Ratio of CaO/K20 > 2 as evidence of a special Rhenish type of medieval stained glass

Abstract

The chemical compositions of more than 300 medieval stained glass samples of different local origin have been published so far. Regarding their characteristic components they can be classified into five types. About 80 % of all investigated samples proved to be potash-lime-silica glasses with roughly equal concentration of CaO and K₂O (type 1). But a small group of little more than 20 samples gave a ratio of CaO/K₂0 > 2 (type 2), all but three of which had been taken from two churches of the Rhine region (St. Catherine Church Oppenheim and Cologne Cathedral). Hence, it seems likely that a corresponding recipe was used there, which was nearly unknown elsewhere in Europe. Investigation of further objects of this region might be of interest for the history of technology.