Acid rain occurs when emissions from coal burning factories and power plants are transformed into sulphuric and nitric acids and fall to earth. In Europe, more than 100,000 stained glass objects, some of them more than 1,000 years old, are threatened by acid rain. Documentary records show that the stained glass in Europe was in a relatively good condition up to the turn of the century and even the Second World War caused practically no damage since all European stained glass was sheltered during that time, but in the last 30 years the deterioration process has accelerated to the extent that a total loss is expected within a few decades. Glass dating from the 8th to the 17th centuries is particularly endangered. Sulphuric acid has an etching effect on the surface and the resulting salts form a chalky crust that accelerate the decomposition process, allowing the paint to peel off. The glass substance finally splits and disintegrates into minute particles. Sulphur compounds also seriously crack old organically treated leather and paper produced after 1750.