Analytical Strategies for Characterizing Organic Paint Media Using Gas Chromatography/Mass Spectrometry

Abstract

Throughout history, artists have experimented with a variety of organic-based natural materials, using them as paint binders, varnishes, and ingredients for mordants in gildings. Artists often use many layers of paint to produce particular effects. How we see a painting is thus the final result of how this complex, highly heterogeneous, multimaterial, and multilayered structure interacts with light. The chemical characterization of the organic substances in paint materials is of great importance for artwork conservation because the organic components of the paint layers are particularly subject to degradation. In addition, understanding the organic content and makeup of paint materials allows us to differentiate between the painting techniques that have been used over history. Applying gas chromatography/mass spectrometry (GC/MS) analysis to microsamples of paint layers is widely recognized as the best approach for identifying organic materials, such as proteins, drying oils, waxes, terpenic resins, and polysaccharide gums. The method provides essential information for reconstructing artistic techniques, assessing the best conditions for long-term preservation, and planning restoration. In this Account, we summarize the more common approaches adopted in the study of the organic components of paint materials. Our progress in developing GC/MS analytical procedures in the field of cultural heritage is presented, focusing on problems that arise from (i) the presence of mixtures of many chemically complex and degraded materials, (ii) the interference of inorganic species, (iii) the small size of the samples, and (iv) the risk of contamination. We outline some critical aspects of the analytical strategy, such as the need to optimize specific wet-chemical sample pretreatments in order to separate the various components, hydrolyze macromolecular analytes, clean-up inorganic ions, and derivatize polar molecules for subsequent GC/MS separation and identification. We also discuss how to interpret the chromatographic data so as to be able to identify the materials. This identification is based on the presence of specific biomarkers (chemotaxonomy), on the evaluation of the overall chromatographic profile, or on the quantitative analysis of significant compounds. GC/MS-based analytical procedures have for 20 years provided important contributions to conservation science, but challenges and opportunities still coexist in the field of organic-based paint materials. We give selected examples and provide case studies showing how a better understanding of the chemical composition of organic paint materials and of their degradation pathways contribute to a better knowledge our cultural heritage, and to its preservation for future generations.

A multidisciplinary approach to the investigation of “La Caverna dell’Antimateria” (1958–1959) by Pinot Gallizio

“…The results can be seen in Table 1, and the chromatogram of sample L12 is shown in Figure 3. The GC/MS analysis shows the presence of all the main fatty acids characteristic of an aged oil, such as suberic, azelaic and sebacic acids, and also common dicarboxylic acids deriving from the drying process of a siccative oil [7]. -the pyrolitic profiles of samples A2 and A19 (Figure 4) showed only the presence of the characteristic molecular markers of a PVAc resin and large amounts of various plasticisers, characteristic additives used in the production of synthetic polymers; no markers of any oil were identified.…”

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

“…It was not possible to determine if yolk or white had been used. The amino acid profiles of egg yolk and egg white proteins are in fact very similar [20], and the P/S ratio could not be used to determine the source of the oil or to verify the presence of the egg lipids. Walnut oil would in fact give rise to similar P/S values as a those of a mixture of egg lipids and linseed oil.…”

“…Pb (lead), Ca (calcium), Al (aluminium), Si (silicon), Fe (iron), Na (sodium), Mg (magnesium), K (potassium), S (sulphur), and Ba (barium) To determine the source of the saccharide materials, the data were compared to a database of saccharide profiles of reference materials [18], and when polysaccharide gums were hypothesised, they were identified using a decisional scheme that was reported in the literature [19]. The interpretation of the chromatograms relative to the lipid-resinous fractions was based on the recognition of molecular markers for resins and waxes and quantitative evaluation of mono and dicarboxylic acids for lipids [20].…”

“…The meaning of the reported parameters are discussed in detail in the literature [20] No. SMK accession no.…”

The chemical-physical knowledge before the restoration: the case of “The Plague in Lucca”, a masterpiece of Lorenzo Viani (1882–1936)

“…Together with the thin layer of painting, Viani was making a point in using a very simple palette, dominated by dark colours: "I must keep in mind to use only few colours with special consideration for ivory black and earth tones such as red, yellow and green, in order to obtain austere and realistic tonalities. Hues such as baby blue, blue, emerald green, orange and all bright colours in general can be deceiving as they evoke sensuality, while black, being a sober hue is undoubtedly an artist's privileged matter, a powerful and fundamental means of expression" [24].…”