First Study of Mercury Content in Archaeological Pottery: Late-Neolithic Penha-Type from NW Spain

In soils, mercury is found bound to organic matter, clays, and iron/manganese oxides, which are also major constituents of archaeological pottery. Although pottery is the most researched cultural material with archaeometric techniques, its mercury content remains largely unexplored. To address this gap, we studied Late Neolithic Penha-type pottery from NW Spain. The Late Neolithic was a period of widespread exploitation and circulation of mercury-bearing resources. A total of 92 samples from five archaeological sites were analysed to determine their mercury, carbon, sulfur, and iron content, as well as their spectroscopic properties (FTIR-ATR). Mercury was detected in all samples, with concentrations ranging from 6 to 1086 ng g^-1. Neither organic matter (C and S) nor iron compounds (Fe) were found to explain Hg concentrations, suggesting that diagenetic mercury incorporation was unlikely. Mercury was found to be related to kaolinite structural transformations, with concentrations decreasing with increasing degree of transformation. Kaolinite transformation depended on firing conditions (temperature and time), pointing to thermal desorption of the mercury present in the clay. The large observed variability most probably resulted from poorly controlled firing conditions. Nevertheless, whether mercury content reflects unintentional incorporation from naturally mercury-rich raw materials or a deliberate selection or addition (e.g., of cinnabar) during pottery manufacture remains to be further explored.