Ancient to modern smelting activity in Tuscany: a petrological and chemical study of iron and copper metallurgical slags

The study of metallurgical slags is an interdisciplinary subject that can lead to implications for archaeology and archaeometallurgy, ore geology and environmental studies as well. The slags, in fact, are the most abundant remaining testimony of ancient to relatively modern smelting processes and the study of the slags is relevant in reconstructing the smelting parameters. Moreover, the chemical composition of the slags, rich in residues of metals derived from the metallurgical process, creates a latent threat for the environmental matrices that surround the slags heaps, due to the weathering of these materials and to the consequent release of potentially toxic elements. On these bases, 17 historical metallurgical sites in Southern Tuscany were investigated and 122 samples were collected, choosing slags derived from both iron and base metal smelting. A selection of the samples was further investigated for bulk chemistry composition as well as mineralogical and petrographic features. New data from underexplored or even yet unexplored smelting heaps of Southern Tuscany and Elba Island have been integrated with the available literature, to build-up a new homogeneous and complete framework of the slags from ancient, medieval and modern smelting sites of both iron and copper type. An additional environmental study was performed to assess the potential release of toxic elements from the slags, using two different leaching protocols: a first series of experiments aimed at investigating the influence of ambient pH and a second series devoted at evaluating the interaction with the vegetation cover. The results underline that leachability potential is related to both mineralogical assemblages and pH values and activity of organic acids, thus demonstrating that the slags are not inert materials and can became environmentally hazardous under specific weathering conditions.