Chemical and metallographic investigation of weapons from the Bronze Age necropolis at Olmo di Nogara, Italy

The present investigation aims to characterise the bronze weapons found in the graves of the Olmo di Nogara necropolis (Verona, Italy) to understand the materials, the metallurgical production techniques and their development during the use of the site. Olmo is an important biritualistic necropolis. The large number of excavated graves provide copious and rich equipments ranging in time from Aeneolithic to Recent Bronze Age (RBA). Small samples for archaeometrical characterization were collected from different points of several typologically defined and well dated weapons: a dagger, ten swords, and two halberds were selected. Each sample was analysed by SEM-EDS in order to obtain point and area chemical analyses, and backscattered electron images, and by reflected-light metallographic analysis. The two Aeneolithic halberds are composed of arsenic bronze alloy with about 5-6 wt % of As. The Middle Bronze Age (MBA) and RBA materials are rather homogeneous tin bronzes, with a variable amount of tin in the range 9-15 wt %. Inclusions of residual copper and copper-iron sulphides were detected, often showing a preferential orientation due to the mechanical working processes. The metallographic analyses show that the α-phase is prevalent in the metal, with the presence of strongly oriented crystallites of the intermetallic γ-phase. Clear evidence of advanced manufacturing techniques, including cycles of thermal annealing and hammering is observed in all samples. Specific textural and compositional features of the finds are discussed, and they indicate a remarkable mastering of different metallurgical processes in the production of the different parts of the weapons.