Plano-convex ingot

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 orientation of concavity.^[1] They are most often made of copper although plano-convex ingots of other materials such as copper alloy, lead and tin are also known.^[2] Plano convex ingots 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.^[3] By the end of the Bronze Age they were found throughout Europe^[4] and in Western and South Asia^[5] and similar ingot forms continue in use during later Roman and Medieval periods.

Manufacture of Bun Ingots

Traditionally bun ingots have been 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 has shown that regular plano-convex ingots are difficult to form within the smelting furnace producing only small ingots or copper prills which need to be remelted (Merkel 1986, Craddock 1995.) The high purity copper bun ingots from the found in Late Bronze Age Britain and the Mediterranean seem to have undergone a secondary refining procedure between

The metallographic structure and high iron compositions of some plano-convex ingots suggest that they are the product of primary smelting (Roman 1990.) Tylecote has suggested that Roman plano-convex copper ingots may have been formed by tapping both slag and copper in one go into a mould or pit outside the furnace (1986, 22-5.) A similar process was described by Agricola in book IX of his De Re Metallica (Hoover and Hoover 1950) and has been replicated experimentally (Bamberger and Wincierz 1990, 133.)

Structure of Bun Ingots

Although all bun ingots share the same basic plano convex 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 the sand can be highly variable in form even on the same site (Weisgerber and Yule 2003, 48) whereas ingots cast in reusable moulds will form sets of identical “mould siblings” (Pulak 2000.)

The composition of the metal and its cooling conditions also has an effect on the bun ingots structure. As the ingot cools gasses are released giving the upper surface a “blistered” texture and if cooling takes place outside of the furnace the outer surface often becomes oxidised . and 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 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 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 artifacts 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

When analyzed, the copper is found to be of very high purity, although earlier examples are sometimes composed of arsenical copper. Tylecote has thus 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 shows a columnar structure which probably indicates slow cooling in a reheating furnace or a warm mold, rather than from pouring into a cold mold (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 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, planoconvex 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