Deterioration of stained medieval glass by atmospheric attack Part 2. Secondary ion mass spectrometry analysis of the naturally weathered glass surfaces

Abstract

Secondary ion mass spectrometry was applied to analyze naturally weathered medieval glass exposed to the ambient atmosphere for more than 600 years. The analytical parameters for avoiding surface charging and mass interferences of 31P with 30Si1H are discussed and the applicability of relative sensitivity factors for the quantification of the raw profiles was examined. Two types of glasses were investigated: those with high amounts of silica, and dark green colored ones with much lower SiO2 contents but with lead and phosphorus oxides in addition to K2O, CaO, MgO and SiO2. Both types of glasses show an apparently unweathered surface. Nevertheless the secondary ion mass spectrometry profiles and nuclear reaction analysis measurements of the glass surfaces reveal a distinct depletion of the network modifiers and incorporation of hydrogen into the silicate structure within a region of 1 to 1.3 µm. The depth profiles show also an enrichment of some of the leached glass constituents (potassium, barium and lead) in the outermost surface domains (300 to 500 nm). The formation of such surface layers may be one of the reasons for the high weathering durability of the dark green colored medieval glasses despite their low silica content. However, it must be kept in mind that the values of the relative sensitivity factors were derived from the intact glass and applied also to describe the depth distributions of the glass constituents in the layers formed during the natural weathering process on the glass surface. Further investigations wih be carried out to increase the accuracy of that quantification procedure.