Elemental and Raman investigation of 13th - 14th and 16th century enamelled glass beakers found in Estonia

Fragments of five enamelled glass beakers found from the Estonian towns Viljandi and Tartu have been studied by using Raman spectroscopy, optical absorption and X-ray fluorescence methods. The beakers of 13th-14th-century dating are made of soda-lime silica glass and by composition and some decoration motifs they are comparable with the analogous finds in Europe (beakers supposedly of Venetian origin), a colourless beaker with an unusual decoration pattern from Viljandi being of a similar composition. The rare and relatively primitive (both by the frit fabrication and enamelling) blue beaker of the same era from Viljandi is also a sodalime glass which contains a minute amount of the colouring agent Co accompanied by the elements Pb and Zn. The composition suggests that this beaker also originates from a workshop somewhere in South Europe (not excluding Venice/Murano), the Co ore being imported from the Occident. The other blue beaker (bowl) of the 16th century, found in Viljandi, belongs to the potash glasses and contains also the colouring agent cobalt, here together with the additives As, Bi, Ti, and Ni. As the Co ore of a similar composition was acquired from the Ore Mountains in Germany, it is likely that this artefact was made in some Central European workshop. An analysis of the yellow enamel on these beakers shows the use of Pb-Sn-O-type colourant on earlier artefacts, while the antimony-rich colourant appears only in the case of the late medieval potash glass. In summary, both the use of Co-colourant of local (European) origin in the blue glass of the 16th century and the occurrence of antimony in the yellow enamel on it, suggest that a marked development of the European mine industry took place in the Late Middle Ages.