Study of the Early Stages of Mn Intrusion in Corroded Glass by Means of Combined SR FTIR/μXRF Imaging and XANES Spectroscopy

Historical glass, especially medieval glass, can undergo weathering under the influence of time and environmental conditions. The aim of this investigation was to better understand the processes involved in this natural degradation process by studying artificially altered glass samples prepared for the use of evaluation of conservation methods. Non-durable glass sensors produced by the Fraunhöfer Institute (type M1.0) were used as a starting material for artificial alteration. These were immersed in acidic (pH = 0, 2, 4) and neutral solutions (1 h - 8 h). In a second stage the glass samples were immersed in a 0.5 M MnCl2 solution (24 h, 48 h and 72 h), allowing intrusion of Mn from the solution into the gel layer. The samples were characterized at different stages with reflectance FTIR spectroscopy, μXRF mapping and μXANES. All measurements were carried out at ESRF, beamline ID21. Reflectance FTIR spectroscopy measurements were performed in the 800 4000 cm-1 range. Cluster analysis of the resulting maps evidenced the rapid growth of the gel layer in strong acidic conditions. The average spectra for each cluster feature show for the original glass a strong Si-O− stretching band between 900 and 1000 cm-1, whereas the gel layer could be identified by the increasing Si-O-Si bands around 1100 and 1250 cm-1. μXRF maps were recorded at different stages of the experiment at energies around the Mn-K edge (6.539 keV) and with a step size of 2 by 2 μ m. These confirm the leaching of K+ and Ca+2 from the glass and the intrusion of Mn from the solution. Mn was found throughout the entire gel layer, but with a concentration gradient peaking at the surface. XANES point measurements were recorded at various points where Mn was present. No spatial variation was found, but linear combination fitting of the spectra with various Mn reference compounds indicated that Mn2+Mn3+2 O4 is the main Mn compound in the gel layer, as was hypothesised by Watkinson et al. The standard corroded glass samples studied here can be used for the evaluation of conservation treatments in follow-up experiments.