Insights into the yellowing of drying oils using fluorescence spectroscopy

L K Cairns,P B C Forbes

doi:10.1186/s40494-020-00403-1

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.

Highlights

Drying oils have been used in paintings as a binding medium since the 15th century [1,2,3]
This study aimed to identify changes in yellowing, during the curing of drying oils, through the use of highsensitivity fluorescence spectroscopy
Poppy seed oil (PO) has three additional moderately absorbing peaks, at 258, 269 and 279 nm, of which the first peak is characteristic of poppy seed oil (PO) and does not appear in either linseed oil (LO) or water-miscible linseed oil (WLO)

Summary

Introduction

Drying oils have been used in paintings as a binding medium since the 15th century [1,2,3]. Linseed-, poppy seed-, as well as walnut- or safflower-oil, which are commonly used in paintings, contain high concentrations of the unsaturated fatty acids, linolenic and linoleic acid (Table 1) [9]. Drying oils are clear or faintly coloured when applied to paintings, but gradually develop a strong yellow discoloration over time. This discoloration is an unavoidable property of oil paint [2]. Together these two compounds are thought to make up the characteristic intense yellow colour [29]

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Conclusion