Slag remains from the Na Slupi site (Prague, Czech Republic): evidence for early medieval non-ferrous metal smelting

International audience ; Archaeological investigation of the Na Slupi site in the central part of Prague (the capital of the Czech Republic) and located in the vicinity of Vyšehrad Castle has demonstrated the presence of early medieval (12th century) large-scale smelting of non-ferrous metals. This study is focused on the investigation of smelting remains (Pb-rich red and black slags) using a combination of geochemical and mineralogical methods in order to understand the technologies and processes used at this site. Glassy red slags are composed of high-temperature Ca-, K- and Pb-rich silicates (wollastonite, larnite, kalsilite, melilite) and Pb-bearing glass, and are associated with less abundant litharge fragments/red-slag-impregnated furnace walls composed of litharge/massicot (PbO) and Cu2PbO2. They indicate that the cupellation process in small furnaces, rather than in crucibles (open-vessels) was used for Ag refining at the studied site. The black slags, found in much lower quantity at this site, are composed of maghemite, fayalite, clinopyroxene, melilite and glass and are S-poor. We believe that they correspond to waste products from the first stage of Ag purification (prior to final cupellation), where slagging additives such as Fe-rich material, silica sand or ashes could be added to remove impurities. The Pb isotopic composition of both types of slags is similar to the isotopic signature of Pb–Zn ores from the vicinity of Cracow (Poland) and supports the hypothesis that Pb was probably traded between Prague and Poland in the 12th century and used for refining precious metals in metallurgical workshops.