Michigan Copper in the Mediterranean (cont.)
By Jay Stuart Wakefield, MES & AAPF

The Uluburun Ingots

In the excellent 30-page 2002 study by Hauptmann et al, on the “Structure and Composition of Ingots from the 1300 BC Uluburun Wreck” (Ref.54) the authors say “the cargo represents the ‘world market’ of bulk metal in the Mediterranean. The wreck contained 354 oxhide-shaped ingots and 121 discoid, or bun ingots, altogether 10 tons of copper (see Fig.4).

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Additionally a ton of tin ingots were recovered, in 120 ingots and fragments, a ratio which roughly corresponds to the ratio of copper to tin in ‘classical’ bronzes.” The cedar hull was badly damaged by a collision with the shore, but some of the wood was preserved by the corrosion products of the copper ingots. These ingots are all now in the Museum of Underwater Archaeology, in Bodrum, Turkey, with the ingots also found in the later date Cape Gelidonya shipwreck. These are more ingots than the total in all other museums and private collections put together. Some oxhide ingots have been excavated in the Minoan ruins of Hagia Triadha in Crete (dated to 1550-1500 BC), and others have been found in Sardinia, Cyprus, the Nile Delta, Turkey and Bulgaria. Researcher Zena Halpern, (Ref.71), reports “I saw heaps of copper ingots in the Maritime Museum in Haifa, Israel”. “Metal bars in the oxhide shape dating from c.1700 BC have been found at Falmouth in Cornwall”, England (Ref.78). Egyptian New Kingdom tomb paintings and temple reliefs depict a great number of copper ingots, but only one has been found in Egypt, as they were consumed there. (Ref.23).

For many years, the archaeological community has thought that lead isotope studies by an Oxford group, Gale et.al.(Ref.23,35,44,56) have proved that the ingots all came from Cyprus. In 1998 the Gale group (Ref.56) reports performing “approximately one thousand [!] lead isotope analyses of ores and ingots, including about 60 Uluburun ingots”. (They did not test a single sample of Michigan copper.) The study reports that the “Uluburun ingots are greater than 99.5% pure copper”.

In the Hauptmann study, a steel chisel was used to cut pieces for surface sampling of 151 of the Uluburun ingots, and three oxhides and one bun were drill cored all the way through (see Fig.2). Their report states that he samples showed porous volume typical of “blister copper”, that “exceeds by far our previous ideas on their inner structure, with void volume reaching 20% or higher, especially in the upper portions of the ingots. In general, cavities like these, called “spratzen”, are caused by the effervescence of gases, such as oxygen, carbon monoxide, and carbon dioxide, by water from burning charcoal. This is in contrast with copper from other periods and other localities... All the ingots contain angular-shaped inclusions of iron-silicate slags, features compatible with natural rocks affected by the impact of high temperatures in the solid state. These can be removed by repeated melting, but, while these were regular steps … at many metallurgical sites all over the middle and southern part of Africa, the Uluburun ingots were not processed in this way. The angular shape of the slag inclusions, the structure, and the existence of iscorite point to a pouring of copper into a mold when the slag was already solidified… Interfaces in the crystalline structure of the ingots points to different batches during casting. Almost all the samples contained cuprite (Cu2O) distributed in changing amounts throughout the ingots, associated with large voids. The cuprite formed by corrosion in the sea does not penetrate for more than 5mm or so. An oxygen-rich atmosphere necessary to produce cuprite in an amount observed does not prevail during the smelting of (roasted) ores. We therefore can eliminate the conclusion that the ingots consist of as-smelted raw copper from a smelting furnace. Most of the ore available on Cyprus is of chalcopyritic composition, and relics of sulfides are quite difficult to completely remove, yet this mixed sulfide does not occur in the copper ingots.”

The Hauptman study concludes that “from a chemical point of view, the purity of the ingots is extraordinary in comparison with other sorts of copper from Wadi Arabah (high lead), from the Caucasus (high arsenic), from Oman (high arsenic and nickel). The ingots are made of pure copper, and all the ingots show a homogeneous composition. From our metallographic investigations, we are able to exclude a conscious purification or even a refining process to produce the ingots. We see few indications that bronze scrap could have been added, due to the very low tin concentration, and would not include gas bubbles and slag inclusions. The ingots provide an explanation for the previously vexing question of how an ingot of a metal as ductile as copper could have been broken up into small pieces such as those excavated by the hundreds in Sardinia. Two characteristics of the Uluburun ingots stand out – the presence of a substantial degree of porosity, and a high concentration of copper oxide inclusions, which made it brittle. Simply dropping the ingots onto a hard surface would easily shatter the ingots.”

A 32 page 1995 study by Budd et al (Ref.55), reviewed all the work to date, and says “all the oxhide ingots are composed of essentially pure copper… No meaningful conclusions on provenance can currently be drawn from a consideration of trace element data for oxhide ingots, ores, and artifacts on Cyprus or Sardinia… It is no surprise that the only oxhide ingot mold ever found, at Ras Ibn Hani, Syria, in 1983 was surrounded by droplets bearing the same isotope signature as the vast majority of the oxhide ingots. The 1989 (Ref.35) Gale report concludes that the Aghia Triadha ingots on Crete “are certainly not made of Cypriot copper”, and the copper source could not be identified. Dickinson, author of the Aegean Bronze Age (Ref.21) “From outside the Aegean came …oxhide ingots. These have all, when tested, proved to be non-Aegean metal.”