Soap – The Unexpected Culprit of Oil Painting Degradation

Soap is a ubiquitous commodity that has been in the hands of humanity for thousands of years. In fact, the oldest known soap is believed to date as far back as 2800 BCE, originating in Ancient Babylonia, where it was used to clean materials like wool (Butler, 2000). Ever since then, the process of soap making has been perfected to the point that it can be considered an art in its own right. However, something relatively unknown is the connection between soap and another type of art: the medium of oil paintings. As it turns out, the lasting appearance of paintings are influenced by numerous chemical and physical processes, which contribute to visual features that are often unforeseen by the artist.

To understand how soap forms on oil paintings, it is first important to consider the reactions that occur to produce soap. In essence, a soap molecule is the salt of a long chain fatty acid, consisting of a long hydrophobic hydrocarbon tail and a hydrophilic anionic head (Amrita, 2013). These molecules are made from the reaction between glycerol and a long chain fatty acid to produce a triglyceride. When the triglyceride reacts with aqueous sodium hydroxide – otherwise known as lye – or another alkaline compound like potassium hydroxide, a glycerol molecule is produced along with soap in a saponification reaction (Figure 1) (Amrita, 2013).

Due to the chemical composition of oil paints, the formation of soap is not uncommon. As their name suggests, oil paints are comprised of a mixture of an oil medium and pigmenting particles that are applied to a ground layer (Senteno and Mahon, 2009). Over time, as the paint is exposed to various conditions, heavy metals like lead, tin, or zinc in the pigments may react with free fatty acids in the oil base medium to form the organic salts that compose soap (Senteo and Mahon, 2009).

A notable example is the 19^th century painting Madame X by John Singer Sargent. By analyzing regions of the painting with Raman microspectroscopy, a process that measures the amount of scattered light that is reflected back at a different energy level (Tabaksblat, Meier, and Kip, 1992), it was apparent that lumpy white protrusions with a diameter of about 10 – 20 micrometers formed because of the reaction of lead white in the ground layer with the fatty acids of the oil binder. The shape of these soap bulges was the result of the intermolecular interactions between the soap molecule and surrounding oil (Figure 2). Moreover, it was shown that darker pigments, such as carbon-based black, absorb more oil binder, contributing to a faster rate of saponification (Senteno and Mahon, 2009).

Additional imaging techniques, such as nuclear magnetic resonance spectroscopy (NMR), have been used to assess painting degradation on a molecular level. Using a zinc white reference sample and paint fragments from John Asger’s Maske, a saponification reaction was observed by studying the chemical shifts in carboxylates, which constitute the anionic head of soap molecules. The results supported that de-esterification and saponification reactions arise from the drying and curing process of paint (Kehlet, et al., 2016).

Overall, the development of intricate analytical technologies elucidates the physical changes and chemical reactions that occur in paint on a microscopic scale. While the formation of soap is one example that yields minute yet visible alterations on a paint film, there may be countless others that impact how the beauty of oil paintings change over time.

References

Amrita, 2013. Saponification – The process of making soap. [online] Amrita University Online Lab. Available at: <http://amrita.olabs.edu.in/?sub=73&brch=3&sim=119&cnt=1> [Accessed 4 Nov. 2017].

Butler, H., 2000. Poucher’s perfumes, cosmetics and soaps. [e-book] Dordrecht: Kluwer Academic Publishers. Available at: <http://priede.bf.lu.lv/grozs/AuguFiziologijas/Augu_resursu_biologija/gramatas/Perfumes%20Cosmetics%20Soaps.pdf> [Accessed 4 Nov. 2017].

Joshi, S., 2017. Cleaning Action of Soap. [image online] Icsepapers. Available at: <https://www.icsepapers.com/blog/Effect%20of%20soap%20in%20cleaning> [Accessed 2 Nov. 2017].

Kehlet, C., Kuvvetli, F., Catalano, A. and Dittmer, J., 2016. Solid-state NMR for the study of Asger Jorn’s paintings. Microchemical Journal, [online] 125. Available at: <http://eds.b.ebscohost.com.libaccess.lib.mcmaster.ca/eds/detail/detail?vid=0&sid=a09794e8-a5bb-4a23-8787-12d2bb3ded93%40sessionmgr102&bdata=JnNpdGU9ZWRzLWxpdmUmc2NvcGU9c2l0ZQ%3d%3d#AN=112016263&db=asx>

Senteno, S.A. and Mahon, D., 2009. The chemistry of aging in oil paintings: Metal soaps and visual changes. The Metropolitan Museum of Art Bulletin, [online] 67, pp.12–19. Available at: <https://www.jstor.org/stable/40588562?seq=1#page_scan_tab_contents>

Tabaksblat, R., Meier, R.J. and Kip, B.J., 1992. Confocal Raman Microspectroscopy: Theory and Application to Thin Polymer Samples. Applied Spectroscopy, [online] 46(1), pp.60–68. Available at: <https://journals.sagepub.com/doi/pdf/10.1366/0003702924444434>

Wahl, G.H. and Gallardo-Williams, M.T., 2011. Experiment 10 – Hydrolysis of Glycerol Tristearate: Preparation of Soap. [image online] WebAssign. Available at: <http://www.webassign.net/sample/ncsumeorgchem2/lab_10/manual.html> [Accessed 2 Nov. 2017].