The combination of an unstable glass composition, fluctuating humidity and a high concentration of organic pollutants is responsible for the widespread alteration of part of the glass collections of the National Museums of Scotland (NMS). The alteration has resulted in the formation of crystalline corrosion at the surface and strong modification of the chemical structure of the glass. The chemical structure before and after alteration of two soda‐silicate glasses from the NMS collection was examined by Raman spectroscopy assisted by electron microprobe. Decomposition of the Raman spectra offered an insight into the modified glass structure and the mechanisms of the alteration. The acidic pollutants, acetic and formic acids, were identified as the main cause of alteration, as they provide a source of H+ ions that enhanced the ion‐exchange reaction. The ion‐exchange reaction in the soda glasses causes leaching of sodium ions out of the structure and formation of silanols (Si‐OH), but leaves the stabiliser ions such as calcium and lead ions undisturbed. The ion exchange is followed by a polymerisation reaction of the silanols inducing the formation of new SiOSi bonds including four‐fold silica D1 rings and the release of molecular water into the structure. The polymerisation reaction is likely to be responsible for the cracking and flaking of the surface through the tensile stresses generated in the glass structure. The alteration process, and in particular the polymerisation reaction, implies that the structural modification of the glass is irreversible. Copyright © 2006 John Wiley & Sons, Ltd.
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