Welsh copper in Brittany ? Results from chemical and isotopic analyses of the Tréboul hoard (Brittany, France)

The eponymous hoard, excavated in 1948 at Treboul (Douarnenez, Finistere), is one of the most important Atlantic Middle Bronze Age I hoards found in the north west of France. It consists of 142 artefacts. Many papers have written about this hoard especially the artefact typologies. The first analyses were carried out in 1958 by J. Briard and J.R. Marechal, however, analyses continued until 1996 with the last ones using ICP-AES. Whilst these analyses indicate a distinctive chemical composition of the metal (high content of nickel and arsenic and low content of antimony, and an high content of lead in some artifacts), the low precision of these measurements does not allow a detailed chemical signature to be defined. Therefore, 78 new ICP-AES analyses were performed in 2014 in Archeosciences Laboratory of Rennes (UMR CReAAH). All functional classes of artefacts were selected. In addition, 5 lead isotopical analyses were performed in Brest (IUEM laboratory) by Neptune multicollector ICP-MS The new data shows that for all trace elements, the chemical variability between artefacts is low resulting in a well-defined chemical signature, which is significantly different from that of the other Bronze Age periods in the northwest of France. Whilst some artifacts are lead-rich, they have the same trace element chemical signature as the low lead bronze artefacts. Similarly, the lead isotopic composition is the same for these two groups of artefacts. Therefore, the hypothesis of an exogen adding of lead is rejected. The comparison of these isotopic and chemical analyses with the ones from Acton Park artefacts from Britain, shows a very similar chemical and isotopic signature which implies that the copper used to produce these Middle Bronze Age I artifacts could have come from the same mine. Furthermore, the chemical and isotopical comparison with Copper ores from the Great Orme copper mine in Britain (North Wales), as well as with metal produced from this mine, highlights a very similar chemical signature. Therefore, it seems that metal used to produce copper-based artifacts in the north west of France and in Britain during the Middle Bronze Age I probably had the same origin, the Great Orme mine. In this mine the copper ores are cut by at least one lead ore vein. The presence of significant content of lead in some of the Treboul artifacts could be linked to this geological association. Was lead added unintentionally as an ore impurity or intentionally to aid smelting or casting? Whilst it is difficult to answer this question, the very variable content of lead amongst the artefacts suggests that the lead was an unintentional additive.