Insights into the yellowing of drying oils using fluorescence spectroscopy

Abstract

Drying oils are commonly used in paintings and are prone to yellow discolouration from aging. Although there are numerous studies aimed at understanding their chemical composition, the yellowing process and its effects on the underlying fluorescence emission of the oils are not fully understood. In this study, four different commercially available oils, namely linseed, water-miscible linseed-, stand- and poppy seed oil, were analysed using UV–visible absorption and fluorescence spectroscopy. Both liquid and cured, solid film oils were analysed. Liquid oils show a structured absorption pattern, of which only two weakly absorbing peaks (λex 300 and 315 nm) result in fluorescence emission (λem 330 and 410 nm). The solid film lacks the structured pattern seen in the liquid oil’s absorption spectrum, showing instead a broad absorption peak. At an excitation wavelength (λex) of 365 nm the cured film normally fluoresces at λem 440 nm but is seen to shift to λem 550 nm as a result of yellowing. Artificial aging techniques, applied to the oils in order to cause a large degree of yellow discolouration, were seen to induce a large bathochromic shift in their fluorescence spectra. A correlation between the degree of discolouration and the shift in fluorescence is demonstrated, giving rise to a quantitative method that can be used to monitor the yellow discolouration. By non-destructively quantifying the degree of discolouration, colour-reconstruction of paintings could be performed to identify what the paintings looked like before degradation.