Raman study of model glass with medieval compositions: artificial weathering and comparison with ancient samples

Potassium-rich ash-based glass is typical of medieval glass windows used in the Central Europe since 1000 A.D. Glasses with medieval-like composition were prepared with different amounts of K2O (15-20-25 K2O weight %) using recipes deduced from archaeometric literature. Raman data on these samples were interpreted on the basis of the Qn units model and of the polymerization index Ip and confirm a close dependence of glass de-polymerization on potassium content. Generally, K-based glasses have high alteration sensitivity to pollutants conveyed by liquid or vapor water (rain, humidity, fog). In this work the glass alteration processes were studied in terms of structural modifications related to the glass-K content. Leaching and network dissolution were investigated by attack with boiling concentrated sulphuric acid, and by exposure to high temperature and high pressure bi-distilled water, respectively. The weight loss and alteration layer thickness were measured. The structural changes in the glass network and the presence of neo-formation crystallized products were determined through linear Raman maps on altered glass cross sections. The more the glass network structure is de-polymerized, the more the surface chemical attack is facilitated. The results obtained for the model glasses are compared with those for a set of ancient K-rich glass fragments.