Copper ingots from a probable Bronze Age shipwreck off the coast of Salcombe, Devon: Composition and microstructure

The seabed site of a probable Bronze Age shipwreck off the coast of Salcombe in south-west England was explored between 1977 and 1982 and from 2004 onwards. Nearly 400 objects including copper and tin ingots, bronze artefacts/fragments and gold ornaments were found, typologically dating either to c. 1300-1150 BC or 1000-800 BC. The 280 copper and 40 tin piano-convex ingots and ingot fragments represent the largest discovery, measured by total weight as well as by quantity, of piano-convex or bun ingots in northwest Europe. The Salcombe copper ingots provided a wonderful opportunity for the technical study of copper ingots in a probable shipwreck context, as opposed to terrestrial contexts of deliberate deposition. The chemical composition of 25 plano-convex copper ingots was determined using inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Two artefacts from the site were also analysed for comparison with the ingots. Following the compositional analysis, a microstructural study was carried out on ten Salcombe copper ingots selected to cover those with different sizes, shapes and variable impurity levels using metallography and scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM-EDS). All the analysed copper ingots are of unalloyed copper with low levels of impurities. Sulphide inclusions are present in all samples and bulk sulphur contents are of 0.32-0.79% in the ingots but lower in the artefacts. The Salcombe ingots were found to have a quite similar impurity pattern to the Hertford Heath (England) ingots (except for iron content). They are distinctly different from the Uluburun ingots, and, to a lesser degree, from Sardinian ingots. The results are inconclusive as to how the Salcombe ingots were made. On the one hand, the very low concentration of iron and the absence of cuprite inclusions suggest that the ingots were primary smelting products of the primitive smelting process rather than produced from re-melting or refining of primary smelting lumps. On the other hand, the dense metal with very low porosity suggests the product of refining and re-casting operations under reducing conditions. However, the small ingots are not likely to have resulted from breaking of large ingots. The chemical compositions of the Salcombe ingots point to British or Western European sources although the connection with other regions cannot be excluded for some of the ingots. Further studies including lead isotope analysis are needed to address the question of provenance of the copper ingots, which would contribute to the re-emerging debates surrounding the European Bronze Age metal trade.