Chemical composition of potassium-glass vessels from the 14th-15th centuries found in Tartu, Estonia

XRF-analysis of 18 fragments of several 14th–15th-century vessel types and an additional fragment of a 16th-century glass vessel, found primarily from the cesspits in the medieval town centre of Tartu, was carried out. The study reveals that these are mostly potassium-calcium silicates, containing 49–56 wt% of SiO2 and a varying high level (from 15 to 20 wt% or even more) of K2O and CaO. Four typological groups are represented: vessels with applied glass threads, high prunted beakers, lower beakers with a spiral glass thread, and beakers with a zig-zag glass thread. It is shown that all these have rather similar chemical composition. Classification by the CaO/K2O ratio hints at a possibility that besides the glass of Bohemian origin, vessels came also from the glasshouses in the present-day Germany. The blue decoration on the basic colourless glass contains Co, Cu, and Fe bearing colourants accompanied by various minor impurities like Zn, Pb, and sometimes Ni (instead of Zn). In addition, in the blue glass of the 16th century dating elements Bi and As were found. Obviously, Co from various sources was used. A fairly high level of Cu and Fe in blue decorations implies that these metals might have been added to the frit separately to get an additional shade, in the form of brass (Cu+Zn alloy) and hematite (Fe2O3, rust) respectively.