ROMAN LEAD SILVER SMELTING AT RIO TINTO The case study of Corta Lago Thesis submitted by Lorna Anguilano For PhD in Archaeology University College London I, Lorna Anguilano confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. ii To my parents Ai miei genitori iii Abstract The Rio Tinto area is famous for the presence there of a rich concentration of several metals, in particular copper, silver and manganese, which were exploited from the Bronze Age up to few decades ago. The modern mining industry has been responsible for both bringing to light and destroying signs of past exploitation of the mines and metal production there. The Corta Lago site owes its discovery to the open cast exploitation that reduced the whole mount of Cerro Colorado to an artificial canyon. This exploitation left behind sections of antique metallurgical debris as well as revealing the old underground workings. The Corta Lago site dates from the Bronze Age up to the 2nd century AD, consisting mainly of silver and copper production slag, but also including litharge cakes, tuyéres and pottery. The project focused on the study of silver production slag from different periods using petrograhical and chemical techniques, such as Optical Microscopy, X-Ray Diffraction, X-Ray Fluorescence, Scanning Electron Microscopy associated to Energy Dispersive Spectrometry and Multi-Collector Inductively Coupled Plasma Mass Spectrometry. The aim of the project was to reconstruct the metallurgical processes of the different periods, detecting any differences and similarities. 3+ The mineral exploited was jarosite, XFe 3(OH)6(SO4)2, where X can be K, Na, Pb, Ag and NH4, and the results show that the system of production was much more similar to iron production than silver. In the slag, the main mineral is fayalite, and the concentration of lead is around 1%. These results and study of the jarosite suggest the possibility of different sources of lead for the collection of silver in the system, and this is the reason for the utilization of the MC-ICP MS for the analysis of the lead isotopes. The results for the isotopes indicate the addition of a second source of lead used as lead metal in the system to increase the amount of lead and improve the collection of silver. The differences in the processes used at different periods are the amount of lead coming from another site that was added, and the level of standardization of the system. While the first difference is evident in a comparison between the pre-Roman process and one of the Republican phases, the second is mainly visible between the pre-Roman and Roman processes. At this stage the aim of the project was to attempt iv to correlate the differences in the processes, metallurgical skills and geological knowledge. v Table of contents Chapter 1 Introduction 1 1.1.The Importance of Iberian metal production in Antiquity 1 1.2.Information on the ancient mines of the Iberian Peninsula and the paucity of information on the ancient production techniques 4 1.3.Archaeo-metallurgical data within cultural and socio-economical reconstruction 5 1.4.Aims and approaches 7 1.5.Slag as primary material for the reconstruction of the metallurgical process 8 Chapter 2 Silver mining and smelting in the Roman Economy 11 2.1. The Rio Tinto silver production within the Roman Economy 11 2.2. Archaeometallurgy and the Economy: potential insights 18 2.2.1. Archaeometallurgy and the Economy in the Rio Tinto area 24 Chapter 3 The technology of Roman silver smelting: galena, cerussite and jarosite 29 Chapter 4 The study sites: Corta Lago and Tharsis 37 4.1. The geology of Corta Lago 38 4.1.1. Geological sequence 39 4.1.2. Mineralised rocks 41 4.1.3. Mineral association 42 4.1.4. The secondary enrichment zone 45 4.1.5. The “hydrothermal deposits” 48 4.2. The Archaeology of Corta Lago 48 4.3. First Archaeometric Studies of the Corta Lago remains 58 4.4. The site of Tharsis 62 4.4.1. Brief summary of the geology and mineralogy of ores at Tharsis 63 4.4.2. The archaeology of Tharsis 65 Chapter 5 Methodology 67 5.1. Analytical Methods and Sample Preparation 67 vi 5.1.1. Sampling strategy 67 5.1.2. Optical Microscopy 69 5.1.3. X-Ray Diffraction 69 5.1.4. X-Ray Fluorescence 71 5.1.4.1. Accuracy and Precision 73 5.1.5. Scanning Electron Microscopy with Energy Dispersive Spectrometry 75 5.1.6. Quantitative analysis with SEM and XRF 80 5.1.7. Multi Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) 99 5.1.8. Lead Isotopes and their use in archaeometallurgy 100 5.1.9. Preparation of the samples as powders 101 5.1.10. Preparation of the samples as mounted polished sections 102 5.2. Interpretative methods 104 5.2.1. Olivine cooling rate 104 5.2.2. Bulk chemical composition, viscosity, melting temperature and reducing conditions 108 Chapter 6 The material: morphological, chemical, petrographic and mineralogical observations 113 6.1. Late Bronze Age “Free silica” slags 120 6.2. Phoenician samples 124 6.2.1. Furnace lining 126 6.2.2. Slag samples 126 6.3. Ibero-Punic tapped slags 132 6.4. Iberian samples 139 6.4.1. Iberian slag samples 140 6.4.2. Iberian litharge samples 142 6.5. Republican phase I tapped slags 143 6.6. Republican phase II plate slags 150 6.6.1. Plate slag chemistry 150 6.6.2. Area analyses and mineralogical association 158 6.6.3. Cooling process hypothesis 161 6.7. Republican phase III tapped slags 162 vii 6.8. Imperial samples 166 6.8.1. Lead Bullion 173 6.9. Ball slags 184 6.10. Preliminary discussion and comparison 193 6.11. Ingots, semi-reacted ores and litharge samples 203 6.11.1. Ingots 203 6.11.2 Semi-reacted ores 207 6.11.3 Litharge 210 6.12. Summary 210 Chapter 7 Corta Lago versus Tharsis 213 7.1. Republican slag 214 7.2. Imperial slag 225 7.3. Comparison: Republican versus Imperial tapped slags at Tharsis 232 7.4. Comparison: Tharsis versus Corta Lago 232 Chapter 8 The isotopic signature: a new approach to the metallurgical process 236 Chapter 9 Discussion: changes in production technologies and organisation 269 9.1. Macro-Morphology 270 9.2. The problem of the Republican phase II plate slags 272 9.3. The addition of “foreign” lead 275 9.4. The two processes during the Republican phase I tapped slag 277 9.5. Lead metal, galena and barite 278 9.6. Silver loss and efficiency of the process 281 9.7 Chemical homogeneity 282 9.8. Smelting process and slag production 283 9.9. The amount of slag produced 285 9.10. The process 285 9.11. Ore variability and technical choice 286 Chapter 10 Conclusion 289 viii 10.1. Morphological changes 290 10.2. Process efficiency 292 10.3. Scale of production 294 10.4. Summary 295 References 298 Appendix 1 Glossary of the chemical compositions of the minerals mentioned in the text 307 Appendix 2 Lead Isotope Analysis 309 Section 2.1. Preparation of the solution for MC-ICP-MS 309 Section 2.2. LIA results 310 ix List of figures Chapter 1 Introduction 1 Chapter 2 Silver mining and smelting in the Roman Economy 11 Chapter 3 The technology of Roman silver smelting: galena, cerussite and jarosite 29 Figure 3.1 Diagram of temperature/oxygen pressure for major mineralogical phases association (MH, magnetite-hematite; NiNiO, Nickel-nickel oxide; FMQ, fayalite-magnetite-quartz; WM, wustite- magnetite; IW, iron-wustite; QIF, quartz-iron-fayalite – Frost, 1991). 32 Chapter 4 The study sites: Corta Lago and Tharsis 37 Figure 4.1 Geological map of the Iberian Peninsula. The symbols on the map indicate: 1. Precambrian, the dotted areas indicate granites; 2. Mesozoic; 3. Cenozoic. (Domergue, 1990, 518). The blue square highlights the location of Cartagena, the red one indicates the location of Rio Tinto, and the green one of Tharsis, the main sites involved in this thesis. 37 Figure 4.2 Detail of the Rio Tinto mines area indicating the position of Corta del Lago (Ca del Lago in the figure) and the position of the main exploitation works (squared areas) and slag heaps (dotted areas) (Domergue, 1990, 531) 38 Figure 4.3 Lago open pit (Rio Tinto). Massive sulphides were present in the pit, forming the core of a syncline, and stockwork is still visible on both sides of the pit. Drifts are of Roman age (Leistel et al. 1998, 10). 39 Figure 4.4 Map of the Iberian Pyrite Belt (Strauss and Gray 1986). Corta Lago is located within the Rio Tinto area, south of Aracena. 44 Figure 4.5 Ore vein in Rio Tinto: the grey area on the left side of the picture is the massive pyrite mineralisation, the red area on the top part is the gossan, and the brownish layer between the two is the jarositic layer (Peña del Hierro – Rio Tinto). 46 Figure 4.6 Corta Lago section as drawn by Brenda Craddock (Rothenberg and Blanco-Freijeiro 1981). The label ‘plate slag’ in the legend does not reflect the label ‘plate slag’ used in this thesis. The term ‘plate slag’ in this map mostly indicates tapped slag, while in this thesis ‘plate slag’ refers to the macro-morphology of the single sample. 50 Figure 4.7 Plan of trenches T1 and T2 and location of the main section at Corta Lago.