Statistical evaluation of potash-lime-silica glass weathering

Two potash-lime-silica model glasses with compositions similar to those of medieval stained glass were exposed at 26 test sites all over Europe and in North America for 3–6 years. The objectives of this large-scale field exposure programme were: (a) a qualitative analysis of the weathering products formed on the surface of the samples in the (environmental) scanning electron microscope in combination with energy dispersive X-ray microanalysis (ESEM/EDX or SEM/EDX, respectively); (b) a statistical evaluation of the weathering phenomena by applying a multiple linear regression (MLR) analysis to find correlations between the degree of weathering, which was measured in terms of surface coverage with reaction products, and environmental parameters such as the concentration (c) of the acidifying gases SO₂, NO₂ and O₃, the temperature (T) and the relative humidity (RH); and (c) the calculation of a time-dependence function of the weathering process of these glasses. Mainly sulfates of calcium and potassium such as gypsum (CaSO₄·2 H₂O), arcanite (K₂SO₄) and syngenite (CaSO₄·K₂SO₄·H₂O) could be identified in the SEM and ESEM. Carbonates, nitrates and many particles deposited on the glass surface were found as well. MLR calculations exhibit significant dependencies of the degree of weathering on T, RH, c(NO_x), c(SO₂) and c(O₃). Applying a time-dependence function of the general form y=a·t^b (t=time) results in a value of approximately 0.42 for the exponent b, which comes close to values expected from various studies in the literature.