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The characterization of paint in famous artworks is valuable for investigating the painting techniques used by artists, for conservation purposes and to help repair damaged paintings. By combining HPLC–ESI-QTOF-MS with traditional analytical techniques, researchers were able to clarify the composition of the oil components and hypothesize the botanical origin of the lipid materials used by Edvard Munch.
Researchers from the University of Pisa, Italy, have devised a new method to characterize complex oil mixtures in paintings using high performance liquid chromatography–electrospray ionization-quadrupole-time of flight mass spectrometry (HPLC-ESI-QTOF-MS).
The characterization of paint in famous artworks is valuable for investigating the painting techniques used by artists and for conservation purposes. Knowledge of the chemical composition of the paint is also useful to help repair damaged paintings. At the beginning of the 20th century a shift occurred in the chemical composition of oil paints. Painters of the 19th century symbolism movement and the early 20th century expressionism movement began to use different mixtures from those used in classical paintings. They blended traditional drying oils, such as linseed or walnut, with less expensive oils, such as castor or safflower. New classes of additives also emerged, including surfactants, metal soaps, and dispersing agents.
This shift in the composition of paint resulted in complex and difficult- to-characterize mixtures, representing a challenge for analytical chemists and researchers within conservation science. Lipid degradation complicated the situation further, especially in older works, and this exacerbated the challenges facing analytical chemists. Lipid characterization of modern paints was primarily performed using infrared spectrometry (FTIR), combined with analytical pyrolysis coupled with gas chromatography–mass spectrometry (Py-GC–MS), and GC–MS following a wet sample treatment. The latter technique was widely used to identify the botanical origin of the oils. However, there are inherent weaknesses in this approach, especially when differentiating oil mixtures because of a reliance on the palmitic to stearic acid ratio (P/S). In theory this ratio compares paint samples with reference samples, using a perceived resistance of saturated acyl chains to physical-chemical reactions during treatment and curing, compared to unsaturated ones. Unfortunately, this parameter is easily affected by environmental contamination as well as by the presence of other lipid sources, such as natural waxes, animal fat, or egg yolk. GC–MS has subsequently been regarded as an unreliable method to characterize oil mixtures.
To overcome the limitations associated with the P/S ratio researchers at the University of Pisa devised an approach combining HPLC–ESI-QTOF-MS with more traditional approaches. This novel technique determined triglyceride profiles in the lipid fractions of paint samples, and therefore the identification of oils used in the production of paint mixtures. To assess the validity of this novel and innovative analytical procedure nine samples from paint tubes, recovered from the atelier of Edvard Munch during his final working years in Ekley, were investigated. The same analytical approach was used to investigate a paint sample from Edvard Munch’s artwork, “New Rays”, part of Munch’s greatest decoration project, completed for the University of Oslo in 1916. By combining HPLC–ESI-QTOF-MS with traditional analytical techniques, researchers were able to clarify the composition of the oil components and hypothesize the botanical origin of the lipid materials used by Edvard Munch. Results obtained found the presence of both linseed and palm oil, and confirmed that paint supplies recovered from Munch’s atelier following his death were indeed used in the creation of the painting “New Rays”.
The use of HPLC–MS lipid profiling represents an important development in the field of cultural heritage and may yet prove to have wider implications. - L.B.
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