Metallographic investigation of Early Bronze Age armbands from Western Switzerland (ca. 2200-1500 BC): new highlights about early manufacturing processes

The Valaisan-type armband (a specific type of bracelet) is a typical metallurgical production from the western area of Switzerland belonging to the classic phase of the Aare-Rhone group (BzA2a, ca. 2000-1800 BC). This investigation aims to (i) characterize the metal composition, (ii) reconstruct the thermomechanical treatments applied during the manufacturing process, and (iii) gather information on the possible exploitation of the local ores coupling metallography and chemical analysis. The results show that each armband is manufactured from a hammered sheet of copper-based alloy, containing either tin (up to 3.0 wt.%) or a combination of antimony, nickel, and silver. In several cases, it is assumed that minor elements are already part of the original ore, suggesting a conscious selection of copper veins. In other armbands, a direct addition ofcassiterite(SnO2) to the copper matrix is hypothesized based on the material composition and features of the inclusions. Microstructural features are coherent with a procedure that combines mechanical deformation (with a total deformation degree between 70 and 76%), annealing, and quenching, coherently with more recent productions (Late Bronze Age). The analysis of inclusions, rarely performed during metallographic investigations, provides precious evidences on thermal treatments applied during the manufacturing process and shows that annealing was carried out at low temperatures.