Isotopic lead characterization of archaeological bronzes from fraga dos corvos (n Portugal)

A set of bronzes recovered from Fraga dos Corvos (Macedo de Cavaleiros) archaeological site, located in Northern Portugal, was analyzed to investigate their lead isotope ratios. The studied metallic artefacts have diverse typologies and include two bar fragments, four fibulae, a pendant and fragments of a possible cauldron. Besides these, two metallurgical remains (nodule and droplet) were also analyzed. Elemental analysis by μ-EDXRF showed that bronzes have a Sn content varying between 5.1 and 13.9% and a Pb content from 1.4 to 6.1%, which allowed to select a suitable methodology for lead separation, prior to isotope ratio determinations. In these alloys the Pb isotope determinations (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb) are influenced by matrix effects that decrease the sensitivity and the reproducibility of the measurements. Analytical procedure consisted in electrochemical Pb separation by anodic oxidation. The determination of Pb isotope ratios was made by ICP-MS with a quadrupole mass filter and provided the first results of Pb isotopic composition of bronze alloys from the Northern Portuguese territory. Isotope ratios determinations were obtained with a relative standard deviation below 0.5%. Pb isotope ratio distributions allowed the identification of different composition patterns indicating probable distinct provenances, although the similar isotopic ratios among some artefacts and the metallurgical remains pointed out to local metallurgical activities.