Abstract
Archaeological investigations of the Colline Metallifere district (Southern Tuscany, Italy) have highlighted several Medieval sites located close to the main Cu-Pb-Fe (Ag) ore occurrences. This study is focused on the investigation of late-medieval slags from Cugnano and Montieri sites using both geochemical and mineralogical methods to understand slag heterogeneities as result of ore differences and technological processes. Matte-rich slags present in both sites (with abundant matte ± speiss and frequent relict phases) represent waste products related to primary sulphide ore smelting to obtain a raw lead bullion. The distribution of slags between the Ca-rich or Fe-rich dominant composition, and the consequent mineralogy, are tracers of the different ore–gangue association that occurred in the two sites. Silver is present only in very small matte-rich slags and ores enclosed within the mortar of the Montieri site; wastes derived from silver-rich mineral charges were probably crushed for the recovery of silver. Matte-poor slags found at Montieri represent a second smelting; raw lead bullion obtained from matte slags (both Fe- and Ca-rich) was probably re-smelted, adding silica and Al2O3-phase-rich fluxes, under more oxidizing conditions to reduce metal impurities. This second step was probably employed for Zn-rich lead ores; this process helped to segregate zinc within slags and improve the quality of the metal.
Highlights
The Colline Metallifere district hosts manyCu-Pb-Zn (Ag) vein deposits, which fed an important mining and metallurgical industry from the Chalcolithic period up to the 1970s
Considering that the presence of both Ag-bearing ore fragments and metallic silver seemed to be restricted to small slags enclosed within mortar, and that those slags did not differ in mineralogy and textures from the bigger cakes from dumps, we suggest that this ore and slag association derived from the treatment of an Ag-enriched mineral charge
We suggest that the practice of adding Al2 O3 -phase as a fluxing agent during smelting and, more commonly, during the refining processes, could help to reduce zinc impurities in the produced metal, and that this was performed especially in those areas where lead- and silver-bearing phases were closely mixed with zinc minerals
Summary
The Colline Metallifere district (near Massa Marittima, southern Tuscany) hosts manyCu-Pb-Zn (Ag) vein deposits, which fed an important mining and metallurgical industry from the Chalcolithic period up to the 1970s. According to archaeological and historical documents, during the medieval period (12th–14th century AD), the metal production of this area was mainly focused on the extraction of copper and silver for coinage. Archaeological campaigns of the renowned castle of Rocca San Silvestro [1,2] highlighted the presence of mining sites intrinsically connected to the process of castle formation. This aspect suggests that during the development of local seigneurships, in this geographical context, much of their economic power was based on controlling and exploiting metals. Particular interest was focused on how local communities controlled and organized metallurgical production during the medieval period [3]. After the castle of Rocca San Silvestro, extensive research was performed on other sites with specific mining vocation, such as Campiglia
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