Synergistic use of Py–THM–GCMS, DTMS, and ESI–MS for the characterization of the organic fraction of modern enamel paints

Introduction
In this study, mass spectrometric techniques (THM–Py–GCMS, DTMS and ESI–MS) have been used to characterize the organic fraction of early twentieth century oil-based enamel paints. Analysis has been carried out on dry and liquid samples from historical reference paints, and on color swatches from commercial paint brochures, with a special focus on Ripolin. This renowned French brand of enamel paints, manufactured for household and other uses since 1897, was reportedly used by many avant-garde artists. Confirming the presence of enamel paint such as Ripolin in early twentieth century artworks scientifically through binding medium analysis is challenging because, until the end of the Second World War, the most widely used house paints were oil-based and chemically similar to artists’ oil paints. In addition, artists often modified/reformulated materials including oils, driers, industrial and house paints to customize their handling and optical properties. The challenge of identifying oil-based enamel paints on the basis of chemical composition is illustrated by the analysis of samples from two paintings by Wassily Kandinsky and Pablo Picasso.

Results
Analysis demonstrated that the organic fraction of Ripolin paints is typically composed of heat-bodied linseed oil in mixture with variable amounts of diterpene (Pinaceae) resin, in accordance with the industrial technical literature of the time. Comparative analyses of Lefranc artists’ tube paints suggested a more variable composition with respect to the type(s) of oil and their pre-treatment, and showed the presence of Pinaceae resin at trace levels only in two cases. Beeswax was detected in one of the tube paints tested.

Conclusions
The results of this study indicate that complementary information can be obtained from the study of liquid paint from cans and dried paint-outs, while the media of painted brochures may differ from the original formulation as those were designed for expediency of drying and aesthetics. The results also elucidate how formulations and processing technology influence the physical and drying properties of oil-based enamel paints, in comparison with contemporary artists’ oil tube paints. Further research on a broader range of reference materials should help in the development of refined strategies for characterizing oil-based paints from the early twentieth century.