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Plano-convex ingot

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Plano-convex ingots, also known as bun ingots, are lumps of metal, circular in plan with a flat or slightly concave top and a rounded base. They are most often made of copper although plano-convex ingots of other materials such as copper alloy[1] , lead[2] and tin[3] are also known. Their characteristic shape is formed when molten metal solidifies within a bowl shaped cavity such as a mould [4] or depression in the ground [5]. Forms of plano-convex ingot are found across a wide chronological and geographical range with the first examples known from the Near East during the 3rd and 2nd Millennia BC . By the end of the Bronze Age they were found widely from North West Europe to South Asia[6] and similar ingot forms continue in use during later Roman and Medieval periods[7]

Manufacture

In the past plano-convex ingots have been seen as a primary product of smelting, formed at the base of a furnace beneath a layer of less dense slag[8].However experimental evidence has shown that regularly shaped plano-convex ingots are difficult to form within the furnace and many plano-convex ingots may have been formed outside of the smelting furnace being tapped or cast into bowl shaped depressions or moulds[9].The composition and structure of the metal within some plano-convex ingots suggests that it may derive from secondary processes such as refining, alloying or the recycling of scrap metal rather than smelting.

Casting in a warm mould or reheating furnace will give the ingot an even columnar structure running in the direction of cooling whereas ingots cast in a cold mould will have a distinctive two stage cooling structure with an outer chilled layer reflecting the rapid cooling of the bottom when it came into contact with the mould [10]. A slightly concave upper surface can be produced if the top of the ingot cools more slowly than the bottom [11]. As the ingot cools gasses are released giving the upper surface a “blistered” texture[12]. The texture and shape of the mould is reflected in the surface of the final ingot as such it is possible to recognise identical “mould siblings” made in the same mould[13].

Britain

Late Bronze Age

By the Late Bronze Age the copper bun ingot, either in a simple form or with a hole in its centre had become the main form of copper ingot replacing the earlier ‘bar ingot’ or rippenbarre . Weights of complete examples average at about 4 kg but examples of up to about 7 kg are known . Many early finds of British LBA bun ingots were unstratified but recently a large number of bun shaped ingots and ingot fragments have been found in hoards alongside bronze artefacts and scrap metal (TAR 2005-6.) Several offshore finds of probable LBA date suggest that copper bun ingots may have been traded by sea during this period

Composition and Structure Where analysed the copper is of very high purity although earlier examples are sometimes of arsenical copper . Tylecote has suggested that they are not primary smelting products and have instead been refined and recast (1986, 22-3.) The macrostructure of a half section example from Gillan, Cornwall showed a columnar structure which probably indicates slow cooling in a reheating furnace or a warm mould rather than pouring into a could mould (Tylecote 1987, 195-6.)

Iron Age and Roman Bun Ingots A second major group of British bun ingots date to the Roman period and are found mostly in the copper rich highland areas of Wales and in Scotland . They are rather heavier than the LBA examples with weights ranging between 12 and 22 kg (Tylecote 1986 20-21 table 10)

A number of these ingots have stamps clearly dating them to the Roman period (Tylecote 1986 20-21 table 10) including an example which reads SOCIO ROMAE NATSOL. The term socio suggests that the ingots were being cast by a private company rather than by the state (de la Bedoyere 1989, 54.) More recently Fraser Hunter has reassessed the context of the Scottish examples and some of the unstamped Welsh examples and argues that they could in fact be Iron Age in date or at least reflect native rather than Roman copper working (Hunter 1999, 338-40.) Although ingots of any sort are not common in the British Iron Age, plano convex or bun shaped ingots are not unknown e.g. a tin ingot which was discovered within the Iron Age hillfort at Chun Castle, Cornwall (Tylecote 1987, 204.)

Composition and Structure of Roman Ingots The Roman Bun Ingots are less pure than the earlier LBA examples and Tylecote suggests that they may be a direct product of smelting (1987, 24.) Although theoretically such an ingot could be formed in the base of the furnace this is problematic in the case of the stamped examples as this would require the furnace to be dismantled or else have a short shaft to allow access for stamping (Merkel 1986, Tylecote 1986, 22.) As a solution the furnace could have been tapped into a mould at the completion of smelting. It is possible that both methods were used as several of the ingots seem to have had additional metal poured onto the top in order to allow stamping (Tylecote 1986, 23-4.)


References

  1. ^ Bass, G.F. (1967) Cape Gelidonya: A Bronze Age Shipwreck US: American Philosophical Society, Maddin, R. & Merkel, J. (1990) ‘Metallographic and statistical analyses’ in Lo Schiavo, F Maddin, R Merkel, J. Muhly, J. D. & Stech, T (eds) Metallographic and statistical analyses of copper ingots from Sardinia P.42-199. Ozieri: Il Torchietto
  2. ^ Waschmann, S. (2009) Seagoing Ships and Seamanship in the Bronze Age Levant (2nd Ed) US: Texas A&M University Press
  3. ^ Tylecote, R.F. (1987) The Early History of Metallurgy in Europe. London: Longman, p.204 Pulak, C (1998) ‘The Uluburun Ship: An Overview’ IJNA 27 (3) p199
  4. ^ Pulak 1998, 196
  5. ^ Weisgerber, G. and Yule, P (2003) ‘Al Aqir near Bahla – an Early Bronze Age Dam Site with Plano Convex Ingots’ in Arabian Archaeology and Epigraphy 14 p.48
  6. ^ Tylecote 1987, 194-209, Janzon, G.O. (1988) ‘Early Nonferrous Metallurgy in Sweden’ In Maddin, R. (ed) The Beginning of theUse of Metals and Alloys US: MIT Press p.104-17, Piggott, V.C. (ed) (1999) The Archaeometallurgy of the Asian Old World :MASCA Research Papers, Begemann, F., Schmitt-Strecker, S. Pernicka, E., and LoSchiavo, F. (2001) ‘Chemical Composition and Lead Isotopy of Copper and Bronze from Nuragic Sardinia’ in EJA 4 p43-85, Hauptmann, A., Maddin, R. and Prange, M. (2002) ‘On the Structure and Composition of Copper and Tin Ingots Excavated from the Shipwreck of Uluburun’ in BASOR 328 p.1-30, Weisgerber and Yule 2003, 48-9
  7. ^ Whittick, G C, & Smythe, J A 1937 'An examination of Roman copper from Wigtownshire and north Wales', Proc Univ Durham Phil Soc, 9/2 (1937), 99-104. Andre, P. (1976) ‘A Copper Ingot From Brittany’ in Bull. Board Celtic Studies 27 p148-53 Tylecote 1987, 202-6
  8. ^ Coghlan, H.H. (1975) Notes on the Prehistoric Metallurgy of Copper and Bronze in the Old World (2nd Ed.) Oxford: OUP Muhly, J.D. Stech-Wheeler, T. and Maddin, R. (1977) ‘The Cape Gelidonya Shipwreack and the Bronze Age Metal Trade in the Eastern Mediterranean’ in JFA 4 p.353-62 Harding, A.F. (2002) ‘The Bronze Age’ in Milisauskas, S (eds) European Prehistory: A Survey New York: Kluwer p.302
  9. ^ Merkel, J (1986) Ancient Smelting and Casting of Copper Oxhide Ingots" in Studies in Sardinian Archaeology II: Sardinia in the Mediterranean, Balmuth, M. (ed). US: University of Michigan Press. P.256, Tylecote 1987, 195-6, Craddock, P (1995) Early Metal Mining and Production Edinburgh: EUP p.202-4
  10. ^ Tylecote 1987, 195-6 fig.6.11
  11. ^ Weisgerger and Yule 2003, 48
  12. ^ Hauptmann et al 2002, 4
  13. ^ Pulak 1998, 196
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