Origin of sulphated grey crusts on glass in polluted urban atmosphere: stained glass Windows of Tours Cathedral (France)

Abstract

The grey crusts covering some places of the weathered stained glass Windows of Tours Cathedral were studied by Analytical Scanning Electron Microscopy These crusts are constituted by a gypsum cement embedding many particles: microspherules (fly ash generated by combustion processes, rounded particles of leached glass, hypersiliceous spherules from tuffeau stone); angular fragments of leached glass; organic objects; siliceous and sulphated aggregates. The particles contained in the sulphated black crusts covering the stone, in the air and in the rain in Tours were studied simultaneously and compared with those of the stained glass Windows' grey crusts. In all cases, similar kinds of fly ash are present demonstrating the action of atmospheric microparticulate pollution both on the stained glass Windows and on the stone. Furthermore, the presence of hypersiliceous particles in crusts on glass and in the rain suggests transfers from stone to glass by rainwater run-off and possibly directly from the atmosphere. The presence of leached glass in the sulphated crusts on glass leads also to conclude on the modifying of the glass surface by the action of the rainwater run-off. Moreover, calcium and sulphur needed to form superficial gypsum crusts come both from the nearby calcareous stone, from the atmospheric gases and particles, and probably partially from calcium contained in unweathered glass.