Chemical analyses of glass fragments found together in a single burial deposit showed that durability is enhanced by a sufficiently high silica content and an optimum total proportion of lime and magnesia. A simulated medieval glass of poor durability was prepared and subjected to various kinds of exposure. Atmospheres of various kinds are less destructive than burial in the ground. Contrary to the opinion of many, atmospheric sulfur dioxide is not an important cause of the deterioration of medieval stained glass windows.