Recently, I was doing some research online, looking for new ingredients for my soaps. I came across a very interesting article, and had to look into it further...
Oil Paintings Can Turn To Soap
The chemical reaction known as saponification, is the process by which soap is made. Saponification, according to Merriam Webster, is the hydrolysis of a fat by an alkali with the formation of a soap and glycerol. This process has been seen in some oil paintings as early as 1912!
Over time, saponification in oil paintings can cause damage both to the overall look of the painting, as well as the actual structure of the painting/canvas itself. It is believed that heavy metals often found in the pigments of oil paints can react with the oils in the paint, allowing soap to form in a layer of paint. This soap layer can then expand to the surface of the painting. Through chemical analysis of paintings, signs of saponification can be found in the deeper layers of a painting, without any visible signs on the surface of the works.
In works of art such as John Signer Sargent's Portrait of Madame X, saponified areas of a painting can deform the surface, with visible bumps and protrusions. In this particular work, the bumps can only be seen in the blackest areas of the piece. It is believed that this is because the artist likely used more medium in those areas.
According to the Metropolitan Museum of Art, the process is still not fully understood. It cannot be traced to any one material, and as of yet, no one knows what triggers the process to start.
While it would likely be frowned upon (and likely have you thrown out), a trip to an art museum for some Monet soap might be a real thing if there is ever a zombie apocalypse.