New Insights into the Ageing of Linseed Oil Paint Binder: A Qualitative and Quantitative Analytical Study

Abstract

This paper presents an analytical investigation of paint reconstructions prepared with linseed oil that have undergone typical 19th century treatments in preparation for painting. The oil was mechanically extracted from the same seed lot, which was then processed by various methods: water washing, heat treatments, and the addition of driers, with and without heat. A modern process lead white (Dutch source, Schoonhoven) and a commercially available vine black were used as pigments. The reconstructions were prepared in 1999, and naturally aged from then onwards. We compared thermogravimetric analysis (TG), which yields macromolecular information, with gas chromatography-mass spectrometry (GC-MS) and direct exposure mass spectrometry (DEMS), which both provide molecular information. The study enabled us to quantitatively demonstrate, for the first time, that the parameters used to identify drying oils are deeply influenced by the history of the paint. In particular, here we show that the ratio between the relative amounts of palmitic and stearic acid (P/S), which is used as an index for differentiating between drying oils, is extremely dependent on the pigments present and the age of the paint. Moreover the study revealed that neither the P/S parameter nor the ratios between the relative amounts of the various dicarboxylic acids (azelaic over suberic and azelaic over sebacic) can be used to trace the sorts of pre-treatment undergone by the oil investigated in this study. The final results represent an important milestone for the scientific community working in the field, highlighting that further research is still necessary to solve the identification of drying oils in works of art.

Characterisation of preparation layers in nine Danish Golden Age canvas paintings by SEM–EDX, FTIR and GC–MS

“…The absence of egg lipids cannot be verified, as egg might be present in relatively minor amounts and consequently not modifying the fatty acid profile of the drying oil. Moreover, it has been demonstrated that fatty acids escape a paint film in time, and considering that palmitic acid evaporates four times quicker than stearic acid [22], the P/S value is expected to significantly decrease over time [23]. The presence of lead white in the preparation layer should make the P/S parameters somewhat more reliable, as lead carboxylates are not volatile.…”

Vibrational Spectroscopies and Chemometry for Nondestructive Identification and Differentiation of Painting Binders

“…As regards drying oils, they should be treated as "fresh" binding media because the drying process has not already been completed (according to previous studies, an oil paint can be regarded as aged only one year after its preparation [17,19]). …”

Transylvanian glass icons: A GC/MS study on the binding media

“…However, samples XI-2 (Nicula), XI-3, XIV-8, XIV-3, XIV-4 (Olt County) and X-1 (Brasov area) show a higher A/P and D parameters than the reference ones for linseed oil. A pre-treatment of the oil or the influence of the pigment has been suggested as responsible for these changes in the profile [12,14]; • samples (XIII-3, XVII-1 and IV-3) showing a lipid profile (A/P < 0.1) ascribable to the presence of egg, already identified from the aminoacidic fraction (see above).…”