Derivatisation Technique for Infrared Spectroscopy – Characterisation of Oxidative Ageing Products in Modern Oil Paint

Abstract

In conservation science the Fourier transformed based infrared spectroscopy (FTIR) is a standard technique to the study of historic paint layers. There are, however, clear limitations when analysing modern oil paint. The spectral information of aged oils is affected from overlapping signals of the binder, the oxidative ageing products, soaps formed during ageing, as well as several pigments and fillers. The distinction of the ageing products such as ketone and carboxylic acid functional groups pose the next problem, as these interfere with the triglyceride esters. Gaseous sulfur tetrafluoride SF4 was used to discriminate overlapping signals in aged oil paint. This derivatisation technique makes it possible to convert carboxylic acids and carboxylate salts into acyl fluoride, while ketones and ester groups remain intact. IR spectra after SF4 treatment show a characteristic splitting of the carbonyl band, allowing the characterisation of the different functional groups formed during oxidative ageing. Furthermore, it is possible to increase the spectral selectivity thanks to the successful elimination of spectral interferences caused by organometallic and inorganic compounds. This sample pre-treatment can be applied to both micro-samples and polished cross-sections, enabling 2D localisation of these compounds in aged oil paint samples using infrared imaging FTIR-FPA (focal plane array). Both artificially aged and real paint samples of twentieth century paintings were investigated to characterise degradation processes in oil paint, including the dripping phenomenon of modern oil paints.