Chemical analysis of 17th century Millefiori glasses excavated in the Monastery of Sta. Clara‐a‐Velha: comparison with Venetian and façon‐de‐Venise production

Abstract

A set of ten Millefiori glass fragments dating from the 17th century, originated from archaeological excavations carried out at the Monastery of Sta. Clara‐a‐Velha (Coimbra, Portugal) were characterized by X‐ray electron probe micro‐analysis (EPMA), Raman microscopy and UV‐Visible absorption spectroscopy. All glasses are of soda‐lime‐silica type. The use of coastal plant ash is suggested by the relatively high content of MgO, K2O and P2O5, as well as by the presence of chlorine. Tin oxide or calcium antimonate were the opacifiers used in the white glasses, cobalt is present in the blue glasses, copper in the turquoise, iron in the yellow and greenish, and iron and copper were found in the opaque red and aventurine glasses. Based on the concentrations of alumina and silica four different sources of silica were identified, allowing the classification of the glasses in the following compositional groups: low alumina (< 2 wt%), low alumina – cristallo (< 2 wt% and SiO2 > 70 wt%) medium alumina (2 ‐ 3 wt%), high alumina (3 ‐ 6 wt%) and very high alumina (> 6 wt%). Comparison with genuine Venetian and façon‐de‐Venise compositions showed that two fragments are of Venetian production, one of Venetian or Spanish production and the remaining are of unknown provenance. In two fragments the glass of the decoration is probably Venetian or Spanish but the glass used in the body is also of unknown provenance.