Arsenical copper and bronze in Middle Bronze Age burial sites of southern Portugal: the first bronzes in Southwestern Iberia

摘要

Middle Bronze Age was a transition period in Iberia, characterised by the emergence of bronzes after more than a millennium of a conservative metallurgy of copper with arsenic. Despite its importance there are no relevant studies on MBA metallurgy in Southwestern Iberia due to the absence, until recently, of known settlements and the scarcity of metals. However, recent archaeological excavations have brought to light important finds dated to the SW Iberian Bronze Age such as new burial monuments and open settlements. About 50 artefacts from hypogea, cists and domestic contexts (pits) from Torre Velha 3 (Serpa) and Monte da Cabida 3 (évora) were analysed by micro-EDXRF, reflected light microscopy, SEM-EDS and Vickers microhardness testing. Radiocarbon dating of their archaeological contexts established a chronology of ~1900-1300 cal BC. Despite presenting different burial practices both sites share the almost exclusive use of arsenical coppers (4.1 ± 1.0 and 4.2 ± 1.5 wt.% As, respectively). However, few awls and a dagger from Torre Velha 3 are among the earliest evidence of bronze in SW Iberia, being dated to the second quarter of the 2nd Millennium BC. These bronzes are similar (9.6 ± 1.2 wt.% Sn) to LBA alloys suggesting trade with a region with a developed bronze metallurgy. The emergence of bronze in SW Iberia during the first half of the 2nd Millennium BC points to an earlier introduction or a more rapid expansion than initially assumed. Nevertheless, these arsenical coppers and bronzes display a similar manufacture involving hammering and annealing cycles. A final hammering increased the hardness, which could be higher for bronzes. Arsenical coppers display variable operational conditions often with poorer thermomechanical work as expected from a prehistoric technology. A bronze dagger with silver rivets evidences the prestige value of early bronzes to MBA communities. Similarly, an arsenical copper dagger with silver coloured rivets shows the ability of MBA metallurgists to replicate prestige objects with indigenous knowledge.