Non-Ferrous Metalworking in England Late Iron Age to Early Medieval
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NON-FERROUS METALWORKING IN ENGLAND LATE IRON AGE TO EARLY MEDIEVAL Justine Cecily Bayley Thesis submitted for the degree of Ph D l JUSTINE BAYLEY NON-FERROUS METALWORKING IN ENGLAND LATE IRON AGE TO EARLY MEDIEVAL Evidence for non-ferrous metalworking in the late Iron Age to early medieval periods comes mainly from archaeological finds, though Roman and medieval writings supply some supporting information. A gazetteer of sites which have produced evidence for metalworking is provided and these finds are categorised and described together with details of the processes in which they were used or produced. The spatial and temporal distributions of finds of all types are also discussed and reasons suggested for some of the patterns that emerge. The processes for which evidence is presented include refining, alloying, melting, casting, smithing and decorating. The majority of the data derives from the working of copper and its alloys though gold, silver, lead, tin and their alloys are also covered. The uses made of different alloys throughout the period of the study is also discussed. A more detailed survey of Roman copper alloy usage is based on analyses of over 3,000 late Iron Age and Roman brooches and nearly 900 other objects. Nearly 1,000 of the brooch analyses were quantitative ones carried out by atomic absorption spectrometry while the remainder were qualitative X-ray fluorescence analyses. Many of the metalworking finds were also analysed qualitatively by X-ray fluorescence. These two strands of evidence are complementary and together provide a mass of evidence for how metals and alloys were worked and the uses that were made of them. 2 NON-FERROUS METAL WORKING IN ENGLAND LATE IRON AGE TO EARLY MEDIEVAL Abstract 2 Contents 3 List of tables 7 List of figures 9 Acknowledgements 13 Pre-fate *5 1 Introduction 16 The present situation 16 Outline of thesis 17 2 Sources of information 20 Excavated material 20 Ancient authors 35 Modern craft practice and metallurgy 38 3 Metal production 40 Ore sources and mining 40 Smelting 42 Ingots 43 4 Refining and alloying 45 Refining 45 Oxidation 46 Cupellation 46 Cupels 50 Parting 53 Alloying 59 Cementation 59 5 Metal melting and casting 61 Crucibles 61 Chronological sequence of metal melting crucibles 64 Moulds 77 Ingot moulds 78 Investment moulds 81 Piece moulds 82 Models and trial pieces 85 Metal waste 88 3 6 Wrought metalworking 89 Processes 89 The archaeological evidence 91 Tools 91 7 Joining 93 Welding and burning together 93 Soldering 93 8 Decoration 95 Relief decoration 95 Applied metal 95 Gilding 96 Tinning 98 Silvering 99 Inlays 99 Overlays 99 Filigree and granulation 101 Non-metallic materials 101 Niello 101 Enamel 102 Applied 'stones' 103 9 Metals and their alloys 104 Nomenclature 104 Alloy composition 105 Ancient names and recipes for alloys 111 Properties of alloys 113 10 Metal usage and alloy selection 119 Gold and silver 120 Lead 121 Tin and pewter 122 Copper and its alloys 122 Iron Age and Roman copper alloys 123 Post-Roman copper alloys 127 Late Iron Age and Roman brooches 133 Group 1 and earlier brooches 140 Group 2 142 Group 3 142 Group 4A 144 4 Group 4B 144 Group 5 146 Group 6 149 Group 7 150 Group 8 154 Group 9 156 Group 10 156 Group 11 159 Plate brooches 161 Penannular brooches 165 Continental brooches and other metalwork 165 Comparing groups of analyses 167 Brass 170 Leaded bronze 173 11 The nature and distribution of metalworking sites 175 The nature of metalworking finds 176 Workshops 177 Geographical distribution of sites 179 Chronological variations 179 Iron Age 179 Roman 182 Gold 185 Silver 188 Lead 190 Tin and pewter 192 Copper and its alloys 192 Early Saxon 197 Middle Saxon 197 Late Saxon 201 Gold and silver 203 Copper alloys 203 Lead, tin and pewter 204 Early medieval 204 Later medieval 206 12 Summary and conclusions 207 5 Appendices 213 A Gazetteer of sites with non-ferrous metal working finds 213 B Analytical methods 286 Sampling for AAS 286 AAS sample preparation 287 AAS analytical procedure 287 Errors in AAS analyses 288 XRF analysis 292 XRF analysis of metal objects 293 XRF analysis of applied decoration 301 XRF analysis of metalworking debris 304 C Analytical results 306 Sites providing objects for analysis 307 Summary of analyses 311 Analytical results for brooches 313 Analytical results for other objects 371 D Illustrations of selected types of Late Iron Age and Roman brooches 390 Bibliography 399 Ancient Monuments Laboratory Reports 399 Other references 409 6 LIST OF TABLES 2.1 Iron Age sites 2.2 Roman sites 2.3 Early Saxon sites (ftk-lkW centuty) 2.4 Middle Saxon sites ( cewtuv'y ) 2.5 Late Saxon sites (mainly lOth/llth century) 2.6 Medieval sites (mainly 12th/13th century) 2.7 Medieval sites (not closely dated) 2.8 Summary of finds from Tables 2.1-2.6 3.1 Areas of England exploited for minerals 4.1 Sites producing litharge cakes and/or cupellation hearths 4.2 Sites producing parting vessels 5.1 Crucible typology 8.1 Gilded Roman brooches (data from Table C.3) 8.2 Overlays on Roman brooches (data from Table C.3) 8.3 Enamelled Roman objects (data from Tables C.3-C.4) 9.1 Alloy composition: boundaries used in assigning alloy names 9.2 Physical properties of modern commercial brass strip and sheet 9.3 Physical properties of modern commercial bronzes 9.4 Physical properties of modern leaded bronzes 9.5 Physical properties of modern gunmetals and leaded gunmetals 10.1 Summary of metals worked (data from Tables 2.1-2.6) 10.2 Summary of analyses of Iron Age and Roman objects other than brooches (data from Table C.4) 10.3 Analyses of Gorhambury objects from closely dated contexts 10.4 Summary of post-Roman analyses 10.5 Means and standard deviations for groups of quantitative analyses in Table 10.4 10.6 Summary of Hull's brooch typology 10.7 Summary of brooch analyses by Group 10.8 Summary of analyses of late Iron Age and Roman bow brooches (data from Table C.3) 10.9 Group 3 brooch types 10.10 Summary of analyses of Roman plate brooches (data from Table C.3) 7 10.11 Summary of analyses of Roman penannular brooches (data from Table C.3) 10.12 Medians, upper and lower quartile ranges for selected types of brass brooches 10.13 Medians, upper and lower quartile ranges for selected types of bronze and leaded bronze brooches 11.1 Temporal distribution of metalworking sites B.l AAS standards used B.2 Multiple analyses of a crossbow brooch B.3 X-ray lines used B.4 Data used to plot Figures B.2-B.5 B.5 Duplicate XRF analyses of 25 Roman brooches from Chelmsford C.l Sites providing objects for analysis C.2 Summary of late Iron Age and Roman analyses included in Tables C.3-C.5 C.3 Analytical results for late Iron Age and Roman brooches (arranged in typological order) C.4 Analytical results for late Iron Age and Roman objects other than brooches C.5 AAS results for objects other than brooches C.6 Analytical results for post-Roman objects 8 LIST OF FIGURES 4.1 Litharge cake fragments from York: Coppergate showing the range of sizes found. Scale 1:1 (drawings by Eddie Moth, York Archaeological Trust) 4.2 Fired clay disc with eroded, lead-rich vitrified upper surface from Winchester: Wolvesey Palace (Bayley and Barclay 1990). Scale 1:2 4.3 Cupels from [1] Winchester: Cathedral Yard (Bayley and Barclay 1990), [2] Verulamium, [3-5] York: Coppergate (Mainman 1990) and [6-11] Lincoln: Flaxengate (Gilmour 1988). [5] was used for gold, the rest for silver. Scale 1:2 4.4 Ellingham diagram showing the range of partial oxygen pressures within which cupels were used. Lead is oxidised to litharge but copper is still present in a sufficiently reduced form to colour vitreous deposits red. 4.5 Sketch drawings of Roman parting vessels from [1] Lincoln, [2] Chichester, [3] Exeter and [4] London. The stippling indicates the luting clay used to seal the lids onto the London vessels. Scale 1:2 4.6 Sherds of parting vessels including lid fragment [top right] from York: 22 Piccadilly. Scale 1:2 (drawings and reconstruction by Eddie Moth, York Archaeological Trust) 4.7 Sketch drawings of cementation crucibles from [1] Canterbury: 7 Palace Street and [2] Colchester: Culver Street. Scale 1:2 5.1 Type 1 crucibles from [1-2] Gussage All Saints (Spratling 1979) and [3] Sutton Walls (sketch after Kenyon 1953). Scale 1:2 5.2 Type 2 crucibles from [1-3] Meare (Bulleid and Gray 1953) and [4] Mucking (Jones 1980). Scale 1:2 5.3 Type 3 crucibles from [1] Doncaster: St Sepulchre Gate (Bayley 1986A), [2] Colchester: Castle (Bayley 1984D) and [3] Colchester: Sheepen. Scale 1:2 5.4 Type 6A crucibles from [1] Heronbridge (Hartley 1954) and [2-3] Verulamium. Type 6 crucibles from [4] Heronbridge (Hartley 1954), [5-7] Verulamium (Frere 1972), [8-10] Gorhambury (Bayley 1990B), [11] Southampton (Addyman and Hill 1969), [12-13] Lincoln: Flaxengate (Gilmour 1988), [14] Winchester (Bayley and Barclay 1990), [15-17] York: Coppergate (Bayley forthcoming), [18-20] Thetford (Bayley 1984E) and [21-23] Cheddar (Biek 1979). [1-10] are Roman, [11-23] Saxon. Scale 1:2 5.5 Type 4 crucibles from [1-2] York: Yorkshire Museum, [3-4] Chester: Hunter Street School, [5] Baldock (Stead and Rigby 1986), [6-7] 9 Colchester: Lion Walk (Bayley 1984C), [8-9] Verulamium (Frere 1972) and [10] Chichester: Greyfriars.