Fluorescence studies of binders used in paintings : ageing and pigment effects

Abstract

A large range of binding materials are used in paintings, some of which are characterised by their fluorescence (e.g. drying oils) and others in which the fluorescence is poorly understood (e.g. synthetic binders). This study aims to identify the fluorescence of binding media (proteinaceous, polysaccharide, synthetic binders and drying oils), as well as determine any changes in fluorescence which are resultant of ageing and pigment effects. Drying oils were identified as having the highest fluorescence intensity of all binders used in this study, and the fluorescence was found to bathochromically shift as the dried film aged (after 2 years of natural ageing). To fully understand the changes in fluorescence, four different commercially available oils (namely linseed, water-miscible linseed-, stand- and poppyseed oil) were analysed using UV-visible absorption and fluorescence spectroscopy. Both liquid and cured, solid film oils were analysed. Liquid oils showed a structured absorption pattern, of which only two weakly absorbing peaks (λex 300 and 315 nm) resulted in fluorescence emission (λem 330 and 410 nm). The solid film lacked 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 was seen to shift to λem 550 nm due to yellowing. Artificial ageing techniques (exposure to UV light, elevated temperatures, and ammonia vapour) were applied to the drying oils to induce the bathochromic fluorescence shifts. As the oils aged a yellow to brown discolouration developed. A correlation between the degree of discolouration and the shift in fluorescence was identified. As the oils change colour so does the fluorescence and the fluorescence shifts from blue (λem 440 nm) to green (λem 550 nm) and sometimes even yellow. In addition to the artificial ageing of drying oils, five different pigments were mixed individually with various binders. The five pigments used in this study were: lamp black, Indian yellow, oriental blue, phthalo blue and alizarin crimson. Characterisation and fluorescence studies of each of these pigments were done, and it was found that alizarin crimson (AC) has a characteristic green fluorescence (λem 550 nm) in solutions that facilitate hydrogen bonding. The pigment-binder mixtures were then analysed using fluorescence spectroscopy to determine whether the pigments caused changes in fluorescence of the binder it was mixed with. It was found that the pigments caused a slight hypsochromic shift in egg-based binding media. However, most binders were unaffected by the non-fluorescent pigments. Interestingly, the drying oils created a newly fluorescent compound with AC, in which three peaks were identified in the fluorescence spectra: the drying oil (λem 450 nm), the green fluorescence of AC (λem 550 nm) and a new orange fluorescent peak (λem 630 nm). The results of this study indicate that fluorescence spectroscopy is a non-invasive technique which can be used in the analysis of paintings. Fluorescence spectroscopy can be used to determine the class of binding medium used, as well as the yellow discolouration of drying oils. Non-fluorescent pigments do not affect the fluorescence of binding media, and thus confirms the validity of fluorescence spectroscopy as an analytical tool in conservation studios.