Evaluation procedure for leaching studies on naturally weathered potash-lime-silica glasses with medieval composition by scanning electron microscopy

In order to study the weathering mechanism and quantify the influence of the various air pollutants on the degradation process, potash-lime-silica (PLS) glasses with a chemical composition similar to medieval stained glass were exposed at more than 20 test sites in Europe and North America to the local environmental conditions within the International Co-operative Programme on Effects on Materials Including Historic and Cultural Monuments of the United Nations Economic Commission for Europe (ICP-Materials, UN/ECE). After exposure periods of up to 6 years the specimens were investigated in the scanning electron microscope in combination with energy dispersive X-ray microanalysis (SEM/EDX) in order to characterize the surface layer (leached layer) of the weathered samples. Cross-sections of the samples were prepared for depth profile analyses using linescan measurements in order to calculate the leaching depths of the network modifier ions. The typical diffusion profiles expected for these elements were observed. The evaluation is based on a numerical integration of the background corrected linescan data. This method proved to be suitable for the high amount of samples (more than 130) and yields acceptable errors of less than 10 rel.%. The first results show leaching depths for K+ and Ca2+ between 0.42 and 1.87 μm (average 1.21 μm for K and 1.11 μm for Ca) for exposure times of up to six years. So both network modifier ions present in the glass were leached out, with K+ showing slightly, but significantly higher leaching depths than the bivalent cation Ca2+. As no continuous increase of the leaching depths with time could be observed, network dissolution must have also occurred during the weathering process. Comparing these results to former leaching studies, the rate of leaching on glasses of this composition seems to decrease significantly after about three or four years of exposure.