Sand and Pebbles: The Study of Portuguese Raw Materials for Provenance Archaeological Glass

Abstract

Portuguese archaeological excavations dated to the 17th century onwards are extremely rich in glass artefacts, with this being a reality from the north to the south of the territory. Contrasting with this reality, no glass production locations from this period have been discovered or excavated so far, which makes the provenance attribution a challenging endeavour. One specific archaeological location, the Monastery of Santa Clara-a-Velha in Coimbra, held one of the largest glass archaeological assemblages dated to the 17th century unearthed in Portugal so far. Due to the large variety of objects’ shapes, glass colours and decorative features, this assemblage is a valuable candidate to hold glass artefacts produced in Portugal. Lacking archaeological excavation on glass furnaces in Portugal, the study of glassmaking raw materials is the most promising research line to investigate the provenance of glass circulating in Portugal. In this study, sand and pebbles from six different locations in the north/centre of Portugal were collected and melted to produce glass samples. The resulting glass samples were chemically characterised using LA-ICP-MS, to obtain the composition of the samples in major, minor and trace elements. The obtained results were compared with the composition of 37 historical samples from the Monastery of Santa Clara-a-Velha previously studied. Additionally, the thermal properties of selected synthesised glasses were analysed by Differential Scanning Calorimetry, allowing new insights on melting temperatures and glass workability. Results indicate that three artefacts with stylistic features attributed to a Portuguese production were locally made with sands collected in Coimbra.