What is hidden underneath the black and brown on Silesian glassware? About the archaeometric challenge in the study of painted decorations on Late-Medieval and Post-Medieval finds from Wrocław, SW of Poland

At the end of the Middle Ages and the beginning of the post-medieval period, painted glass vessels were not uncommon and are found during archaeological excavations, especially in urban sites. In the first quarter of the 21st century in Wroc ł aw, a considerable collection of painted glassware was obtained during excavations: some of these artefacts were characterised by multicoloured painted decorations, while others seemed to be made using a modest colour palette, mainly brown and black. It was decided to take a closer look at the latter vessels and establish the cause of such differences. For this purpose, non-destructive and minimally invasive analytical tools were employed, commonly used to study archaeological artefacts, such as microscopic observations in visible light (OM), X-ray fluorescence spectrometry (XRF) or scanning electron microscopy coupled with an energy-dispersive spectrometer (SEM-EDS). In the course of archaeometric research, the results confirmed the assumptions about the significant contamination of painted decorations by mineral substances, especially iron and sulphur. The deposition of these elements on the surface of paint caused a permanent change in the colour of decorations on glass vessels.