The causes and prevention of deterioration of medieval painted window glass are described. The main cause of decay is the leaching of potassium ions out of the silicate lattice of the glass by moisture present in the environment. Moisture can be prevented from reaching the glass either by coating the cleaned glass with an impermeable resin by lamination, or by isothermal glazing, where the ancient glass is re-hung 60-80 mm inside modern glass. Another major cause of deterioration considered is natural and artificial sulfur dioxide present in the atmosphere, which produces opaque white crusts consisting of gypsum and/or syngenite on the surface of the glass. Lost painted designs may be revealed by photographing the glass using specularly reflected light, or polarized light at the Brewster angle of incidence (56%). An accelerated aging test used by the British Glass Industry Research Association is described; this uses a conven tional Soxhlet apparatus operating with water at 81°C.