ON COBALT USAGE IN THE OLD RUSSIAN PRODUCTION OF LEAD GLASS
In the Old Russian glassmaking, beginning in XI century and based on the production of lead glass, cobalt was used very rare. Until recently only three assured cases of cobalt usage for blue coloration of bracelets and beads were fixed. In addition, two cases of probable cobalt coloration were published in the middle of the preceding century, but owing to weakness of analytical methods of those times, the presence of cobalt was under detection limit. During the last twenty years another seven samples containing cobalt were analyzed by the author. These bracelets and beads were excavated in Old Russian towns and cities such as Suzdal, Vladimir, Smolensk, Tver, Dmitrov. All artifacts occurred from the strata of pre-Mongolian time dated to XI-XIII centuries and manufactured of potash lead glass. In addition, one blue bracelet of this type comes from Bolgar, the city of medieval Volga Bulgaria, and was analyzed in Kazan University. These analyses together with the two earlier ones are present in the table. The earliest Russian glass article, coming from Kievan St. Sophia Cathedral founded in 1036, only with a high portion of doubt can be determined as a product of Russian workshop. As opposed to all other analyzed artifacts it is a piece of tessera from one of the earliest Russian temples, and demonstrates an extremely high content of cobalt side by side with enhanced content of sodium. This glass, as well as two other artifacts, contains antimony, probably used as opacifier. Another common feature of all samples with antimony is higher content of calcium. One artifact has an enhanced content of manganese, whereas in two others the blue coloration had not been achieved. In spite of some new information for cobalt usage these facts indicate that cobalt blue colorant was imported to Russia occasionally from different sources and tradition of its usage in the pre-Mongolian time had not been developed.