In addition to oil and pigment, the oil paints in paintings by great artists such as Sandro Botticelli and Leonardo da Vinci often also contain proteins, for example from egg yolk. However, how and why the old masters added egg to their paints was previously unclear. With their own experiments, researchers have now demonstrated that the added proteins affect the colors in a variety of ways: On the one hand, they make the color stronger and thus facilitate certain painting techniques. On the other hand, they prevent cracks and creases from forming during drying, the colors yellowing or absorbing moisture from the air. The results could also be helpful for the preservation of famous works of art.
Famous painters of the Renaissance usually mixed their colors themselves. To get oil paints, they mixed crushed pigments with oil as a binder. While art historians originally assumed that these were the only constituents of the paints, modern analyzes have also detected proteins in the paint layer of the paintings by artists such as Sandro Botticelli, Leonardo da Vinci, Albrecht Dürer and Rembrandt. However, how and why the old masters used this addition was previously unclear.
Color production according to an old recipe
A team led by Ophélie Ranquet from the Karlsruhe Institute of Technology (KIT) has now used its own experiments to understand how different protein contents affect the properties of the colors. "To better understand Old Master paintings, their techniques and art historical developments, we prepared and studied oil paints in which either the pigments were completely encased in proteins or that contained only a small amount of added egg yolk," the team reports. "Our research work is a proof of concept and shows that such protein-containing binders can be important additives."
When producing the paints, Ranquet and her colleagues followed a traditional recipe that was written down in Italy around 1300. The pigment is first mixed with egg and water and dried in the sun before oil is added. "This recipe ensures that the pigments are completely encased in a protein layer," according to the research team. "Other methods of production, on the other hand, lead to different results, for example if you first mix small amounts of egg yolk on the pallet to create the colour." In this case, the proteins form bridges between the individual particles and link them to form stable networks.
Improved processing and preservation
"This mixture produces very solid colors that are well suited for the impasto painting technique," the researchers report. With this technique, the paint is applied thickly so that the brush strokes are clearly visible. In addition, the proteins prevent cracks or wrinkles from forming in the paint layer during drying. These are still a problem in da Vinci's early paintings, including his famous "Madonna in the Rocks". Later paintings, on the other hand, show a smoother layer of paint - probably thanks to the addition of proteins to the paint.
In addition, Ranquet and her colleagues found that in paints made according to the traditional recipe, the proteins form a protective layer around the pigment particles and prevent them from absorbing water from the humidity in the air. "Egg also acts as an antioxidant," explains the team. "Compared to oil alone, the egg-infused paints are less susceptible to oxidative degradation, which could improve the preservation of valuable works of art." The egg proteins also counteract the yellowing of the paintings, which is usually caused by light-induced oxidation of the pigments.
“Our study shows how artists might have used proteinaceous materials to affect the impasto of their fresh oil paints, overcome humidity issues and create layers of paint stable to wrinkling and oxidative degradation, such that we still admire their masterpieces today can,” the authors write. "This provides new insights that may contribute to better conservation and preservation of priceless works of art."
Source: Ophélie Ranquet (Karlsruhe Institute of Technology) et al., Nature Communications, doi: 10.1038/s41467-023-36859-5