The deterioration of glass on exposure to atmosphere (weathering) varies with environmental parameters, time of exposure as well as with glass chemical composition. The analysis of 13-14th century glasses of stained windows in France and Germany identified 4 potash-lime-silica glass compositions. To investigate their weather weathering mechanisms and the impact of the environment on the conservation, glasses reproducing the original compositions were melted in the laboratory and their surfaces exposed in field for up to 12 months in different positions near the corresponding original stained glass windows. Furthermore, in order to ascertain the contribution of the glass composition to the weathering process, a laboratory test was developed to compare the durability of the four glasses.
