Leaching studies on naturally weathered potash-lime–silica glasses

Within the international, EU-supported research project MULTI-ASSESS more than 150 glass samples with a chemical composition similar to medieval stained glasses were exposed in six European cities (Athens, Krakow, London, Prague, Rome and Riga) to the local environmental and climatic conditions. After exposure periods of 6 or 12 months the samples were investigated in the scanning electron microscope with energy dispersive X-ray microanalysis (SEM/EDX). Surface analyses indicate that the main weathering products were sulphates such as syngenite (CaSO4 · K2SO4 · H2O) and gypsum (CaSO4 · 2H2O). Chlorides, organic compounds and/or carbonates or nitrates of the elements K, Ca and Na were predominantly detected on the 6-months samples, whereas Si-containing compounds appeared particularly on the 12-months samples. Linescan measurements performed on the cross-sectioned glass samples allowed for the determination of leaching depths of the network modifier ions according to an evaluation procedure recently presented. The average leaching depths after 6 months of exposure were d(K) = 0.88 ± 0.43 μm, d(Ca) = 0.62 ± 0.37 μm, d(Na) = 0.34 ± 0.32 μm and d(Mg) = 0.16 ± 0.18 μm. After 12 months an average increase of the leaching depths between 38% (K) and 63% (Mg) is observed. No leaching could be determined for the network former elements Al and P. A clear influence of the environmental conditions at the atmospheric test sites on the degree of weathering was observed. In a first approximation, the leaching depths exhibit a linear dependence on a general pollution factor comprising the concentrations of the acidifying gases SO2, NO2 and O3.