Mr. Willenbacher, as a chemical engineer you deal with paints and coatings. But in a straight in Nature Communications published study, you were involved in is about egg yolks. So-called tempera paints were already being mixed with it in ancient times, for example for mummy portraits in Egypt. What is egg yolk doing in paint?
The classic egg tempera consists of water, egg yolk and color pigments. The water acts as a solvent and evaporates as the paint dries. The egg yolk, on the other hand, remains in the dried paint as a binding agent – it embeds and protects the pigments. Tempera colors are therefore considered to be particularly durable and retain their color strength for centuries.
Tempera colors were still very popular throughout the Middle Ages. Then they got competition. Renaissance and Baroque masters began to paint in oils. What are the advantages of oil paints over tempera?
Oil paints appeared in Afghanistan in the seventh century and only found their way to Europe in the late Middle Ages. There they gradually prevailed over the egg yolk colors thanks to their superior color properties. In contrast to tempera, oil paints consist only of vegetable oils, such as linseed or walnut oil, and pigments. The oil is both a solvent and a binding agent. When it dries, it becomes resinous as the unsaturated fatty acids react with atmospheric oxygen. However, this crosslinking reaction is quite slow and often lasts several days. In contrast to the quick-drying tempera paints, oil paints allow the artist to paint wet-on-wet and to create fine color transitions.
In your study, you and your co-authors investigate why many oil paintings – such as those by Dürer, Vermeer or Rembrandt – still contain egg yolk. Did the artists deliberately add it to their colors?
Thanks to modern analysis, it has long been known that some oil paintings from this period contain small amounts of egg yolk or other animal proteins. Initially, these traces were still considered impurities. While we know exactly what’s in the colors, it’s difficult to understand why and how certain additives like egg yolk were added. Because the formulations of the colors were the artists’ best-kept secrets and were only passed on within the workshops. Unfortunately, this custom also meant that much of this knowledge was lost over the course of the 19th century.
Have you been able to find out why egg yolk was added to oil paints anyway?
We found that the way you incorporate the yolk has a very big impact on the color properties. If you mix the egg yolk with the pigment before adding the oil, the proteins in the egg yolk first coat the micrometer-sized color pigments. Since many pigments, such as white lead, catalyze the resinification of the oil, the egg yolk proteins on the pigment surface prevent premature drying in this variant. The paint stays liquid longer and can be processed longer.
And what effect does the egg yolk have if you stir it into the oil paints afterwards?
In this case, the yolk behaves quite differently, since it does not dissolve well in oils: the small amount of yolk therefore forms small droplets that collect between the dispersed color pigments to minimize the contact surface with the oil. My group described this physical effect for the first time a good ten years ago – but at that time it had no connection to oil paints. A stiff network is formed that is held together by capillary forces. As a result, the paint becomes a viscous paste and allows, for example, the “impasto” painting technique, in which the brushstrokes remain even after drying. Without egg yolk, this technique is only possible if more pigment is added, which means that transparent colors are no longer accessible.
Da Vinci’s “Madonna of the Carnation”
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Bild: https://press.springernature.com/a-holistic-view-on-the-role-of-egg-yolk-in-old-masters-oil-paint/24644384
Did the painters already know about the complex relationships between the processing properties of a color and its composition?
They learned through trial and error. A fine example is Leonardo da Vinci’s “Madonna with the Carnation”, which is the only work by the Italian artist to be exhibited in Germany in the Alte Pinakothek. It shows the Madonna, who despite her young age has wrinkled cheeks – but not on purpose. The work is one of da Vinci’s first oil paintings and testifies to his lack of experience in handling oil paints. The wrinkles are due to the fact that he mixed the light color for her skin with very little pigment, i.e. too thin. While the top layer seemed to dry normally, bottom layers stayed liquid longer and bled over time. The result is wrinkled cheeks, so to speak, an artificial aging of the depicted Madonna. If da Vinci had added egg yolk to the paint, the Madonna’s cheeks would probably still be wrinkle-free today. But he learned from his mistake, later works no longer show this mistake.