Analysis of Urnfield Period Bronze Droplets Formed during Casting

Abstract

The Late Bronze Age copper mining site “Gasteil Cu I” (ca 1050–850 BC) is located in the district of Neunkirchen, Lower Austria. Since 2010 five excavation campaigns took place and many artifacts as well as corroded metallic droplets were discovered. Two of these droplets were investigated by metallography. At the droplet ́s surface tin was measured, indicating that these droplets are formed during bronze casting. The small droplet is severely corroded but in its core the original bronze alloy is still present. The analysis showed about 10 wt.% Sn. The microstructure is characterized by a Cu-Sn solid solution and a Cu-Sn intermetallic phase. Additionally small amounts of Cu2S were observed in the metallic core. The corroded rim contains oxides and hydroxides of Cu and Sn and other impurities like Ca, P, Si, S and Fe. Analyzing the corrosion products, Sn concentrations up to 40 wt.% were measured. This enrichment can be explained by a simultaneous formation of insoluble SnO2 and Cu ions, which were transported to the surface. Surprisingly, the other large droplet has a corroded core and metallic phases are remaining at the rim. The microstructures of the corrosion products reflect the original casting microstructures of the bronze.