers to a particular weight and then finally (iv) the pieces were stamped with dies or punches having symbols. The present hoard was recovered from the Suktel river valley in Sakma area of Bolangir District, Orissa, India identified to be the ancient site of ‘Suktimati-pura’, the ancient capital city of the Chedi dynasty to which the ancestors of Kharavela belonged. The site is datable to 2nd century BC. The Suktel river has mingled with the Tel river near the confluence of the Mahanadi and the Tel river at Sonepur and is referred in the Seravanijia Jatak of Buddhist literature as ‘‘Telavahanadi’’. It is believed that extensive trade activities were conducted through the Tel river. The finding of ancient silver punch-marked coins in an ancient river valley with developed metal technology, as evident from the current study, is of immense academic importance. Punchmarked silver coins do not reveal any unilateral line of development so far as the manufacturing techniques are concerned and the employment of one technique or the other has very little significance in chronological terms. In fact, the practise of punching small symbols on one side of the coins, leaving the other side completely blank, persisted in ancient time. The first step of
Fig. 1. Both sides of silver punch-marked coins.
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Table 1 Concentration (in weight %) of elements in punch-marked coins. Elements
Coin 1
Coin 2
Coin 3
Coin 4
Coin 5
Coin 6
Coin 7
K Ca Ti V Cr Mn Fe Co Ni Cu Rb Ag Pb Au
2.6 0.87 1.97 2.6 1.5 1.3 3.4 0.45 1.9 14.3 0.46 61.4 1.9 1.2
4.1 1.3 2.2 2.4 1.6 1.3 3.6 0.63 2.1 11.4 0.51 63.5 2.4 0.9
1.9 1.2 2.0 2.0 1.6 0.73 2.9 0.49 0.46 7.9 0.33 67.9 0.89 2.7
1.0 0.54 1.1 1.9 1.4 0.42 4.6 0.86 0.84 6.5 0.27 74.2 1.1 4.3
1.2 0.31 1.4 1.2 1.1 0.71 6.2 0.71 0.56 4.4 0.30 81.7 2.1 6.2
0.95 0.46 0.89 1.8 1.3 0.93 7.3 0.38 1.3 9.7 0.42 65.0 0.7 5.3
1.11 3.8 0.73 0.7 1.3 1.2 6.8 0.66 0.41 18.5 0.44 56.3 0.5 0.7
Fig. 3. Ag and Cu percentage in the analysed punch-marked silver coins.
Fig. 2. Negative correlation between Ag and Cu.
a numismatist’s work, the visual examination, may not always be sufficient to classify the coins, which requires elemental analysis. The present work was mostly aimed at ‘how effectively’ external PIXE can be used for characterising ancient punch-marked coins. The results in the current study show large variations in concentrations of elements from sample to sample and the results are shown in Table 1. 4. Results and discussion From the results it can be inferred that coins were primarily silver coins with copper, iron, gold and lead as the other major constituents. The content of silver in the studied punch-marked coins was found between 56.3% and 81.7% and that of copper was found between 4.4% and 18.5%. The strong negative correlation between Ag and Cu (Fig. 2) is an indication that the substitution of silver by copper was made and copper was deliberately added with silver for hardening purposes and economic reasons, as it is less expensive than silver. Also since small pure silver coins were physically inconvenient and prone to loss, copper was perhaps used as a carrier or matrix to give the coins their desired size and, convenient weight for the purpose of circulation. It is well known that in ancient India there was a paucity of silver and hence silver might have been obtained from foreign countries of western Asia like Persia and Afghanistan for the manufacture of punch-marked coins. The percentage of Ag and Cu in the analysed coins is shown in Fig. 3. Moreover there is a strong positive correlation between Ag and Au (shown in Fig. 4). Hence it can be concluded that as the percentage of Ag in the coin increases, then the concentration of
Fig. 4. Positive co-relation between Ag and Au.
Au increases with a decrease in Cu amount. The variation of Ag, Cu and Au percentage in the analysed coins is shown in Fig. 5 in a 3D diagram. The relative variation of Ag, Cu, Au and Pb in each coin is shown in the bar diagram (Fig. 6; the elements having less than 1% are not shown in the figure). The analysis shows that the coin having the highest percentage of silver has the highest percentage of gold but has the lowest percentage of copper (Coin 5). Similarly the coin having the lowest percentage of silver has the highest percentage of copper, and the lowest percentage of gold (Coin 7). The percentage of gold in the analysed coins varied between 0.7% and 6.2%, which shows that this metal may have been deliberately added to the coin alloys and may be used as a provenance indicator. Also gold is usually taken as an indicator of the origin of the silver used, as the metal does not oxidise during the melting of ore to obtain silver. The absence of Zn and Sn in the analysed coins indicates that Cu was added as pure metal not as a Cu alloy. The content of lead varies from 0.5% to 2.4% in the studied coins, which indicates that the silver refining process was not so good during ancient time in India, as silver is originally obtained in the lead ore ‘argentiferrous galena’. There was a range of variation regarding the weight of the punch-marked coins, which was due to the rise and fall in the price of silver. The weight of each coin was between 2.5 and 3.5 g, which indicates that the coins were dated before 50 AD (Radhakrishnan, 1998). Besides the main group of elements such as Ag, Cu, Au, and Pb, which are related to silver ores and their
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Fig. 5. 3D diagram of Ag–Cu–Au concentration.
Fig. 6. Variation of Ag, Cu, Au and Pb in each coin.
metallurgy, we have also detected other elements, like K, Ca, Ti, V, Cr, Mn, Co, Ni and Rb as minor/trace elements. These minor/trace components originate mainly from the different ores or from the (geographically and chronologically) varying manufacturing processes used for silver production (Linke et al., 2004). Civici et al. (2007) analysed a large number of silver coins (about 120) of the 3rd century BC from Kreshpan hoard (Albania) of different groups by energy dispersive X-ray florescence (EDXRF). The analysis showed that Ag, Cu, Pb, Au and Bi are the main group of elements with Ag concentration in the range of 88.3–99.3%. In addition to the main group of elements they had also detected trace elements like Cl, Ca, Fe, Br and Zn. Kallithrakas-Kanotos et al. (2000) analysed Alexander silver coins of 4th century BC by EDXRF technique, where the concentration of Ag varied between 95.3% and 98.3%. They have also detected Cl, Fe, Au, Pb, Ti, Cr, Mn, Ni, Cu, Zn and Bi as minor/trace elements. In the current study there is a wide variation of Ag percentage in the Indian punch-marked silver coins, which indicates that the numismatics policy was not uniform, further indicating the fragmentation of power of the rulers giving rise to various production processes. The symbols and the weight of these analysed coins show that the coins were produced during the Mouryan period (except Coin 5, which might have been
made before the Mouryan period). According to Jaina tradition, Koutilya issued eight billion debased silver coins called karshpan to fill the treasury. The manufacturing and use of coins in such a large scale led to the deterioration in the quality of coins (wide variation of Ag percentage). In our recent studies on the analyses of punch-marked coins of Bengal (Chattopadhyay et al., 2009), we found that the analysed silver coins were of high purity (Ag 78.78–92.08%, Cu 1.78–14.12%). Moreover, if we compare the punch-marked silver coins with the Kreshpan hoard silver coins and Alexander silver coins, some interesting conclusions can be drawn. Trace elements like K, V, Co and Rb are present in the punch-marked coins but are absent in the other categories of coins. Bismuth, which is present as a minor element and trace/minor element in Kreshpan hoard silver coins and Alexander silver coins, respectively, is absent in punchmarked silver coins. Again Zn is absent in the studied Indian punch-marked coins, but is present as a trace element in the Kreshpan hoard and Alexander silver coins.
5. Conclusion Seven silver punch-marked coins were analysed for their elemental compositions. The external PIXE set-up was very useful for the elemental analysis of our punch-marked coins. Ag, Cu, Au, Pb and Fe were found to be the main constituents of the silver punch-marked coins. The presence of trace/minor elements like K, Ca, Ti, V, Cr, Mn, Co, Ni and Rb have also been detected. Gold was found in all the coins and varied between 0.7% and 6.2% indicating the better economic condition of that civilisation. The present elemental analysis results were compared with the Kreshpan hoard silver coins and Alexander silver coins, which were minted at the same time period as the punch-marked coins and some important conclusions were drawn. The compositional analysis indicates clearly that external PIXE can be used effectively for the non-destructive quantitative analysis of ancient coins.
Acknowledgements The authors are indebted to the staff members of Ion beam laboratory for their help during the course of the present work.
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