Plano-convex ingot

Last updated

Plano-convex ingots are lumps of metal with a flat or slightly concave top and a convex base. They are sometimes, misleadingly, referred to as bun ingots which imply the opposite concavity. [1] They are most often made of copper, although other materials such as copper alloy, lead and tin are used. [2] [3] [4] [5] [6] The first examples known were from the Near East during the 3rd and 2nd Millennia BC. [7] By the end of the Bronze Age they were found throughout Europe [8] [9] and in Western and South Asia. [10] [11] [12] Similar ingot forms continued in use during later Roman and Medieval periods.

Contents

Manufacture

Traditionally bun ingots were seen as a primary product of smelting, forming at the base of a furnace beneath a layer of less dense slag. However, experimental reconstruction of copper smelting showed that regular plano-convex ingots are difficult to form within the smelting furnace, producing only small ingots or copper prills that need to be remelted. [13] [14] High purity copper bun ingots found in Late Bronze Age Britain and the Mediterranean seem to have undergone a secondary refining procedure.

The metallographic structure and high iron compositions of some plano-convex ingots suggest that they are the product of primary smelting. [15] Tylecote suggested that Roman plano-convex copper ingots may have been formed by tapping both slag and copper in one step into a mould or pit outside the furnace. [16] A similar process was described by Agricola in book IX of his De Re Metallica [17] and has been replicated experimentally. [18]

Structure

Although all bun ingots share the same basic morphology, the details of their form and the texture of their convex base is dependent on the mould in which they cooled. Bun ingots made in purpose-dug depressions in sand can be highly variable in form even on the same site, [19] whereas ingots cast in reusable moulds will form sets of identical “mould siblings”. [20]

The composition of the metal and its cooling conditions affect structure. As the ingot cools gases are released giving the upper surface a “blistered” texture and if cooling takes place outside of the furnace, the outer surface often becomes oxidised. Casting in a warm mould or reheating furnace gives the ingot an even columnar structure running in the direction of cooling, whereas ingots cast in a cold mould 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. A slightly concave upper surface can be produced if the top of the ingot cools more slowly than the bottom.

Britain

Late Bronze Age

By the Late Bronze Age, the copper bun ingot, either in a simple form or with a hole in its center, had become the main form of copper ingot, replacing the earlier ‘bar ingot’ or rippenbarre. Weights of complete examples average ~4 kg, but examples of up to about 7 kg are known. Many early finds of British LBA bun ingots were unstratified but recently bun-shaped ingots and ingot fragments have been found in hoards alongside bronze artifacts and scrap metal. [21] Several offshore finds of probable LBA date suggest that copper bun ingots may have been traded by sea during this period.

Composition and Structure

The copper is of high purity, although earlier examples are sometimes composed of arsenical copper. Tylecote suggested that they are not primary smelting products and instead were refined and recast. [22] The macrostructure of a half section example from Gillan, Cornwall shows a columnar structure that probably indicates slow cooling in a reheating furnace or a warm mold, rather than from pouring into a cold mold. [23]

Iron Age and Roman period

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 heavier than the LBA examples, with weights ranging between 12 and 22 kg. [24]

Some have stamps clearly dating them to the Roman period [24] including an example that reads SOCIO ROMAE NATSOL. The term "socio" suggests that the ingots were cast by a private company rather than by the state. [25] Fraser Hunter reassessed the context of the Scottish examples and some of the unstamped Welsh examples and argues that they could in fact date to the Iron Age or at least reflect native rather than Roman copper working. [26] Although ingots of any sort are not common in the British Iron Age, planoconvex or bun-shaped ingots exist, e.g. a tin ingot discovered within the Iron Age hillfort at Chun Castle, Cornwall. [4]

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. [27] Theoretically such an ingot could be formed in the base of the furnace. However, 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. [28] [29] 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. [30]

Related Research Articles

<span class="mw-page-title-main">Brass</span> Alloy of copper and zinc

Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other within the same crystal structure.

<span class="mw-page-title-main">Smelting</span> Use of heat and a reducing agent to extract metal from ore

Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a chemical reducing agent to decompose the ore, driving off other elements as gases or slag and leaving the metal base behind. The reducing agent is commonly a fossil fuel source of carbon, such as coke—or, in earlier times, charcoal. The oxygen in the ore binds to carbon at high temperatures due to the lower potential energy of the bonds in carbon dioxide. Smelting most prominently takes place in a blast furnace to produce pig iron, which is converted into steel.

<span class="mw-page-title-main">Pig iron</span> Iron alloy

Pig iron, also known as crude iron, is an intermediate product of the iron industry in the production of steel which is obtained by smelting iron ore in a blast furnace. Pig iron has a high carbon content, typically 3.8–4.7%, along with silica and other constituents of dross, which makes it brittle and not useful directly as a material except for limited applications.

<span class="mw-page-title-main">Crucible</span> Container in which substances are heated

A crucible is a ceramic or metal container in which metals or other substances may be melted or subjected to very high temperatures. While crucibles were historically usually made from clay, they can be made from any material that withstands temperatures high enough to melt or otherwise alter its contents.

<span class="mw-page-title-main">Ingot</span> Piece of relatively pure metal

An ingot is a piece of relatively pure material, usually metal, that is cast into a shape suitable for further processing. In steelmaking, it is the first step among semi-finished casting products. Ingots usually require a second procedure of shaping, such as cold/hot working, cutting, or milling to produce a useful final product. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods. Precious metal ingots can be used as currency, or as a currency reserve, as with gold bars.

<span class="mw-page-title-main">Copper extraction</span> Process of extracting copper from the ground

Copper extraction refers to the methods used to obtain copper from its ores. The conversion of copper consists of a series of physical and electrochemical processes. Methods have evolved and vary with country depending on the ore source, local environmental regulations, and other factors.

<span class="mw-page-title-main">Lost-wax casting</span> Process by which a duplicate metal sculpture is cast from an original sculpture

Lost-wax casting is the process by which a duplicate metal sculpture is cast from an original sculpture. Intricate works can be achieved by this method.

<span class="mw-page-title-main">Reverberatory furnace</span> Metallurgical furnace

A reverberatory furnace is a metallurgical or process furnace that isolates the material being processed from contact with the fuel, but not from contact with combustion gases. The term reverberation is used here in a generic sense of rebounding or reflecting, not in the acoustic sense of echoing.

<span class="mw-page-title-main">Cupellation</span> Refining process in metallurgy

Cupellation is a refining process in metallurgy where ores or alloyed metals are treated under very high temperatures and have controlled operations to separate noble metals, like gold and silver, from base metals, like lead, copper, zinc, arsenic, antimony, or bismuth, present in the ore. The process is based on the principle that precious metals do not oxidise or react chemically, unlike base metals. When they are heated at high temperatures, the precious metals remain apart, and the others react, forming slags or other compounds.

<span class="mw-page-title-main">Blowing house</span> Tin smelting building formerly used in southwest England

A blowing house or blowing mill was a building used for smelting tin in Cornwall and on Dartmoor in Devon, in South West England. Blowing houses contained a furnace and a pair of bellows that were powered by an adjacent water wheel, and they were in use from the early 14th century until they were gradually replaced by reverberatory furnaces in the 18th century. The remains of over 40 blowing houses have been identified on Dartmoor.

<span class="mw-page-title-main">Ferrous metallurgy</span> Metallurgy of iron and its alloys

Ferrous metallurgy is the metallurgy of iron and its alloys. The earliest surviving prehistoric iron artifacts, from the 4th millennium BC in Egypt, were made from meteoritic iron-nickel. It is not known when or where the smelting of iron from ores began, but by the end of the 2nd millennium BC iron was being produced from iron ores in the region from Greece to India, and Sub-Saharan Africa. The use of wrought iron was known by the 1st millennium BC, and its spread defined the Iron Age. During the medieval period, smiths in Europe found a way of producing wrought iron from cast iron using finery forges. All these processes required charcoal as fuel.

In metallurgy, refining consists of purifying an impure metal. It is to be distinguished from other processes such as smelting and calcining in that those two involve a chemical change to the raw material, whereas in refining, the final material is usually identical chemically to the original one, only it is purer. The processes used are of many types, including pyrometallurgical and hydrometallurgical techniques.

Liquation is a metallurgical method for separating metals from an ore or alloy. A mixture of metals is melted together with a third, after which the mixture is separated by liquid extraction. This method was largely used to separate silver from copper using lead, but it can also be used to remove antimony minerals from ore, and refine tin.

<span class="mw-page-title-main">Copper Hoard culture</span>

Copper Hoard culture describes find-complexes which mainly occur in the western Ganges–Yamuna doab in the northern part of the Indian subcontinent. They occur in hoards large and small, and are dated to the first half of the 2nd millennium BCE, although very few derive from controlled and dateable excavation contexts. The copper hoards are associated with the Ochre Coloured Pottery (OCP), which is closely associated with the Late Harappan phase of the IVC. Associations with the Indo-Aryan of the second millennium BCE have also been proposed, though association with the Vedic Aryans is problematic, since the hoards are found east of the territory of the Vedic Aryans.

<span class="mw-page-title-main">Archaeology of Oman</span>

The present-day Sultanate of Oman lies in the south-eastern Arabian Peninsula. There are different definitions for Oman: traditional Oman includes the present-day United Arab Emirates (U.A.E.), though its prehistoric remains differ in some respects from the more specifically defined Oman proper, which corresponds roughly with the present-day central provinces of the Sultanate. In the north, the Oman Peninsula is more specific, and juts into the Strait of Hormuz. The archaeology of southern Oman Dhofar develops separately from that of central and northern Oman.

<span class="mw-page-title-main">Oxhide ingot</span> Mediterranean Late Bronze Age metal slabs

Oxhide ingots are heavy metal slabs, usually of copper but sometimes of tin, produced and widely distributed during the Mediterranean Late Bronze Age (LBA). Their shape resembles the hide of an ox with a protruding handle in each of the ingot’s four corners. Early thought was that each ingot was equivalent to the value of one ox. However, the similarity in shape is simply a coincidence. The ingots' producers probably designed these protrusions to make the ingots easily transportable overland on the backs of pack animals. Complete or partial oxhide ingots have been discovered in Sardinia, Crete, Peloponnese, Cyprus, Cannatello in Sicily, Boğazköy in Turkey, Qantir in Egypt, and Sozopol in Bulgaria. Archaeologists have recovered many oxhide ingots from two shipwrecks off the coast of Turkey.

Metals and metal working had been known to the people of modern Italy since the Bronze Age. By 53 BC, Rome had expanded to control an immense expanse of the Mediterranean. This included Italy and its islands, Spain, Macedonia, Africa, Asia Minor, Syria and Greece; by the end of the Emperor Trajan's reign, the Roman Empire had grown further to encompass parts of Britain, Egypt, all of modern Germany west of the Rhine, Dacia, Noricum, Judea, Armenia, Illyria, and Thrace. As the empire grew, so did its need for metals.

<span class="mw-page-title-main">Tin sources and trade in ancient times</span>

Tin is an essential metal in the creation of tin-bronzes, and its acquisition was an important part of ancient cultures from the Bronze Age onward. Its use began in the Middle East and the Balkans around 3000 BC. Tin is a relatively rare element in the Earth's crust, with about two parts per million (ppm), compared to iron with 50,000 ppm, copper with 70 ppm, lead with 16 ppm, arsenic with 5 ppm, silver with 0.1 ppm, and gold with 0.005 ppm. Ancient sources of tin were therefore rare, and the metal usually had to be traded over very long distances to meet demand in areas which lacked tin deposits.

Experimental archaeometallurgy is a subset of experimental archaeology that specifically involves past metallurgical processes most commonly involving the replication of copper and iron objects as well as testing the methodology behind the production of ancient metals and metal objects. Metals and elements used primarily as alloying materials, such as tin, lead, and arsenic, are also a part of experimental research.

<span class="mw-page-title-main">Lead smelting</span> Process of refining lead metal

Plants for the production of lead are generally referred to as lead smelters. Primary lead production begins with sintering. Concentrated lead ore is fed into a sintering machine with iron, silica, limestone fluxes, coke, soda ash, pyrite, zinc, caustics or pollution control particulates. Smelting uses suitable reducing substances that will combine with those oxidizing elements to free the metal. Reduction is the final, high-temperature step in smelting. It is here that the oxide becomes the elemental metal. A reducing environment pulls the final oxygen atoms from the raw metal.

References

  1. ↑ Weisgerber, Gerd; Yule, Paul (May 2003). "Al-Aqir near Bahla' - an Early Bronze Age dam site with planoconvex 'copper' ingots". Arabian Archaeology and Epigraphy. 14 (1): 48. doi:10.1034/j.1600-0471.2003.00003.x. ISSN   0905-7196.
  2. ↑ Stieglitz, Robert R.; Bass, George F. (October 1970). "Cape Gelidonya: A Bronze Age Shipwreck". Journal of the American Oriental Society. 90 (4): 541. doi:10.2307/598833. ISSN   0003-0279. JSTOR   598833.
  3. ↑ Maddin, R.; Merkel, J. (1990). "Metallographic and statistical analyses Ozieri: Il Torchietto". In Lo Schiavo, Fulvia; Maddin, Robert; Merkel, John; Muhly, James D.; Stech, Tamara (eds.). Analisi metallurgiche e statistiche sui lingotti di rame della Sardegna. Il Torchietto. pp. 42–199.
  4. 1 2 Tylecote 1987, p. 204.
  5. ↑ Pulak, Cemal (1998-08-01). "The Uluburun shipwreck: an overview". International Journal of Nautical Archaeology. 27 (3): 199. doi:10.1111/j.1095-9270.1998.tb00803.x. ISSN   1057-2414.
  6. ↑ Wachsmann, Shelley (1998). Seagoing ships [and] seamanship in the Bronze Age Levant. Texas A & M University Press. ISBN   0-89096-709-1. OCLC   884517291.
  7. ↑ Piggott, V.C. (1999) ‘The development of metal production on the Iranian Plateau: An Archaeometallurgical Perspective’ in The Archaeometallurgy of the Asian Old World :MASCA Research Papers, 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
  8. ↑ Tylecote 1987, pp. 194–209.
  9. ↑ Janzon, G.O. (1988-08-30). "Early Nonferrous Metallurgy in Sweden". In Maddin, Robert (ed.). The Beginning of the Use of Metals and Alloys. Cambridge, MA, USA: MIT Press. pp. 104–17. ISBN   978-0-262-13232-9.
  10. ↑ Rothenberg 1990, chapter 5
  11. ↑ Piggott, V.C. (1999). "The development of metal production on the Iranian Plateau: An Archaeometallurgical Perspective". In Pigott, Vincent C. (ed.). The archaeometallurgy of the Asian old world. Univ. Museum. ISBN   0-924171-34-0. OCLC   216894068.
  12. ↑ Weisgerber, Gerd; Yule, Paul (May 2003). "Al-Aqir near Bahla' - an Early Bronze Age dam site with planoconvex 'copper' ingots". Arabian Archaeology and Epigraphy. 14 (1): 24–53. doi:10.1034/j.1600-0471.2003.00003.x. ISSN   0905-7196.
  13. ↑ Merkel 1986
  14. ↑ Craddock 1995
  15. ↑ Roman 1990
  16. ↑ Tylecote 1987, pp. 22–5.
  17. ↑ Hoover and Hoover 1950
  18. ↑ Bamberger and Wincierz 1990, 133
  19. ↑ Weisgerber and Yule 2003, 48
  20. ↑ Pulak 2000
  21. ↑ TAR 2005-6
  22. ↑ Tylecote 1987, pp. , 22–3.
  23. ↑ Tylecote 1987, pp. 195–6.
  24. 1 2 Tylecote 1987, pp. 20-21 Table 10.
  25. ↑ de la Bedoyere 1989, 54
  26. ↑ Hunter 1999, 338-40
  27. ↑ Tylecote 1987, pp. 24.
  28. ↑ Merkel 1986
  29. ↑ Tylecote 1987, pp. 22.
  30. ↑ Tylecote 1987, pp. 23–4.

Sources

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.