The introduction of the tin opacified glaze technique in Switzerland at the end of the 15th century CE – A SEM study of stove tiles from the canton Bern

Abstract

Late 15th century tin glazed stove tile fragments from Bern, Fraubrunnen and Nidau are among the oldest witnesses of this advanced glaze technique in Switzerland and coexist with traditional lead glazed tiles. On a single tile, both glazes can also occur (two-glaze technique). In this SEM-BSE and -EDS study, bodies, slips and glazes of 14 tin and 2 lead glazed tiles were examined in order to constrain the raw material sources, the recipes, the application technique and the firing parameters. Mostly all ceramic bodies are of the traditional low Ca type. With the exception of two samples, they all have a slip that either belongs to the traditional quartz-rich, CaO-poor and SiO2-rich type I or a new CaO-rich type II. The latter is the earliest known use (so far) in tin-glazed stoves from Switzerland. Slip I can be found under both glazes, slip II only under tin glazes. Uncovered or slip covered bodies were biscuit fired before glazing. Firing temperatures were mostly in the range 950–1050 °C. Important chemical exchanges between the liquid glazes took place during firing. The lead glazes digested minerals of the latter, while lead infiltrated the porous ceramic substrate. The transparent monochromatic lead glazes are green with Cu, or yellow and brown, depending on the Fe content and the presence or absence of a slip. The opaque tin glazes have SnO2 concentrations of 6–17 mass%. Objects with two-glaze technique show 4 of the 6 possible combinations of slip and glaze types. This indicates that the potters did not follow a unique (new) recipe, but tried many mix of old and new techniques.