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Secret of tempera’s pleasing properties is how egg yolk interacts with pigment

Enlarge / Detail of Michelangelo’s unfinished painting The Virgin and Child with Saint John and Angels (tempera on wood, ca. 1497), aka The Manchester Madonna. The outlines of two angelic figures on left are rendered in green earth tempera underpaint.

Tempera is a painting medium that has been used to decorate everything from early Egyptian sarcophagi to India’s rock-cut temples. The medium was particularly popular with medieval and early Renaissance artists until it was eventually supplanted by oil-based paints. But while there has been a great deal of research on the chemistry of oil paints, tempera has been largely neglected in the scientific literature, according to researchers at the Sorbonne University in France.

To remedy that, the Sorbonne scientists recreated recipes for tempera written down by a medieval Tuscan painter, the better to analyze the flow properties and molecular organization of the paints, according to a recent paper published in the journal Angewandte Chemie. The aim is to gain a more precise understanding of the underlying chemistry in order to aid in ongoing conservation efforts on tempera-based artworks from the Middle Ages in particular.

Tempera in this context refers to a fast-drying paint in which colored pigments are mixed with a water-soluble binder—traditionally egg yolk, often augmented with an agent such as a few drops of vinegar to prevent cracking once the tempera has dried. The powdered pigment and distilled water would be mixed with binder directly onto the palette, or in a bowl. Liquid myrrh was sometimes added to offset the tempera’s rather pungent odor. Artists had to keep adding water as they worked because the medium dried so quickly, and in those days, tempera could not be stored because the yolk would begin to cure, thickening the paint. While tempera fell out of favor after 1500, it is periodically rediscovered. For instance, 20th-century artists like Jacob Lawrence and Andrew Wyeth used tempera in their work.

Traditional egg tempera works best on a stiff wooden panel rather than canvas, since it is prone to crack and chip on the latter. It’s exceptionally long-lasting, and while it holds less pigment than oil paints, the colors can confer an aesthetically pleasing jewel-like quality upon the finished painting. It doesn’t blend well; color mixing, even today, is typically achieved via cross-hatching techniques or overlays. That’s why it’s often compared to pastels or colored pencils.

Enlarge / Stages in the recreation of a 15th-century recipe for tempera in the lab.
Agathe Fanost

According to the authors of the Angewandte Chemie paper, most research on tempera paints to date has focused on identifying the binders used or investigating how tempera paintings might degrade over time. The researchers expressed surprise that, given the central role the technique played in the art of painting in Europe, more attention had not been paid to the molecular interactions between the binder (egg yolk) and different pigments. “The structure and composition of yolk has been studied in the agro-food industry but although it has been used in paint for centuries, its structure, organization and properties have never been studied in any depth in paint,” they wrote.

Medieval painters used a variety of pigments, some of which were quite toxic (cinnabar, orpiment, and lead white in particular). The researchers wisely avoided those and chose to focus on a green earth (terra verde) clay-based pigment for their experiments. Green earth was ubiquitous in medieval Italian wood-panel paintings as a base layer for gilding and an underlay for flesh tones. This can be clearly seen in Michelangelo’s unfinished The Virgin and Child with Saint John and Angels (ca. 1497), aka the Manchester Madonna (see top image), now housed in London’s National Gallery. Specifically, the outlines of two angelic figures on the left are rendered entirely in green earth underpaint.

The authors used recipes for green earth tempera paints recorded in the treatise Il libro dell’arte by a 14th-century Italian painter named Cennino Cennini. (It’s essentially a handbook documenting pigments, brushes, and various techniques and tricks, as well as some lifestyle advice for aspiring young painters.) One tempera batch was a mixture of green earth pigment and egg yolk, while another was a mixture of green earth suspended in water for comparison purposes. The researchers then applied each paint onto a canvas.

For their analysis, the authors combined two complementary techniques: rheology (relating to how a material deforms) to measure flow properties, and NMR relaxometry to characterize the properties and structures of each mixture. (Nuclear magnetic resonance imaging uses magnetic moments of atoms to measure physical and chemical properties of materials, and the relaxation rate of nuclear spins provides a kind of fingerprint for molecular dynamics.)

Visual comparison of two different paint mixtures. (top left) Green earth pigment suspended in water. (top right) Green earth egg yolk tempera. (bottom left) Water mixture applied to canvas. (bottom right) Egg yolk tempera applied to canvas,
Enlarge / Visual comparison of two different paint mixtures. (top left) Green earth pigment suspended in water. (top right) Green earth egg yolk tempera. (bottom left) Water mixture applied to canvas. (bottom right) Egg yolk tempera applied to canvas,
Agathe Fanost et al., 2021

While both batches exhibited the expected shear-thinning—in which a fluid’s viscosity decreases with increased stress, such as hitting a ketchup bottle to get the condiment to flow more freely—the tempera paint had higher viscosities. “This demonstrates an important synergistic effect between green earth and egg yolk in tempera paint,” the authors wrote. “Its consistency is considerably higher than that of a green earth water dispersion.”

This is due to a network that forms between the proteins in the egg yolk, the water molecules, and the clay particles in the pigment. The resulting synergy is what enhances the spreadability and coverage of the egg yolk tempera. The use of the yolk as a binder also increases the resulting tempera’s elasticity.

“This study does not claim to solve or answer all questions with regard to the importance of the binder type on the ability of paint to be applied to a surface,” the authors concluded. “Rather, it shows the great interest of combining these two techniques, applied to artistic paint materials, to give new insights in the network binder-pigment and put oneself in the painter’s shoes.”

DOI: Angewandte Chemie, 2021. 10.1002/anie.202112108  (About DOIs).

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