The Application of Trace Element and Isotopic Analyses to the Study of Celtic Gold Coins and their Metal Sources. Chris Bendall Johann Wolfgang Goethe University-Frankfurt 2003 The Application of Trace Element and Isotopic Analyses to the Study of Celtic Gold Coins and their Metal Sources. Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften vorgelegt beim Fachbereich Geowissenschaften der Johann Wolfgang Goethe-Universität in Frankfurt am Main Von Chris Bendall Oxford Frankfurt (2003) ii vom Fachberiech Geowissenschaften der Johann Wolfgang Goethe-Universität als Dissertation angenommen Dekan: Gutachter: Datum der Disputation: iii I would firstly like to thank those people and institutions which provided coins and gold samples for analysis, they include: Celtic coins: S. Berger; Historisches Museum, Frankfurt Dr. K.-J. Gilles; Rheinisches Landesmuseum, Trier Johan van Heesch;Cabinet des Medailles, Biblioteque Royale, Brussels Gino Languini; Wallendorf Francois Reinert, M.A. ; Musée National d’Histoire et d’Art, Luxemburg Dr. David Wigg-Wolf ; Fundmünzen der Antike, Johann Wolfgang Goethe- Universität (Includes the coins excavated from the Martberg, Sanctuary site) Gold Samples: Bruno van Eerdenbrugh; Belgium Dr. Beda Hofmann; Naturisches Museum Bern, Switzerland Werner Störk; AG Minifossi, Stuttgart Secondly, I would like to thank everyone within the Institute of Mineralogy, Uni- Frankfurt, who have all helped in one way or another to make the study possible. And last but not least my beautiful family Adi and Avi who are more amazing and wonderful than anything else. iv Table of Contents Abstract (English) 1 Abstract (German) 3 German Summary 5 I. Introduction 13 II. Aims 14 III. Synopsis 14 1.0 Numismatic Context and Ancient Metallurgical Methods 16 1.1 Numismatics 16 1.1.1 Celtic society 16 1.1.2 The Celts and their coins: origins and use 16 Phase 1 (3rd century B.C.) 19 Phase 2 (c.200-125 B.C.) 20 Phase 3 (125-60BC) 20 Phase 4 (60-20 BC) 21 1.2 Metallurgy and the Colour of Money 22 1.3 Metal Refining Processes 24 1.3.1 Gold Refining 24 1.3.2 Silver 24 1.3.3 Copper 25 1.4 Casting and Striking 26 2.0 Analytical Methods Chapter 28 2.1 Introduction 28 2.2 Sample preparation 28 2.3 EPMA 29 2.3.1 Coins 30 2.4 Trace Element Fingerprinting by LA-ICPMS 33 2.4.1 Element Fractionation 35 2.4.2 Standardistion 36 a) Nebulised Solution Calibration 36 b) 100% normalisation 36 2.4.3 Quantification 37 a) Using solution standards 37 v b) Choosing the Internal Standard 39 c) Data Reduction 39 d) The precision and accuracy of the method 39 2.4.4 Limitations of the technique 40 2.4.5 Conclusion: 41 2.5 Pb Isotopes 41 2.5.1 Solution Procedure 42 2.5.2 Laser Ablation Method 44 2.6 Cu Isotopes 46 2.6.1 Chromatographic and Mass Bias Considerations 47 a) Chromatographic considerations 47 b) Correcting for Instrument Mass Bias 48 c) Spike addition and measurement procedure 49 d) Reproducibility 49 2.6.2 Laser Method 51 2.6.3 Conclusions 53 2.7 Silver and Copper coinages 54 2.7.1 Sample preparation 54 2.7.2 EPMA 54 2.7.3 Isotopic anaylses 55 2.8 Natural Gold Samples 56 2.8.1 Sample Preparation 56 2.8.2 EPMA 56 2.8.3 Trace Elements 56 3.0 Coin Chapter 57 3.1 Coin Weights 57 3.2 Coin Alloys 59 3.2.1 Scheers 23 63 3.2.2 Scheers 18 63 3.2.3 Scheers 16 64 3.2.4 Rainbow Cups 65 3.2.5 Scheers 30/I 66 3.2.6 Scheers 30/IV 67 vi 3.2.7 Series 30/V POTTINA 68 3.2.8 Scheers 30/VI ARDA 69 3.2.9 Flans 70 3.2.10 Silver and Copper coinages 71 3.2.11 Discussion 74 3.3 Evidence for Techniques from Metallographic Studies 75 3.4 Trace Elements 78 3.4.1 Determining Useful Traces 78 3.4.2 Observing changes in metal sources 79 3.4.3 Correlations 80 a) The correlation of Te with Ag 80 b)The correlation of Sb and Ni with Cu 82 c) The Pt and Au correlation 84 3.4.4 Summary 86 3.5 Pb Isotopes 87 3.5.1 Results 87 3.5.2 Controls on mixing lines 88 3.5.3 Regional comparison 90 3.6 Cu Isotopes 91 3.7 Discussion 93 3.8 Summary 96 3.9 Recommendations for future work 98 4.0 Geochemistry of Gold Sources 99 4.1 Introduction 99 4.2 Gold Deposits Types and Classification 99 4.3 Placer gold; characteristics and classification 101 4.3.1 Gold Morhpohology and Classification 101 4.3.2 Gold Compositions. 102 4.3.3 Gold Rimming 102 a) Previous Studies 102 b) Observations from this study 106 4.3.4 Inclusions 107 vii 4.4 Trace Elements 109 4.4.1 Selecting elements unaffected by transport effects 110 4.4.2 Visualising the data 114 4.4.3 Summary 114 4.5 Gold Sample Localities 114 4.5.1 Germany 114 a) The Rheingold 114 b) Eder, Hessen (M-7) 118 c) Schwarza, Thüringen (M-17) 118 4.5.2 Switzerland 119 4.5.3 France 122 4.5.4 Belgium 125 4.5.5 Scotland and Italy 126 4.6 Discussion 126 4.7 Pb Isotopes 128 4.7.1 Results 130 4.7.2 Conclusions 131 4.8 Gold Chapter; Summary and Conclusions 132 References 139 Appendices 127 Curriculum Vitae 141 On CD-Rom - Appendix 1 : Coin Catalogue Appendix 2: Gold Samples Appendix 3: Averaged EPMA results of the coin alloys Appendix 4: EPMA results of gold samples Appendix 5: Trace element data; gold coins Appendix 6: Trace element data; gold samples Appendix 7: Pb isotope data; coins and the gold samples Appendix 8: Cu isotope data; coins and the gold samples viii The application of trace element and isotopic analyses to the study of Celtic gold coins and their metal sources. The focus of this study were Celtic gold coins excavated from the Martberg, a Celtic oppidium and sanctuary, occupied in the first century B.C. by a Celtic tribe known as the Treveri. These coins and a number of associated coinages, were characterised in terms of their alloy compositions and their geochemical and isotopic signatures so as to answer archaeological and numismatic questions of coinage development and metal sources. This required the development of analytical methods involving; Electron Microprobe (EPMA), Laser Ablation-ICP-MS, solution Multicollector-ICP- MS and LA-MC-ICP-MS. The alloy compositions (Au-Ag-Cu-Sn) were determined by EPMA on a small polished area on the edge of the coins. A large beam size, 50µm (diameter), was used to overcome the extreme heterogeneity of these alloys. These analyses were shown to be representative of the bulk composition of the coins. The metallurgical development of the coinages was defined and showed that the earlier coinages followed a debasement trend, which was superceded by a trend of increasing copper at the expense of sliver while gold compositions remained stable. This change occurred with the appearance of the inscribed “POTTINA” coinage, Scheers 30/V. Two typologically different coinages, Scheers 16 and 18 (“Armorican Types”) were found to have markedly different compositions which do not fit into the trends described above. A Flan for a gold coin, which may indicate the presence of a mint at the Martberg, was found to have an identicle weight and composition as the Scheers 30/I coins, which preceeded the majority of the coins found at the Martberg in the coin development chronology. The trace element anaylses were made by Laser Ablation-ICPMS using an AridusTM desolvating nebuliser to introduce matrix matched solution standards to calibrate the measurements, which were then normalised to 100%. Quantitative results were obtained for the following elements: Sc, Ti, Cr, Mn, Co, Ni, Cu, Zn, Se, Ru, Rh, Pd, Ag, Sb, Te, W, Ir, Pt, Pb, Bi. The remaining elements remain problematic as they produced incorrect standardisations mainly due to chemical effects in solution such as adsorption onto the beaker walls or oxidation : V, Fe, Ga, Ge, As, Mo, Sn, Re, Os, Hg. Changes in the sources of Au, Ag and Cu were observed during the development of the coinages through the variation of trace elements, which correlate positively with the major components of the coin alloys. Changes in the Pt/Au ratios show that the Scheers 23 coins contain distinctly different gold from the later coinages and that the Scheers 18 gold source was also different. Te/Ag was used to show that the Sch.23 coins also contained different silver and some subgroups were observed in the Sch. 30/V coins. A major change in copper source is indicated by the sudden increase of Sb and Ni with the introduction of the Sch. 30/V coins (POTTINA), which can be linked to a similar change in copper observed in the contemporary silver coinage, Sch. 55 (with a ring). Lead isotopic analyses were made by solution- and Laser Ablation – MC-ICP-MS, The laser technique proved to be in good agreement with the solution analyses with precisions between 1 and 0.1%o (per mil). The development of the laser method opens the way for easy and virtually non-destructive Pb isotopic determinations of ancient gold coins. The results showed that Sch. 23 is very different from the following coinages, Sch. 16 and 18 are also different, forming their own group, and all the later “Eye” staters (Sch. 30/I-VI) lie on a mixing line controlled by the addition of copper from a Mediterranean source, probably Sardinia or Spain.