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.
Highlights
Traditional linseed oil based paint consists of a heterogeneous mixture of organic and inorganic compounds, frequently arranged in a complex multi-layered structure
MS and thermogravimetric analysis (TG) of paint layers prepared with the same oil and processed according to various pre-treatments, using two different pigments: lead white, and vine black
A comparison of the data obtained from the paint layers prepared using the differently pre-treated oils, and the different pigments enabled us to draw several important conclusions:
Summary
Traditional linseed oil based paint consists of a heterogeneous mixture of organic and inorganic compounds, frequently arranged in a complex multi-layered structure. Further chemical changes occur: hydrolysis of the ester bonds, formation of new oxygen containing functional groups, oxidative cleavage of the fatty acid hydrocarbon chains, and metal-ion coordination of the fatty acid groups of the cross-linked material and non cross-linked fractions [1,5,6]. Several phenomena can take place during drying and ageing as a result of the interaction of the binder with the pigments. The nature of the pigment present appears to have a crucial role in the chemical composition of an aged oil paint [12], and is often responsible for some well known degradation phenomena [13,14,15,16,17,18,19,20]. For example it is reported in the literature that double bonds isomerise when the oil is thermally treated [25,26], and, as a consequence, the relative amounts of suberic and sebacic acids increase with respect to azelaic acid [6,23,24,27]
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