This pigment has been used throughout human history and appeared in iconic works of art, such as Michelangelo’s "Manchester Madonna"

Before oil paint became popular in the Renaissance, tempera paintings were favored in many cultures around the world. This egg-yolk-based paint is also called tempera, and was used to design murals in ancient China, Mycenae, Greece, Egypt, and Babylonia. Legendary artists Leonardo Da Vinci, Raphael and Sandro Botticelli prefer tempera over other media because of its fast-drying characteristics and the ability to make the flesh of their subject appear Opaque and luminous.

Because this type of paint cannot mix well with other paints, it is not as popular as acrylic paint, watercolor paint, and oil paint, but artists still use it today. So, what makes tempera painting supreme among the giants of the Renaissance? Jennifer Ouellette of Ars Technica reported that researchers at Sorbonne University in France may have cracked the code.

In a new study published in the journal Angewandte Chemie last month, scientists analyzed how egg yolk protein interacts with pigments to create a pleasing tempera aesthetic. 

Tempera paint is widely used because of its durable multi-purpose applications. The paint is not affected by humidity or temperature and can be used to create various transparent and opaque effects. After drying, its satin finish is similar to modern acrylic paint.

Egg yolk paint is prepared by mixing colored powder pigments with water-soluble binders (eggs in this case). According to Ars Technica, the paint is then finished with a few drops of vinegar to prevent the paint from cracking when it dries. Because paint dries quickly, artists must constantly add water while working. Compared with paper or canvas, tempera works best on solid wood surfaces that are not easy to crack. This color can also be used to decorate mummy coffins in ancient Egypt, wood panels from the Byzantine era, and the walls of early Christian tombs.

According to a statement from the French National Center for Scientific Research, in order to understand the molecular structure behind the 15th-century tempera paint, researchers recreated a recipe recorded by the Italian painter Cennino Cennini in a manual called Il libro dell'arte.

Because other pigment colors come from toxic minerals, the team used a clay-based "terra verde" pigment in the experiment. Green soil is widely used as the foundation and bottom layer of skin tone. This pigment was used as the skin tone in Michelangelo's unfinished painting "The Virgin and Child with St. John and the Angel" ("Mcchester Madonna"). 

In the experiment, the scientists compared a batch of tempera paints that used egg yolks and green earth with another mixture in which the egg yolk was omitted but the paint was suspended in water. Each variety is painted on the canvas and analyzed using rheology to measure the flow characteristics of the paint. According to Ars Technica, the team used nuclear magnetic resonance (NMR) relaxation measurements to measure the physical and chemical properties of colors.

Although the viscosity of both mixtures decreases with increasing stress, also known as shear thinning, coatings containing egg yolk have a higher viscosity or thickness. According to the university’s statement, the researchers suspect that this difference is due to the bond network between the egg yolk, water molecules, and the clay particles in the paint, which makes the egg yolk mixture denser than the water-based mixture. The team's tempera paint also provides more coverage and flexibility.

The research team hopes to use the new findings to help protect medieval tempera artworks.

Elizabeth Gamillo is a daily correspondent for the Smithsonian Institution and a science reporter in Milwaukee, Wisconsin. She has written for "Science" magazine and served as the diversified voice of AAAS science journalism intern in 2018.

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