Late Bronze Age slags from Trentino (Italy): Intepretation of the copper smelting model

At the end of the second millennium BC the extractive metallurgy of copper in North-Eastern Italy achieved a peak of technological efficiency and mass production, as evidenced by the large number of metallurgical sites and the total amount of slags resulting from smelting activities. Though several smelting processes have been proposed, an agreed interpretation of the whole process is lacking. Based on a collaboration with the Archaeological Heritage Office of the Autonomous Province of Trento and thanks to their recent excavations(directed by Dr. F. Nicolis and Dr. P. Bellintani,with the collaboration of E. Silvestri), over 70 slags from the Luserna, Transacqua and Segonzano sites (Trentino, Italy) have been characterized by means of OM, XRPD, XRF and SEM-EDS. A few metallurgical smelting experiments were carried out in order to explore both the slagging and the matting processes. Three different types of archaeological slags have been distinguished from the mineralogical and chemical points of view: the two well-known Alpine LBA “coarse” and “flat” slag types, and a new intermediate type of slag here defined as “massive”. The three slag groups differ in the size and relative amount of the unreacted sulphides, the highly Cu-enriched matte, and the metallic copper. Theratio between therestitic quartz and the newly formed fayalite and magnetite is also a distinctive parameter, possibly related to the maximum temperature involved in each step. It is proposed that the three types of slags are related to three distinct metallurgical steps: 1) the coarse slags are the product of the initialroasting/slagging operations, where a Cu-poor matte is produced, mainly of bornitic composition, 2) the massive slags are the result of the major matting process, where a Cu-rich matte (covellite-chalcocite composition) is produced, and 3) the thin flat slags are the product of the final refining process, formed directly at the surface of the raw molten metal.