Why Did Old Masters Use Eggs in Oil Paintings?

A new study explores how artists may have added yolk to alter the properties of their paints

Lamentation over the Dead Christ
Researchers think old masters like Sandro Botticelli, who painted Lamentation Over the Dead Christ, may have mixed egg into their oil paints to alter certain qualities. Fine Art Images / Heritage Images / Getty Images

Leonardo da Vinci seems to have taken “put an egg on it” to heart. 

In 15th-century Italy, oil paints largely replaced egg-based tempera paints. But despite the shift, da Vinci and other old masters experimented with paints that mixed egg and oil together. Now, scientists are learning more about why.

In a new study published in the journal Nature Communications, researchers created their own paint recipes to better understand the role of egg yolk on Renaissance canvases.

“Usually, when we think about art, not everybody thinks about the science which is behind it,” says Ophélie Ranquet, a chemical engineer at the Karlsruhe Institute of Technology in Germany and lead author of the study, to Science News’ Jude Coleman.

Initially, oil achieved primacy over tempera—a mixture of egg yolk, powdered pigment and water formulated by ancient Egyptians—for aesthetic and practical purposes: It creates more vivid colors and smoother color transitions. It also dries slowly, so it can be used for longer after the initial preparation.

Oil paint, however, isn’t without its flaws. The colors darken more easily over time, and the paint is more susceptible to damage from light exposure. It also has a tendency to wrinkle as it dries—evidence of which can be seen on the faces in da Vinci’s Madonna of the Carnation, the researchers note.

To get the best of both worlds, some old masters may have combined the two techniques. One example the researchers point to is Lamentation Over the Dead Christ, which Sandro Botticelli painted with mostly tempera, though oil paint with egg yolk proteins has been detected in the background and certain other sections.

While these proteins could be simply cross-contamination between paints in the workshop, Ilaria Bonaduce, an analytical chemist at the University of Pisa and a co-author of the study, says artists like Botticelli were likely making an intentional choice. 

“I am quite convinced that they did not know the chemical and physical explanations of what they were doing, but they knew very well what they were doing,” she tells the Guardian’s Nicola Davis.

To explore the effects of adding egg yolk to the palette, researchers crafted two egg-oil paint mixtures to test against regular oil paint. One contained fresh egg yolk mixed with oil paint, while the other was made by grinding pigment into a diluted egg yolk, which was then dried and dispersed in oil. This process, which results in protein-coated pigments, is based on a late medieval text that researchers say provides the only known recipe directly linking the addition of a protein-heavy binder to oil painting.

“Our results show that even with a very small amount of egg yolk, you can achieve an amazing change of properties in the oil paint, demonstrating how it might have been beneficial for the artists,” Ranquet tells Jacopo Prisco of CNN.

In tests, the egg-boosted oil paints suffered less yellowing, wrinkling and humidity-related problems. Additionally, the researchers found that adding yolk in different concentrations changed the paint’s stiffness.

“By changing preparation technique, you can change the properties of the paint,” says Bonaduce to the Guardian. “Thus two paints with the same composition can have different microstructures and this will result in different properties.”

Understanding how old masters mixed their paints sheds light on key chapters from art history; it also helps researchers more effectively preserve centuries-old artworks, says Maria Perla Colombini, an analytical chemist at the University of Pisa, who was not involved in the study, to CNN.

The research also offers a new perspective on the artistic decisions behind famous works, says Ken Sutherland, director of scientific research at the Art Institute of Chicago, who was also not involved with the study, to Science News

“The more we understand how artists select and manipulate their materials,” he says, “the more we can appreciate what they’re doing, the creative process and the final product.”

Get the latest stories in your inbox every weekday.