The high degree of corrosion occurring in medieval glass painting requires preservation measures. Tests were made on simulated medieval glass, which was kept in water, for the suitability of the most varied coating materials, with wax mixtures combined with adhesive agents based on silane proving best suited. They are already being applied to window preservation in the Erfurt Cathedral to protect the dark opaque enamel. Two sections of the stock of medieval glass painting have been subjected to a total of 32 kinds of treatment and are to be checked annually.