A geochemical survey of the Antas Valley, Sardinia: Medieval metallurgy and modern slag recycling?

• Lead contamination was detected in the soil of the Antas valley in Sardinia using portable X-ray fluorescence devices. • Three concentrations of lead-bearing black slag have been located along a river. • Analysis of the slag revealed a high lead content (around 75%) and a very low zinc content (less than 1%). • It is possible that this slag is the result of recycling of medieval slags carried out in the 19th century. • The presence of high concentrations of zinc in the soil is probably due to run-off from the zinc-rich dolomite that characterises the geology of the region. Geochemical surveys of the Antas Valley in Sardinia, Italy, have revealed significant zinc and lead concentrations along the Antas River floodplains, suggesting the presence of medieval ore-processing workshops which are otherwise hard to detect. While the zinc concentrations were found to be dispersed and probably related to the erosion of zinc-rich dolomite, the lead concentrations were more localised on the banks of the river, suggesting an anthropogenic origin. Three large concentrations of lead were found to coincide with deposits of black glassy slag, a by-product of ore processing. Analysis of the slag revealed a high lead content (around 75%) and very low zinc content, which along with historical research suggests either medieval ore processing for silver and/or 19th-century slag recycling. The near absence of zinc in the slag supports the 19th-century recycling hypothesis, as this period saw an increased demand for silver, lead and zinc and the development of processes to extract it from older slag. Further research, including dating of the slag and excavation of the lead-enriched areas, is required to confirm the origin of the slag deposits.