Weathering phenomena on naturally weathered potash-lime-silica-glass with medieval composition studied by secondary electron microscopy and energy dispersive microanalysis

Samples of two model glasses with chemical compositions similar to medieval stained glass were exposed to the natural environment at 23 test sites for a period of 6 months, I and 2 years within an exposure programme of the "W orking Group of Effects on Materials, Including Historic and Cultural Monum ents" of the Economic Commission fur Europe of the United Nations. During t he exposure the environmental data were measured and collected at each test site in order to enable a correlation between the pollutant load in the am bient atmosphere and the weathering phenomena of the glass samples. After t he exposure, which was performed in a sheltered as well as in an unsheltere d mode for gaining information about the influence of dry and wet depositio n of ail pollutants, the glass samples were analysed in the scanning electr on microscope with energy dispersive microanalysis (SEM/EDX). Model glass M 1 - a potassium rich glass - was covered to a high amount with crystalline weathering products (above all syngenite), whereas glass M3 - a glass with a higher Ca and Si content - turned out to be more stable against weatherin g This glass was covered with gypsum and arcanite crystals but in total to a less extent than glass M1.