Decay of medieval stained glass at York, Canterbury and Carlisle Part 2. Relationship between the composition of the glass, its durability and the weathering products

Abstract

The chemical compositions of specimens of weathered medieval stained glass from York Minster and Canterbury and Carlisle Cathedrals (discussed in part 1 of this paper) have been expressed in terms of three reduced variables, "SiO2", "R2O" and "RO", which represent the glass network formers, and the monovalent and divalent network modifiers, respectively. It is shown that the durability of the glass - assessed on the basis of its physical condition - and the identity of the weathering products present on the outer surface of specimens, may be broadly related to the composition of the glass. The types of weathered surfaces which occur on specimens of decayed stained glass in England, and the compounds present in the weathering crusts, are compared with those previously reported for glass from churches on the Continent of Europe — principally in France and Germany — and are found to be closely related. Hydrated silica, however, has been more frequently recorded as a component of the crusts on ancient glass in England. Any influence of climate on the way in which glass weathers could not be detected, nor could the effect of the orientation of the window.