By Gottfried Frenzel
Scientific American, Supplement: Science and the Arts (1995)
Introduction: Light has long served religion as a symbol. It has signified creation (“Let there be light” was the first command of the Creator) as well as salvation (John the Evangelist saw the Heavenly Jerusalem illuminated as if made “of jasper” and its walls ” like clear glass”) The earthly reflections of such visions, achieved throughout the Middle Ages by means of light, were the period’ s most brilliant works of art: the stained glass windows of Romanesque and Gothic chapels, churches, minsters and cathedrals. For almost a millennium, in the case of the earliest stained-glass windows, the glass escaped major damage. Even the catastrophe of World War II inflicted harm that was within bearable limits. In fact, stained glass all over Europe was removed to safety. Today, however, its total destruction is threatened, not by war but by air pollution. If stained glass windows are kept in their present state of preservation, their total ruin cno be predicted within our generation.
A few examples will illustrate the threat. The stained-glass windows of Cologne Cathedral, in the immediate vicinity of the city’ s main railway station, have been unusually vulnerable. They were endangered by exterior weathering and air pollution as early as the mid 19th century. Seen from outside the building, the windows now look like sheets of chalky plaster. Continuous etching by air pollutants has corroded the exterior surface of the glass, reducing its thickness year by year and giving the decomposed surface a so-called weathering crust. The process of destruction starts anew as each rain washes the crust away. Meanwhile the colored glass itself breaks into tiny particles. The particles fall out of each panel: thus the window disintegrates.