Walking through the halls of the Former College of San Ildefonso in Mexico City, visitors encounter the first mural the Mexican government commissioned from Diego Rivera, titled Creation and painted in 1922 and 1923. The sprawling work depicts the creation of humankind and features allegorical representations of the arts and of Christian virtues. But look at it with a chemist’s eye and another story emerges from the materials Rivera used to make it.
In an interview conducted while he was painting the mural and in writings published later, Rivera described using encaustic, a technique that involves mixing pigments with a heated binder and later applying heat to the painted surface. Among other components such as elemi resins and solvents, his recipe included beeswax, a traditional ingredient in ancient Greek encaustic painting, and copal, an aromatic tree resin with a long history of human use in Mexico dating to pre-Hispanic times.
In a new study published in ACS Omega, researchers used a suite of analytical techniques to examine microscopic samples of the mural. They indeed detected chemical markers of copal resin, but found no evidence of beeswax, suggesting that Rivera may have relied primarily on the former as the mural’s binding medium (DOI: 10.1021/acsomega.6c00983).
“The fact that he experimented with copal shows he may have been trying to ‘inject’ his Western art with ancient American substances, making it physically more ‘Mexican,’ ” James Oles, an art historian at Wellesley College, says in an email.
To determine what remained of Rivera’s reported recipe in Creation, the researchers first used his recipe to recreate the paints in the laboratory. They then used nuclear magnetic resonance spectroscopy to characterize the molecular structure of the binder and gas chromatography/mass spectrometry to separate and identify individual organic compounds. They compared the chemical fingerprints they identified in their paint with the samples from the mural, which contained triterpenoids associated with copal resin, including lupeol and α- and β-amyrin, but none of the compounds expected from beeswax.
Because Rivera and his assistants heated, or cauterized, sections of the mural with a blowtorch, researchers considered whether the high temperatures might simply have destroyed the beeswax. When subjecting their recreated paints to similar heating, however, they found that beeswax signals weakened but remained detectable even after the samples began to burn.
Knowing the exact composition of a mural can influence how conservators care for it. Jane Spooner, a wall paintings specialist at the Courtauld Institute of Art in London, says information about a mural’s origin materials and physical history can help diagnose its condition and determine which treatments are safe. A water-soluble paint medium, for example, could rule out aqueous cleaning methods, while organic or light-sensitive pigments might require controlled lighting.
“Without visual examination, historical research, or an understanding of the context of the mural painting and conservation methodologies, scientific results remain just sophisticated information,” Spooner says. “It is when combined with these other kinds of information that scientific analysis becomes a really powerful tool for understanding and caring for mural paintings.”
Although Oles does not think the discovery will change our understanding of Rivera’s work, he’s excited by the process. “I love these chemical discoveries because I can use them to show young students that art history connects to many disciplines, including the sciences.”
