Lost colour on corroded stained glass can be recovered with the normarski differential interference contrast technique and specular reflection. Microsandblasting can be used for the removal of the weathering crust on clear glass from the inside, however the coloured parts should be protected with rubber latex. The author suggests that sulphates in the weathering crust of mediaeval glass are due to the presence of natural sulphur gases in the atmophere. Case studies from English churches. -- ICCROM