Traceability of iron from Elba Island: new data from experimental Archaeometallurgy

Iron deposits from Elba Island (Tuscan Archipelago) were extensively exploited since the 1st millennium BC: both raw iron ore and smelted blooms were extensively traded throughout the Mediterranean region. However, archaeological evidence of Roman and pre-Roman (Etruscan) ore exploitation and extractive metallurgy on Elba Island is extremely scarce mostly due to the past century's slag recovery for re-use in modern blast furnaces. Recent studies of hematite-rich ores from Elba showed a prominent and apparently unique (at an European scale) co-enrichment in W and Sn that may represent a provenance marker not only for the ore itself (remains of which are commonly found at many ancient smelting/smithing sites) but, possibly, also for iron blooms and metallurgical slag. Within the frame of the multidisciplinary research Project "AITHALE" (from the Greek name of Elba Island), we have performed a series of archaeometallurgical experiments primarily to investigate the extent of Sn and W partitioning during the various steps of the chaîne opératoire of bloomery iron production. On March 2013 at the Bacino stope, Rio Marina mine (Elba Island) a furnace was prepared entirely made of local clay; forced draught was provided by an air compressor connected through a clay tuyere with the furnace. After 1h preheating, about 45 kg of pre-roasted hematite-rich ore (taken from the nearby Bacino stope) and charcoal were regularly added to the top of the furnace in roughly a 1:1 ratio. A total of some 30 kg of fayalite-rich slag were tapped from the furnace in five separate steps. At the end of the experiment about 5 kg of iron bloom were obtained. The hematite-rich ores, analyzed by ICP-MS, have bulk contents of Sn and W of, respectively, 75 and 656 ppm; EMPA analyses showed that W is present both in solid solution in hematite and as micrometric crystals of scheelite (CaWO4). The W/Sn ratio of the slag's silicate melt (?20) is much higher than that of the bloom (?3), indicating that under the operating conditions of this experimental process W preferentially partitions into the slag. Smelting experiments of Sn/W-rich hematite from Terranera mine (Elba Island, to the south of Rio Marina district) were also carried out in a Deltech DT-31VT-OS2 vertical quench furnace (at the Petrology Laboratory of the Dip.to Scienze della Terra, Firenze) under controlled redox and temperature conditions and variable ore:charcoal: (siliceous) flux ratios. In one experiment, we obtained metallic iron with appreciable amounts of dissolved Sn (0.2-0.3 wt%, EPMA analysis), but with W still below the instrumental detection limit (0.05 wt%). This confirms the preferential partitioning of W into the silicate slag. In both types of experiments the slags resemble in their mineralogical and textural features to analogue materials found at ancient ironworking sites, as for instance Baratti-Populonia, the most important site in ancient Italy for iron production during the Etruscan and Roman periods.