A Raman spectroscopic study of pollution-induced glass deterioration

White crystalline deposits were present on a large number of 19th and 20th century British glass artefacts in the National Museums of Scotland collections. Analysis of these deposits by ion chromatography showed that sodium and formate were the dominant ions. Raman spectroscopy identified sodium formate anhydrate phase II as the main phase, with sodium formate phase I', sodium sulfate decahydrate and possibly potassium formate present at minor levels. Minor levels of sodium, chloride and potassium were detected by scanning electron microscopy, suggesting the presence of sodium chloride and potassium chloride. Elemental analysis of the glass artefacts by x-ray fluorescence spectrometry and electron microprobe analysis revealed that most artefacts had high levels of sodium or (more rarely) potassium and low levels of calcium. Measurements and simulation experiments showed that the degradation was caused by the combination of high levels of formic acid, and possibly formaldehyde, generated inside the display cases and storage cupboards and high relative humidity. This study shows that the combination of micro-Raman spectroscopy, ion chromatography and scanning electron microscopy is ideal for the analysis of glass corrosion products induced by pollutant gases.