Effect of organic acid vapors on the alteration of soda silicate glass

Organic acids were previously shown to be involved in the alteration of historic soda silicate glasses in humid atmospheres under museum storage conditions. The present study investigates the role of these pollutants on the visual, compositional and structural modification of soda silicate glasses. Replica glasses aged in humid or humid/acidic atmospheres under accelerated conditions were examined and compared using light microscopy, electron microprobe and Raman spectroscopy. The characteristic modifications induced by each atmosphere are described. In the acid polluted atmosphere the leaching process created a layer that retained the transparency of the glass but with a chemical structure hydrated and more polymerized following the loss of alkali and the associated non-bridging oxygens, and the formation of new bridging bonds. In an unpolluted humid atmosphere, the dissolution process caused disruption of the silicate network at the glass surface and formation of an opaque gel layer. The hydrated silicate species and the cations in this gel layer subsequently polymerized to form a new amorphous material, hydrated and more depolymerized than the original glass. This investigation confirms that organic acid pollutants are responsible for the modifications observed on altered historic soda silicate glasses in the collections of the National Museums of Scotland.