Provenance study of rubies from a Parthian statuette by PIXE analysis

.H J&I __ __

NlONl

BB ELSEVIER

B

Beam Interactions with Materials 8 Atoms

Nuclear Instruments

and Methods

in Physics

Research

B 136-138

(1998) 846-850

Provenance study of rubies from a Parthian statuette by PIXE analysis

Abstract The aim of this work was the characterization of three red inlays of a Pdrthian statuette (3rd century B.C.) originating from Babylon, Mesopotamia. The external beam setup of AGLAE was used in order to determine the chemical composition of the inlays. PIXE analysis confirmed the expected nature of the cabochons: ruby, and not colored glass as previously reported. According to these results. this archaeological object contains one of the oldest rubies left today. Moreover, quantitative analysis of trace elements allowed us to infer the geographic origin of these gems. PIXE analyses of a batch of 57 rubies from nine well-known sources were carried out in order to establish a geochemical database. The use of multivariate statistical methods enables us to restrict the possible provenance to either Ceylon (Sri Lanka) or more likely Burma (Myanmar). 0 1998 Elsevier Science B.V.

1. Introduction Ion beam analysis (IBA) techniques are particularly suitable for the chemical analysis of gemstones because of their non-destructive character, their high sensitivity and the use of an external beam. For several years, our laboratory has successfully applied these techniques to antique jewelry [1,2]. The present study concerns a Parthian statuette (3rd century B.C.) originating from Babylon, Mesopotamia which is now located at the Louvre museum. This remarkable object, made of alabaster, represents the pagan goddess Ishtar, equivalent to Aphrodite in Greek mythology (Fig. 1). Her eyes and navel are formed by three encrusted clear red stones which some curators be*Corresponding

author.

0168.583X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved PIISO168-583X(97)00894-X

lieve to be rubies although they have been considered to be red glass [3] by others. The external beam of the AGLAE IBA facility [4] at the Laboratoire de recherche des mu&es de France has been used to characterize these inlays, because the size of the statuette (25 cm) and its fragility (the gemstones and headdress are stuck in place) excluded any analysis under vacuum. In-air PIXE/PIGE investigation established that these inlays were natural rubies. The origin of the rubies of this antique statuette was quite problematic since there are no known sources of rubies in Mesopotamia or surrounding countries [5]. By comparing the chemical composition of these rubies to those of a collection of rubies coming from well established sources, we have been able to give the probable geographical origin for these gemstones.

T. Culliguro

et cd. I Nd.

Instr.

und Mdz.

in Phys. Rex B 136-138

(I YWI 846--X50

847

cancelled by the octahedral crystal field and transitions can occur between the fundamental level, A?, and the excited levels 4Ti, 4T7 and ‘E. This gives two optical absorption bands centered at 2.2 and 3.0 eV and a fluorescence line at 1.79 eV, and therefore the red color. The PIXE technique is well suited for the characterization of this material (Al,03) containing less than 1% of metallic impurities, since it has an excellent sensitivity for the chromophoric transition elements (Ti, V, Cr, Fe, etc). Simultaneously, the light elements such as Be, B and F which might be present, can be detected with PIGE. Moreover, the small diameter of the proton beam on target (co.5 mm) means that large inclusions in the gem can be included or excluded during the measurement. PIXE studies have been conducted for several years using macro-beams [8,9] and micro-beams [I 0,111 to study trace elements in red rubies in order to determine their geological or synthetic nature, and their provenance. The ability to detect artificial treatments aimed at coloring rubies or at hiding cracks (defects) have also been proposed

1121.

Fig. I. Picture

of the Parthian

statuette.

2. Rubies and ion beam analysis

Gemstone ruby, like sapphire, is alumina (gAl,Oj) colored by the presence of impurities: chromium causes the red of ruby while iron and titanium are responsible for the blue color of sapphire [6]. The red color for the ruby is due to the presence of Cr3+ ions replacing aluminum in the A&O3 structure [7]. Indeed the degeneracy of the 3d orbitals (Cr3+ electronic configuration is [Ar]3d”) is

The external beam facility of AGLAE [13] was operated with 3 MeV proton beam. Collimated with a O.l-mm hole within the vacuum, the beam passes through a lo-urn thick Al exit foil and a 10 mm helium atmosphere path, leading to 0.5 mm diameter on the target. The beam intensity was monitored by detecting the protons backscattered from the exit foil. Each spectrum was obtained with a dose of 0.46 uC and a current between 0.5 and 1 nA. Using two Si(Li) and an HPGe detectors, a wide range of elements can be detected simultaneously. The Si( Li) detector devoted to low energy X-rays detection has an ultrathin window (Kevex SuperQuantum). This and the use of helium to replace the air on the X-ray path from the target to the detector allowed quantitative analysis major and minor elements (8 < Z < 30). and therefore permitted the gem identification. Trace elements (Z > 20) were measured with the second Si(Li) detector which had a larger solid angle and a 50-urn Al filter to absorb the low energy X-rays of the matrix.

848

T. Cal&m)

et ul. 1 Nucl. Instr. und Merh. in Plys.

mina content and hence the gemstones were clearly identified as rubies. Table 1 summarizes the results obtained on the three spots analyzed in each eye and the navel. The composition of the three rubies have a good similarity: they were probably extracted from the same location. The analysis of Ca and S was difficult because the beam halo possibly reached the statuette body made of alabaster (CaS04) when the small gems were analyzed. The surprisingly high amount of Cu, Zn and Pb obtained in some analyses could be due to inclusions or residues left by polishing tools, often made of Cu-based alloys. In order to determine the geographical origin of the rubies in the Statuette, a comparative study of the elemental composition with rubies of known provenance has been performed. The different sources and the number of gems from each location were the following: Afghanistan (4, Jagdalek), Myanmar ex Burma (11, Mogok), Cambodia (1, Pailin), India (3, Ja’ipur; 3, Mysore), Kenya (1, Tsavo), Madagascar (l), Sri Lanka, ex Ceylon (31, Ratnapura), Thailand, ex Siam (1, Chantaburi), Vietnam (1, Luc Yen). Although deposits in Cambodia, Kenya, Madagascar and Vietnam have been exploited only recently, samples from these deposits were included for the sake of comparison and

3. Results and discussion

The X-ray spectra obtained on the statuette’s gemstones are shown in Fig. 2. The spectrum from the detector for the light elements exhibits a dominant Al line and a few weak lines due to metallic elements (Ti, V, Cr, Fe). The spectrum recorded by the detector for the trace elements displays lines due to metallic elements mentioned above with better statistics and, in addition, lines due to Ga, Cu and at some spots Zn and Pb. Spectra processing by the GUPIX software [14] yielded a 99% alu-

Fig. 2. X-ray spectra of the left eye of the statuette the simultaneous use of two Si(Li) detectors. Table I PIXE results of the analyses

obtained

Rex B 136-13X (1998) N46tLSO

by

of the rubies from eyes and navel Fe

Cu

Zn

Ga

75 245 75 270 105 285

1150 1160 1360

140 245 160

< < II

< < <

I38 < 144 < 147 <

85 267

1223

182

II

<

143 <

135 95 1015 155 85 II75 160 190 1370

180 660 300

641 409 25

212 1710 <

23 9 I7 7 37 <

150

380

358

961

26 <

1180 1330 1490

< 166 400

53 42 80

85 53 121

166 82 211 30 I84 43

67 320 1333

283

58

86

187

52

4

2

2

6

K

160 260 810

< < 135

< < <

150 330 500

<

410

<

<

327

99.0 97.8 99.0

< < <

210 400 510

< I70 110

< < 175

4917

98.3

<

373

<

<

Navel #I Navel #2 Navel #3

1850 1120 1570

98.2 98.6 97.7

750 640 1280

2610 1550 3320

555 380 840

510 415 710

2010 1260 2840

60 280 60 325 80 355

Average

1847

98.3

890

2493

592

545

2037

300

80

95

45

35

Al>01

Si

Left eye #I Left eye #2 Left eye #3

2990 2480 2290

99.2 99.2 99.0

< < <

Average

2567

99.3

Right eye #I Right eye #2 Right eye #3

3460 9080 2210

Average

LOD Concentrations under LOD.

240

140

s

in ppm except AI?@ in weight%.

Ca

Cr

Cl

Mg

Ti

740 620 1250 870

20

LOD are the limits of detection

V

123 1187

13

IO

given by the GUPIX

2

program.

Pb

The sign < means

T. Cdliguro

et ul. I Nucl. Instr. und Meth. in Ph~s. Rrs. B 136-13X (19%)

for testing the criteria of chemical composition used to determine the rubies’ provenance. Except for raw samples from India and Afghanistan, all stones were cut. Two or three analyses were performed on each ruby, depending on its homogeneous or heterogeneous appearance, yielding a total of 130 analysis for the amount of Ti, V, Cr, Fe, Cu, Ga and Pb. The results summarized in Table 2 indicate markedly different amounts of Ti, V, Cr and Fe in the rubies coming from different locations, with a good agreement in the amounts for rubies from the same location. Rubies from Kenya and Madagascar exhibit strikingly different composition from those of Asian origin. In the latter, iron seems to play a dominant role, in agreement with previous studies [9, lo]. Box and whisker diagram (Fig. 3) show that Thai and Cambodian rubies have a very high concentration of this element (several thousands of ppm) in contrast to Vietnamese and Burmese rubies (less than 100 ppm). The other sources have intermediate concentrations of Fe. Concentration values for Ti, V and Cr from different sources partially overlap. Plots Fe vs. Cr (Fig. 4) and Cr vs. V show more clearly the differences between the rubies investigated. The data obtained for the gemstones of the statuette lay in a zone between values corresponding to those of Ceylon and those of Burma. A concentration of some 50 ppm for Ga, a chemical homologue of Al, has been found in all rubies, but its correlation with the rubies’ provenance is not significant [15]. As mentioned by other authors, tracTable 2 Elemental

composition

Afghanistan Burma Thailand Cambodia Vietnam Sri Lanka Madagascar India Kenya Statuette Standard

deviation

of different

provenances

Fig. 3. Box and Whisker ditrerent sources.

846X_%

plot of the Fe content

Fig. 4. Plot of Fe vs. Cr for different

es of Cu and Pb were their amounts did not source. The occasional was due to inclusions

of rubies from

locations

found in some rubies, but seem to be linked to their occurrence of Zr and Hf of zirconia. Gamma ray

in ppm

Ti

V

Cl

Fe

CU

Ga

Pb

203 (113) 164 (280) 131 (31) 61(11) 114 (21) 187 (108) 458 (45) 291 (240) 37 (6) 89 (43)

63 (40) 199 (139) 13 (7) 15 (8) 23 (8) 53 (27) 28 (17) 102 (76)

3900 ( 1854) 2418 (1693) 3585 (814) 3424 (148) 3302 (276) 611 (327) 4777 (358) 2590 (991) 2661 (280) 1313 (137)

710 (376) 73 (37) 3168 (619) 2590 (241) 89 (4) 713 (339) 4757 (82) 2279 (430) 4 (13) 155 (164)

5 (7) 19 (68) < < < i < < < 137 (222)

43 (9) 67 (33) 37 (II) 31 (2) 58 (3) 57(18) 58 (4) 134 (64) 240 (29) 116(70)

8 (4) 7 (12) i < i \ i i i 19 (26)

I (2) 236 (93)

is given in parentheses.

850

T. Culliguuro et ui. I Nucl. Instr. and Meth.

spectra did not show evidence of either F or B, which excluded any treatment for crack filling. This was an expected result for an antique gem. Since it is difficult to identify the geographical origin of a gemstone by the amount of only one impurity, multivariate statistical methods have been applied to the whole set of measurements. Hierarchical clustering, principal component analysis and discriminant analysis led to concordant results, namely: the rubies of the statuette have a composition very close to those of Burma and to a lesser extent to those of Sri Lanka.

4. Conclusion The analysis of ancient rubies stored in museums is a promising archaeological research field, because the history of the use of rubies is hardly known, and their introduction into Europe and the Near-east is relatively recent (Greek-Roman era). The present PIXE study of antique gemstones from a Parthian statuette indicates that (1) they are some of the oldest rubies left today, and (2) they probably came from Burma or Sri Lanka. These results are in agreement with Sanskrit texts written in IV-Xth century B.C. [ 16.171stating that rubies were extracted from deposits in India and Sri Lanka. New improvements of the external beam line of AGLAE [18] will allow more extensive investigations which might help to discriminate between the two possible sources of these rubies. Indeed. an external beam focused down to a 30-urn size will be well suited to the characterization of inclusions within these rubies, which have not been analyzed yet.

Acknowledgements

We wish to thank Mrs. D. Piat, A. Jarry, and G. Gropiron for providing the reference rubies.

in Phys. Rex

B 136-138

(1998) 846-850

Thanks are also due to J. Solomon and B. Moignard for running the accelerator, designing and building the accurate sample positioning system, and to H. Hamon for the development of the spectrum acquisition system. We are grateful to Mrs. A. Caubet and Dr. J.-C. Dran for his constant help and support.

References G. Querre. A. Bouquillon. Analusis 23 (1995) 25. G. Querre, A. Bouquillon. T. Calligaro. M. Dubus. J. Salomon. Nucl. Instr. and Meth. B 1091110 (1996) 686. B. Andre-Salvini. Les pierres precieuses de I’Orient ancien, des Sumeriens aux Sassanides.. Dossiers du musee du Louvre. editions de la Reunion des Musies Nationaux. 1995. [41 M. Menu. T. Calligaro. J. Salomon, G. Amsel. J. Mouhn. Nucl. Instr. and Meth. B 45 (1990) 610. [51 R.J. Forbes, Studies in Ancient Technology. vol. 7. Brill. Leiden, 1963. Corundum, ButterworthssHeinemann. [61 R.W. Hugues. London. 1990. 171 K. Nassau. The Physics and Chemistry of Color, Wiley. New York. 1983. PI S.M. Tang, S.H. Tang, T.S. Tay. A.T. Retty. Appi. Spect. 42 (1988) 44. [91 S.M. Tang. S.H. Tang. K.F. Mok. A.T. Retty. T.S. Tay. Appl. Spect. 43 (1989) 219. PO1 T. Osipowicz, T.S. Tay. I. Orlic, S.M. Tang. F. Watt, Nucl. Instr. and Meth. B 104 (1995) 590. [I 11 J.L. Sanchez. T. Osipowicz. S.M. Tang, T.S. Tay. T.T. Win Nucl. Instr. and Meth. B 130 (1997) 682. [131 T. Themelis. The Heat Treatment of Ruby and Sapphire. Gemlabs inc., 1992. J. Salomon. Nucl. Instr. []?I T. Calligaro. J.D. MacArthur. and Meth. B 109/110 (1996) 125. [I41 J.A. Maxwell, J.A. Campbell. W.J. Teesdale. Nucl. Instr. and Meth. B 43 (1989) 218. iI51 H.A. Hanni, W.B. Stein. Z. Dt. Gemmol. Ges. 31 (1982) 2555259. [I61 SM. Tdgore. Mani-mala or a Treatise on Gems, Bose. Calcutta.

I171 L. Finot. 1896. [I81 J. Salomon

1897. E. Bouillon

(Ed.).

Lapidaires

et al. these proceedings.

Indiens.

Paris.