Impact of biogenic exudates on the dissolution and browning of stained glass windows

Abstract

The browning phenomenon is a pathology affecting Mn-bearing medieval stained-glass-windows in potash-lime-silicate glass system. In order to unravel the potential implication of microorganisms in the appearance of this pathology, three model glasses respectively containing no MnO, 1 wt% and 2 wt% MnO were altered at circumneutral pH, with and without organic exudates (oxalic acid (OA) 1000 μM and siderophore desferrioxamine B (DFOB) 50–1000 μM) likely to be produced by bacteria and fungi. In the absence of exudates, the dissolution rates are inversely dependent on the Mn content of the glasses (0.8, 0.5 and 0.4 g m−2d−1 respectively for no MnO, 1 wt% and 2 wt% MnO glasses). In contact with exudates, an opposite trend is observed. The prevalent mechanisms are interpreted as a strong ligand-promoted dissolution for DFOB (dissolution rate increase up to 270%) and a dominant proton-promoted dissolution for OA (dissolution rate increase up to 60%). When DFOB and OA are added together, the effect of DFOB on the dissolution rate is prevailing, while OA effect is tangible on the stoichiometry of the dissolution of the alteration. These results suggest that an indirect biological activity could be involved in the mobilization of Mn from a Mn-bearing glass, thus playing a role in the appearance of the browning phenomenon.