Because of its low silica content, medieval stained glass hydrolyzes easily. Excess moisture results in a leaching of potassium from the glass, higher pH, a breakdown of the silica network, and the formation of decomposition products on the surface. Two methods of preventing deterioration are proposed: 1) coating with a transparent resin impervious to water and 2) rehanging behind modern glass to allow air circulation. The drawbacks of both solutions are mentioned. -- AATA