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.
