Provenance studies of 18th century potassium-rich archaeological glass from Portugal

The archaeometric study of a group of colourless glass fragments recovered from two archaeological excavations performed in Lisbon, Portugal is here reported. The archaeological interventions were performed at Rua do Arsenal (LRA), where the ruins of the Côrte-Real Palace were partially discovered, and at the Roman Theatre Museum (LTR), where the remains of a middle-class house dated to the 18th century were found. The recovered glass fragments are dated between the end of the 17th century and the 18th century, and were chemically characterised with the aim of discussing its provenance. Micro particle induced X-ray emission (μ-PIXE) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were the chosen analytical methods to characterise the glass composition. All twenty-five fragments proved to be of a potassium-rich composition, where the choice of raw materials seems to have been made with extreme care. This is reflected in general by a composition with low levels of impurities, such as alumina and iron oxide, and high levels of silica. Two compositional groups were identified, that besides differing in other components have different contents of arsenic oxide. None of the groups could be undoubtedly compared with the few known potassium-rich compositions from coeval glass production centres in Europe. The majority of the analysed samples could only be compared, in terms of major oxides, with the potassium-rich production from the Coina Glass Factory in Portugal.