Deterioration of stained medieval glass by atmospheric attack Part 1. Scanning electron microscopic investigations of the weathering phenomena

Abstract

In studies on the deterioration of naturally weathered stained silicate glasses used in medieval glass paintings, Scanning Electron Microscopy (SEM) together with an energy-dispersive x-ray microanalyzer were used to obtain information about the morphological and chemical changes which have taken place during exposure to the ambient atmosphere for nearly 700 years. Generally, medieval stained glass shows a low chemical durability due both to its high content of modifier ions such as potassium calcium and magnesium as well as to a much lower silica content than is normal for common modern glass. Depending on the chemical composition and particularly on the silica content of the glass, three types were observed for the Austrian medieval glasses investigated: crusted, pitted and apparently unweathered glass. In addition to those glasses with a high silica content and K2O, CaO and MgO as the main components, the dark green window glasses, containing additionally lead and phosphorus oxides, also show a smooth and scarcely weathered exterior surface despite their low silica content. A weathering mechanism governed by an ion exchange process can be assumed from the analytical results. The alkali ions such as potassium as well as the alkaline earth ions (calcium, magnesium) and, in the case of the dark green stained glass, lead are displaced by hydrogen-bearing species. Gypsum and syngenite were identified by x-ray diffraction analysis as crystalline weathering products formed on the glass surface by reactions of the leached glass constituents with components of the ambient atmosphere. Typical for the deterioration of medieval glass is pitting corrosion, where numerous cracks are formed concentrically within a domain of several microns. It could be shown by the SEM investigations of the cross sections of the specimens that inhomogeneities just beneath the glass surface as well as an atmospheric attack at isolated points may be the main reason for that type of glass deterioration.