17th century blue enamel on window glass from the cathedral of Christ Church, Oxford: Investigating its deterioration mechanism

A chance discovery beneath Christ Church Cathedral, Oxford has brought to light some very well-preserved 17th century van Linge enamel-painted glass fragments. Deterioration was in most cases minimal and largely limited itself to the blue areas of the fragments. Here, cracking in both the enamel and the glass surface beneath it was observed, which resulted in detachment of the painted enamel and of some of its underlying glass in the more severely affected areas. This study investigates the deterioration mechanisms involved and demonstrates that cracking was probably brought about by different levels of expansion and contraction experienced by the paint layer constituents in response to fluctuations in temperature. Differences between the thermal expansion coefficients in the enamel paint and glass are thought to induce the formation of microcracks within the enamel. Certain physical properties of the blue enamel were expected to increase these thermal expansion coefficient differences, making this colour more susceptible to deterioration, resulting in the selective detachment observed in these painted fragments.