Why the old masters used egg yolks in their oil paints

Even mixing oil paints is an art - but a lot of old knowledge about it has been lost over the centuries. From around the 15th century, painters such as Leonardo da Vinci, Sandro Botticelli, Albrecht Dürer or Rembrandt mixed proteins - such as egg yolk - into their oil paints.

In an experimental study, a German-Italian research team has now determined the advantages of this addition. These include better brushability, fewer wrinkles and cracks when drying, and greater chemical resistance of the colors.

It is generally known that oil is sufficient as a binder for the color pigments in oil paints, writes the group led by Ophélie Ranquet and Norbert Willenbacher from the Karlsruhe Institute of Technology (KIT) in the journal "Nature Communications". However, traces of proteins such as eggs were often detected in painting samples. "For a long time, people thought it was contamination," says Willenbacher. But the regular appearance of such components indicated that there was intention behind it.

"There's clearly more to making a color than just mixing the pigment with a binder," the team writes. Even in the Middle Ages, Italian painters mainly used egg tempera – i.e. pigments mixed with water and egg yolk as a binding agent. But as the 15th century progressed, oil paints came into use, and this probably caused problems, at least initially.

As a possible example, the team cites the picture "Madonna with the Carnation" - an early oil painting created around 1475 by the then still young da Vinci. The face of the Madonna shows wrinkles, so-called wrinkles, which are believed to have arisen when the paint dried. "That never happened to da Vinci," explains Willenbacher. "We suspect that he later added egg yolk to the oil paints."

Botticelli's painting "The Lamentation of Christ" from around 1490 verifiably contains egg yolk in several sampled areas. Even in tiny amounts, the yolk influences the stiffness of the colors, especially if they only have low pigment concentrations.

In the study, the team also tested different approaches to mixing pigments with linseed oil and egg yolk. Egg yolk can either be mixed into the already mixed oil paint, or the ground color pigments are first mixed with egg yolk, then dried and then rubbed with linseed oil so that they are encased in a protein layer in the oil.

This protects paints from absorbing moisture from the environment and also allows a higher pigment content in the paint with good brushability at the same time. A higher pigment content, in turn, leads to higher resistance to cracking and reduces yellowing and other color changes.

Especially with the first recipe - i.e. the subsequent addition of egg yolk - very stiff stains are created that can be applied thickly and at the same time very easily with the brush. In addition, the strength of the lower layers during drying prevents the paint surface from cracking, thus preventing wrinkles and cracks. But that's not all: the yolk contains antioxidants that preserve the original color for a long time, the team writes.

"This analysis forces us to think differently," says co-author Patrick Dietemann of the Bavarian State Painting Collections in Munich. "It makes sense to use egg additives in oil paints."

The group writes that old masters such as Botticelli, da Vinci or Rembrandt used protein-based binders in different ways for their artworks. With these techniques they would have avoided problems and "give us the opportunity to still admire their masterpieces today".

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