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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.

Megilp was made by combining a lead-treated drying oil with a mastic varnish and adding the product to paint on the palette, to make a thixotropic medium. Various forms of megilp were used by many artists from the mid-eighteenth to the nineteenth century, both for impasto and for glazing, but undesirable properties of darkening and cracking were soon reported. The present study began with a survey of the published recipes for megilp and the properties described, followed by their formulation in practice, with linseed oil prepared with lead acetate or lead oxide driers combined with mastic varnish in various proportions. These megilp formulations were then combined with modem commercial oil paints, painted out, and aged both naturally and artificially. Their surface morphology on drying and after aging was characterized by SEM-EDX, and thermoanalytical techniques, FTIR and DTMS were used to investigate and compare the different proportions used in various formulations, both for megilps alone and for megilps mixed with paint. It appears that a one-to-one or a resin-rich mixture reacts to form a new entity with properties distinct from the oil or resin alone, regardless of oil source or drier type. Oil-rich mixtures do not form 'true' megilps, and break down both in storage and in the paint film. [ABSTRACT FROM AUTHOR]

[Display omitted] • Identification of asphalt markers with pyrolysis mass spectrometry techniques. • Cooking with drying oil leads to the disappearance of asphalt markers. • Py-GCxGC/MS used for the first time on asphalt containing oil paints. Bitumen brown oil paint reconstructions based on 19th century production records from the British colourman Winsor & Newton were analysed using thermally assisted methylation with tetramethylammonium hydroxide (Py-TMAH-GC/MS), and pyrolysis comprehensive two-dimensional gas chromatography (Py-GCxGC/MS). Reconstructions were compared with the starting material, Trinidad Lake asphalt (TLA), to determine how detectable this asphalt is after heat processing in lead treated linseed oil. The use of brown paint containing asphalt/bitumen (the names are used interchangeably) was blamed for severe film-forming defects in 18th and 19th century oil paintings, yet this material has rarely been identified in earlier studies of historical paintings. This research offers a possible explanation for the paucity of evidence as it reveals that asphalt markers identified in the TLA disappear in the first stage of reconstructing bitumen brown oil paint. [ABSTRACT FROM AUTHOR]