If you compare how Pablo Picasso’s 1907 painting Femme (Époque des “Demoiselles d’Avignon”) looked 50 years ago with how it looks now, you might notice something strange. Like a rotting banana, a formerly yellow section in the upper left corner is now brownish while another yellow section in the painting has maintained its brilliant lemon hue. According to a new study, this difference is because Picasso used two different yellow paints: they contain different sized cadmium sulfide particles and have degraded at different rates (Anal. Chem. 2019, DOI: 10.1021/acs.analchem.8b04914). The study shows that analyzing the photoluminescence of the pigment particles in paint may help art conservators identify the start of color degradation in other paintings and protect artworks against it.
Many artists in the late 19th and early 20th centuries used similar CdS paints as Picasso, and scientists and art conservators have seen many such paintings decay. Although scientists understand some of the mechanism behind the fading, much is still not understood about why some of these paints degrade and others don’t, says Daniela Comelli, an optical spectroscopist at the Politecnico di Milano. Comelli and colleagues wanted to better understand the degradation process. In analyzing Femme, the researchers also had a unique opportunity to compare faded and unfaded paints within the same painting.
Working with researchers from the Getty Conservation Institute, Fondation Beyeler, and the National Research Council’s Institute for Photonics and Nanotechnologies, Comelli analyzed two different sections of the painting: one where the once yellow paint has turned brown and one where it hasn’t. The team used noninvasive analytical techniques to look at the intact painting and also took small paint samples for other analyses. The researchers used photoluminescence spectroscopy to look closely at the CdS particles, and the method revealed that the brown section emitted more intense and shorter wavelength light than that of the unfaded yellow paint.
This difference in emission can be traced to what are known as trap states, which arise from defects in the crystalline structure of the CdS particles in the faded paint, Comelli says. These states allow electrons to get trapped in ways that increase the intensity and shorten the wavelength of the particles’ photoluminescence. The difference between the emission spectra of the brown paint and yellow paint led Comelli and her team to conclude that the brown, degraded paint contains smaller CdS nanoparticles with more defects whereas the unfaded yellow paint contains larger CdS grains. The researchers are not sure if the original paint already contained nanosized crystals or if the degradation reduced their size, Comelli says, but this is part of their future work.
Around the turn of the 20th century, when Picasso painted Femme, new pigments were becoming available and suppliers were trying different ways to synthesize paint, Comelli says. Picasso was an experimental kind of guy, she says. It’s possible he was playing with different paints to see their effect on the canvas. It’s also possible that he used the paint that eventually degraded because it was cheaper.
Many labs have worked on the CdS fading problem, and many people were aware of the importance of these trap states in the degradation of CdS paints, but this paper helps to explain exactly how they arise, says Erich S. Uffelman, an inorganic chemist at Washington and Lee University.
This information can help art conservators understand if other paintings are going to degrade in a similar way to Femme, Comelli says. “For conservation, probably once the paint has degraded it’s not possible to go back. But if we are able to detect a sign of degradation, we are better able to save the paint” by, for example, adding a protective layer on top, she says.