Determining the provenance of glass beads from a Franciscan Indigenous reduction (XVII century) in Argentina. The use of X-ray computed tomography to determine production processes

Abstract

In order to elucidate the provenance of fourteen European glass beads recovered from the historical colonial site of San Bartolom´e de los Chan´a (XVII century) located in Argentina, South America, scanning electron microscopy and X-ray energy dispersive spectroscopy studies were performed. The study of the relationships between key oxides in the glass beads – magnesia vs. lime, alumina vs. iron oxide and magnesia vs. potash, successfully indentifies the Netherlands as their likely provenance. In addition, characteristics of the beads production in Dutch workshops support this proposal. These conclusions align with historical records, which indicate that Dutch beads were more readily accessible to the Spanish for trade with Indigenous populations in the region. Additionally, light microscopy (optical) and prompt gamma neutron activation analysis were employed as complementary characterisation techniques. X-ray computed tomography studies, using Heidelberg illumination and analysis of the X-ray absorption coefficient through the beads, emerged as a powerful non-destructive method for inferring production techniques. This approach distinguishes between the use of a rotating mandrel and the application of successive glass layers formed after blowing. Furthermore, the identification of successive superimposed layers within the beads, exhibiting variations in doping elements and compositional gradients, was achieved.