The stability of the Ravenscroft's glass. Influence of the composition and the environment

George Ravenscroft designed the lead silicate glasses in 17th-century; however, the first pieces produced showed degradation pathologies after few years. This study reproduces one of the glasses produced by George Ravenscroft and analyses its chemical stability in comparison to two other glasses based on the original composition but with the addition of 5 mol % of PbO and CaO, respectively. The three glasses were exposed to an environment of high humidity and different concentrations of formic acid (0, 100 and 300 ppm), which is the main contaminant in display cases and wooden cabinets. The hygroscopic capacity of the glass was analysed by gravimetry, infrared spectroscopy and µ-Raman spectroscopy. The results show that the acidic environment promotes the ion exchange reaction, accelerating the leaching of alkaline ions, which has a great influence on the durability of the glass. The glasses with a higher content of stabilizers presented a greater stability, especially the glass with calcium.