Identification of an original non-terpenoid varnish from the early 20th century oil painting "The White Horse" (1929), by H. Menzel

Different mass spectrometric approaches were used to identify an original non-terpenoid varnish. Direct temperature-resolved MS and pyrolysis-GC/MS mainly showed the phenolic components, whereas thermally assisted (trans)methylation with tetramethylammonium hydroxide (TMAH) strongly enhanced the evidence for tung oil as part of the varnish. Transethylation studies of the fatty acids confirmed the TMAH data. The degree of hydrolysis of the oil network was found to be low. No evidence was found for a direct link between the drying oil and phenolic resin. On the basis of the MS information, the aged varnish is identified as an open-structure tert-butylphenol-formaldehyde resin which entangles the tung oil polymer.