New data on the process leading to the formation of corrosive products have been obtained through electron microprobe observations of glass showing only slight traces of weathering. The problem of cleaning ancient stained glass was studied using glass from Strasbourg, which was subjected to various processes designed to remove corrosive products. The final surface condition of the glass, which had originally been covered by a film of gypsum and syngenite, was observed using a scanning electron microscope. It was possible to compare the effectiveness of the various methods judged in terms of the quality of the cleaning. Synthetic glass with compositions comparable to those found in old glass have been developed with a view to identifying and isolating all the factors involved in the corrosion of ancient stained glass. See also related entry in this section, under Collongues, R. -- AATA