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 (λ ex 300 and 315 nm) result in fluorescence emission (λ em 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 (λ ex) of 365 nm the cured film normally fluoresces at λ em 440 nm but is seen to shift to λ em 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.

Evaluation of Fatty Acid Methyl Esters (FAME) as a Green Alternative to Common Solvents in Conservation Treatments

“…The untreated canvases revealed visible brush strokes. Under ultraviolet light, the oil-painted canvas exhibited a subtle fluorescence indicative of oil aging, as discussed by Cairns et al [34], while canvases painted with acrylic showed no fluorescence [35].…”

Advances in the characterisation and identification of mastic (Pistacia sp.) resin in archaeological samples by GC-QToF-MS

“…By contrast, KW 144 is dark orange in colour, suggesting a possible correlation between the light colour of the resin and its good preservation state, 3 although the presence of the oxidised oil cannot be ruled out as contributor of the darker colour of KW 144. 56 …”

“…By contrast, KW 144 is dark orange in colour, suggesting a possible correlation between the light colour of the resin and its good preservation state, 3 although the presence of the oxidised oil cannot be ruled out as contributor of the darker colour of KW 144. 56 The resin lumps in the Fitzwilliam Museum collection (F45A) provide an instructive comparator for this material. Although of uncertain context, their excavation origin at Abydos indicates a dry preservation environment in contrast to waterlogged condition of the Uluburun containers.…”

UV-Excited Fluorescence as a Basis for the In-Situ Identification of Natural Binders in Historical Painting: A Critical Study on Model Samples

“…At least the first aspect is undoubtedly evident in Figure 1a, which shows how the fluorescence intensity of linseed oil is particularly intense after only 2 years of aging in a pictorial layer containing lead white. In this regard, it should be considered that the mock-up samples analyzed have been mainly kept away from light, a condition that seems to favor the accumulation of the species responsible for the emission [14]. This effect is much less evident in the case of walnut oil, which also generally appears less fluorescent than linseed oil (Figure 1e).…”

“…The latter are mainly based on oxidation reactions, which lead, among other things, to the yellowing of the dried oil, with condensation of fatty acids oxidized at more than one position to give quinone-type structures, or possibly condensation with nitrogen-containing compounds, such as, for example, atmospheric ammonia or proteins belonging to the oil itself [13]. It has been shown that there is a correlation between yellowing and fluorescence of these binders [14][15][16]. Indeed, quinone-like structures can act as fluorophores with emission maxima between 380 and 520 nm [17], and fluorescence properties have been suggested for conjugated imines [11].…”