University of Bradford eThesis This thesis is hosted in Bradford Scholars – The University of Bradford Open Access repository. Visit the repository for full metadata or to contact the repository team © University of Bradford. This work is licenced for reuse under a Creative Commons Licence. THE CLASSIFICATION AND INTERPRETATION OF TIN SMELTING REMAINS FROM SOUTH WEST ENGLAND A study of the microstructure and chemical composition of tin smelting slags from Devon and Cornwall, and the effect of technological developments upon the character of slags. Volume I of II Albertine MALHAM BSc (Hons), MPhil submitted for the degree of Doctor of Philosophy Department of Archaeological Sciences University of Bradford 2010 Albertine MALHAM THE CLASSIFICATION AND INTERPRETATION OF TIN SMELTING REMAINS FROM SOUTH-WEST ENGLAND KEYWORDS: Tin Smelting Blowing house Slag Furnace Devon Cornwall Medieval Post-medieval Artefacts relating to tin smelting from tin mills or ‘blowing houses’ in Devon and Cornwall, plus material from smelting sites that cover a range of dates from the Bronze Age through to the 19th Century, were examined: these include metallic tin, furnace linings, ore samples and slag. Analysis of tin slags from over forty sites was carried out, to determine microstructure and chemical composition. Techniques employed included optical and scanning electron microscopy, X-ray fluorescence and ICP mass spectrometry. Analysis indicates that slag appearance and composition are heavily influenced by local geology. Composition, particularly iron content, is shown to have a strong effect on slag melting point and viscosity, and the implications for the purity of metal produced are discussed. Bringing together the evidence provided by slag chemistry, documentary sources and smelting remains in the archaeological record, changes in tin smelting technology through time, and the consequences thereof, are considered. i Table of Contents VOLUME I Page No Abstract i Table of Contents ii List of Figures ix Acknowledgements xviii CHAPTER 1: INTRODUCTION 1.1: Introduction 1 1.2: Previous Work 4 1.2.1: Tin slag 4 1.2.2: Tin Ores 15 1.2.3: Furnace Linings 19 1.2.4: Tin Metal 19 1.3: The Geological Setting 24 1.3.1: Tin Minerals 24 1.3.2: Tin Deposits: Veins, Shoad and Stream Tin 24 1.3.3: Impurities in Tin Ores 30 1.4: Historical Background 31 1.4.1: The Bronze Age (c.2500 BC to c.700 BC) 31 1.4.2: The Iron Age (c.700 BC to AD 43) 44 1.4.3: The Romano-British Period (AD 43 to AD 450) 54 1.4.4: The Dark Ages (AD 450 to AD 1000) 66 1.4.5: The Mediaeval Period (AD 1000 to AD 1500) 74 1.4.5.1: Documentary Evidence for Tinworking 74 1.4.5.2: Methods of Ore Extraction 87 ii 1.4.5.3: Smelting 91 1.4.6: The Post-mediaeval Period (AD 1500 to AD 1700) 102 1.4.6.1: Production from the 16th Century 102 1.4.6.2: Methods of Ore Extraction 106 1.4.6.3: Ore Processing 111 1.4.6.4: Smelting 117 1.4.7: The 18th and 19th Centuries (AD 1700 to AD 1900) 145 1.4.7.1: Production from the 18th Century 145 1.4.7.2: Ore Extraction 148 1.4.7.3: Ore Processing 151 1.4.7.4: Smelting in the 18th and 19th Centuries 154 The Blowing House Blast Furnace 155 Furnace Design 155 The Blowing Process 161 The Reverberatory Furnace 164 Reverberatory Furnace Design 165 The Reverberatory Smelting Process 175 1.4.7.5: The Distribution of 18th and 19th Century Smelting 182 Sites 1.4.7.6: Archaeological Evidence for 18th and 19th Century 184 Tin Smelting 1.4.8: 20th Century to Present Day 186 CHAPTER 2: EXPERIMENTAL METHODOLOGY 2.1: Collection of Samples 189 2.1.1: Tin Slag 191 2.1.2: Tin Ores 203 iii 2.1.3: Furnace Wall Material 204 2.1.4: Tin Metal 206 2.2: Methods of Analysis 207 2.2.1: Analysis of Slags 207 2.2.2: Analysis of Ores 208 2.2.3: Analysis of Furnace Wall and Lining Materials 210 2.2.4: Analysis of Metallic Tin 211 2.3: Assessment of Analytical Techniques 213 2.3.1: X-Ray Fluorescence 213 2.3.2: Energy Dispersive Scanning Electron Microscopy 214 2.3.3: Induction Coupled Plasma Mass Spectrometry 216 2.3.4: X-Ray Diffraction 218 CHAPTER 3: RESULTS 3.1: Results of Slag Analysis 220 3.1.1: Physical Characteristics of Tin Smelting Slags 220 3.1.2: Optical Microscopy of Slags 224 3.1.3: Imaging of Slags using the Scanning Electron Microscope 230 3.1.4: Compositional Analysis of Slags 243 3.1.4.1: Bulk Analysis of Slag 243 3.1.4.2: Analysis of Slag Phases 286 Flow Bands 286 Feathery/Needle-like Phases 288 Dendrites 289 Crystalline Phases 291 Isolated Crystals 295 3.1.4.3: Analysis of Mineral Inclusions 297 iv 3.1.4.4: Analysis of Metallic Prills 299 3.1.5: Investigation of Melting Range and Viscous Behaviour 311 3.2: Results of Ore Analysis 314 3.2.1: Description of Ore Samples 314 3.2.2: Chemical Analysis of Ore Samples 316 Scanning Electron Microscopy 316 Laser Ablation ICPMS 323 X-ray Diffraction 330 3.2.3: Summary of Ore Analysis 339 3.3: Results of Furnace Wall Material Analysis 341 3.3.1: Physical Characteristics of Furnace Wall Material 341 3.3.2:. SEM Imaging and X-ray Mapping of Furnace Wall Materials / 345 Furnace Linings 3.3.3: Chemical Analysis of Furnace Linings 355 3.4: Results of Metallic Tin Analysis 360 3.4.1: Description of Metallic Tin Samples 360 3.4.2: Optical Microscopy of Metallic Tin Samples 361 3.4.3: Results of Chemical Analysis of Metallic Tin Samples 364 EDX-SEM Analysis 364 Laser Ablation ICPMS 366 X-ray Diffraction 371 3.4.4: Summary of Tin Metal Analysis 375 VOLUME II CHAPTER 4: DISCUSSION 4.1: Slag Analysis 377 4.1.1: Factors Determining the Appearance of Slag 377 v 4.1.1.1: Morphology, Microstructure and Viscosity 377 4.1.1.2: Mineral Inclusions 382 4.1.2: The Effect of Slag Chemistry on Tin Smelting 388 4.1.2.1: Losses of Tin Oxide to the Slag 388 4.1.2.2: The Effect of Iron 392 Iron Oxides in Blowing House Slags 397 Iron Oxides in Early and Reverberatory Smelting 400 Slags Metal Quality in Prills 406 4.1.2.3: The Effect of Other Metallic Elements in Slags and 410 Prills Calcium (Lime) 410 Titanium 412 Copper, Arsenic and Antimony 412 Tungsten 414 Molybdenum 420 4.1.3: The Evidence for Slag Recycling and its Implications for the 420 Study of Archaeological Slags 4.1.3.1: Slag Reprocessing in Blowing Houses 421 4.1.3.2: Slag Reprocessing in Reverberatory Furnaces 424 4.1.4: Application of Slag Analysis in the Identification of Unknown 427 Slags Metherel 427 Yes Tor Bottom 431 Ditsworthy 433 Lether Tor Farm 434 vi Wapsworthy Newtake 435 Drakeford Bridge 438 4.2: Ores 441 4.3: Furnace Wall and Lining Materials 444 4.4: Metallic Tin 454 4.5: The Interpretation of Early Smelting Remains 464 4.6: Pre-Blowing House Smelting in the Mediaeval Period 470 4.6.1: Case Study: The Interpretation of Crift Farm as a Smelting Site 470 4.6.2: The Crift Farm Smelting Site and its Place in the Cornish 479 Economy 4.6.3: Early Mediaeval Smelting as a Two Stage Process: The De 481 Wrotham Correspondence CHAPTER 5: CONCLUSION 5.1: Conclusion 487 5.2: Further Work 496 APPENDICES Appendix 1: Glossary of Terms 498 Appendix 2: Reported Slag Finds not included in this Work 507 Appendix 3a: Descriptions of Selected Slag Samples 509 Appendix 3b: Descriptions of Selected Slag Samples from Upper Merrivale 515 Appendix 4: Composition of Tin Smelting Slags determined by EDX-SEM 522 Appendix 5: Chemical Composition of Flow Bands 566 Appendix 6: Chemical Composition of Feathery/Needle Phases 584 Appendix 7: Chemical Composition of Dendrites 596 Appendix 8: Chemical Composition of Crystalline Phases 607 Appendix 9: Chemical Composition of Isolated Crystals in Non-crystalline 620 vii Slags Appendix 10: Composition of Ore Samples determined by ICPMS 623 Appendix 11: Composition of Tin metal samples determined by ICPMS 633 Appendix 12: Binary Phase Diagrams 638 Appendix 13: Analysis of Recently Discovered Slag from High Down and 641 East Okement Farm BIBLIOGRAPHY 657 ADDITIONAL MATERIAL ON CD-ROM IMAGE GALLERY File 1: Photographs of Tin Smelting Slags File 2: Optical Microscope Images of Tin Smelting Slags File 3: Photographs of Tin Ores File 4: Optical Microscope Images of Tin Ores File 5: Photographs of Furnace Wall Fragments File 6: SEM Images and X-ray Maps of Furnace Linings File 7: Photographs of Metallic Tin File 8: Optical Microscope Images of Metallic Tin CATALOGUES Catalogue of Smelting Sites Catalogue of Tin Metal Finds RAW DATA viii List of Figures Figure Page No 1.1 Map of European Tin Deposits 24 1.2 Amount of tin metal coined in Devon and Cornwall between 83 1250 and 1500 1.3 Crazing stone from Crift Farm 91 1.4 Stone ore crushing tools from Crift Farm 95 1.5 Plan of the Crift Farm Excavation 96 1.6 The Crift Farm smelting site (remains of Building B 97 and C) 1.7 The Crift Farm smelting site (remains of Building A and 97 slag heap) 1.8 Amount of tin metal coined in Devon and Cornwall 103 between 1500 and 1700 1.9 Mortar Stone from Week Ford Tin Mill 115 1.10 Simplified section diagram of a furnace or ‘hearth’ based on 119 the account of Beare 1586 1.11 Section diagram of a German tin smelting furnace based on the 122 account of Agricola 1556 1.12 Remains of the furnace at Lower Merrivale blowing house 135 (viewed from front), with upright side walls of granite blocks and a displaced float stone forming its base 1.13 Mouldstone at Lower Merrivale tin mill showing 139 notch at edge of cavity ix 1.14a & b Mouldstones at Upper Merrivale tin mill showing 140 sample moulds 1.15 Amount of tin metal coined in Devon and Cornwall 146 between 1700 and 1837 1.16 General plan and section of an 18th Century Reverberatory 166 Furnace 2.1 Map of Southwest England showing location of sites