Blue decoration on glass Objects: Unravelling origins and Significance in late medieval – Early modern Portugal

Late medieval and Early modern Portuguese archaeological contexts have brought to light several colourless objects with blue thread application. Usually related to cylindrical glasses and pedestal goblets, this decoration can be found in several features: one or more lines applied horizontally or in a spiral, frills forming festoons or blue lines combined with a ribbed decoration. The presence of these objects reveals a connection with the upper strata of society, as they were considered luxurious and could be identified as imported, raising questions about their provenance. This study examines 20 glass objects adorned with blue decoration from five Portuguese archaeological sites in Lisbon, Almada, and Setúbal, dating from the 14th to 16th centuries. Through a combination of micro particle-induced X-ray emission (μ-PIXE) and inductively coupled plasma mass spectrometry with laser ablation (LA-ICP-MS), the chemical composition of the glass was examined. Stylistic investigations and analytical techniques shed light on how the raw materials used in objects with this type of decoration differ between older and more recent artefacts, indicating changes in society’s taste and the raw materials used, hence the place of production. The chemical composition of the analysed fragments, which falls within the soda-silica-lime group, gives clues to their origin and leads to a debate about whether these objects are imported or locally produced. This interdisciplinary approach combining archaeological findings, stylistic analysis and chemical analysis, offers an understanding of the cultural and technological dynamics that shaped glass production in medieval and Renaissance Europe.