Egg Yolk Gives Tempera Paint Its Enhanced Coverage and Spreadable Properties

Before oil paints rose to popularity during the Renaissance, tempera paint was favored among many of the world's cultures. Also called egg tempera, this yolk-based paint was used to design murals in ancient China, Mycenaean Greece, Egypt and Babylonia. Legendary artists Leonardo DaVinci, Raphael, and Sandro Botticelli preferred tempera over other mediums for its quick-drying nature and its ability to make their subjects' flesh appear opaque and luminous.

Because the pigment doesn't mix well with other paints, it is less popular than acrylics, watercolor and oil-based paints, but it's still used by artists to this day. So, what made tempera reign supreme among the Renaissance giants? Researchers at Sorbonne University in France may have cracked the code, reports Jennifer Ouellette for Ars Technica.

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

Tempera was widely used because of its durable, multi-purpose applications. The paint was not affected by humidity or temperature and could be used to create various transparent and opaque effects. Once dried, its satin luster resembles modern acrylic paints.

The yolk-based paint is prepared by mixing colored, powdered pigments with a water-soluble binder—in this case, eggs. Then, the paint is finished off with a few drops of vinegar to prevent cracking once the paint dries, Ars Technica reports. Because the paint dries so fast, artists have to keep adding water as they work. Rather than paper or canvas, tempera works best on solid wooden surfaces where it's less prone to cracking. The color is also found adorning mummy caskets of ancient Egypt, wood panels from the Byzantine era, and the walls of early Christian catacombs.

To understand the molecular structures behind 15th-century tempera paints, researchers recreated recipes recorded in a handbook called Il libro dell'arte by Italian painter Cennino Cennini, according to a French National Centre for Scientific Research statement.

Since other pigment colors were derived from toxic minerals, the team used a clay-based "green earth" (terra verde) pigment for their experiments. Green earth was widely used as a base layer and an underlay for skin tones. The paint was used as a skin tone underlay in Michelangelo's unfinished painting, The Virgin and Child with Saint John and Angels ('The Machester Madonna'). 

For the experiment, scientists compared one batch of tempera paint using egg yolks and green earth to another mixture that omitted egg yolks and instead suspended the pigment in water. Each variety was brushed onto canvas and analyzed using rheology to measure the paint's flow properties. The team used nuclear magnetic resonance (NMR) relaxometry to measure the physical and chemical properties of the color, Ars Technica reports.

While both mixtures' viscosity decreased with more stress, also called shear thinning, the paint that contained egg yolks had a higher viscosity, or thickness. Researchers suspect this difference is due to a network of bonds between the egg yolks, water molecules and clay particles in the pigment, making the yolk-based mixture more dense than the water-based mixture, according to the university statement. The team's egg tempera also provided more coverage and elasticity as well.

The research team hopes to use the new findings to help preserve tempera-based artworks from the Middle Ages.

Elizabeth Gamillo | | READ MORE

Elizabeth Gamillo is a daily correspondent for Smithsonian and a science journalist based in Milwaukee, Wisconsin. She has written for Science magazine as their 2018 AAAS Diverse Voices in Science Journalism Intern.