Editor’s Note: Don’t miss out on CNN’s Wonder Theory science newsletter for fascinating discoveries, scientific advancements, and more. A recent study suggests that famous artists like Leonardo da Vinci, Sandro Botticelli, and Rembrandt may have intentionally used proteins, specifically egg yolk, in their oil paintings. Although trace amounts of protein residue have been found in classic oil paintings, it was previously assumed to be contamination. Published in the journal Nature Communications, this new study sheds light on the technical knowledge of these “Old Masters” and their painting techniques. By incorporating egg yolk into their works, these artists achieved not only aesthetic changes but also long-lasting effects. Compared to the ancient Egyptian medium called tempera, which uses egg yolk, powdered pigments, and water, oil paint offers more intense colors, smooth color transitions, and a longer drying time. However, oil paint has its drawbacks, including vulnerability to color darkening and light damage. Since the process of making paint was experimental and artisanal, these artists may have added egg yolk, a familiar ingredient, to the newer type of paint that emerged in Central Asia in the seventh century and spread to Northern Europe and Italy in later centuries. The study recreated the paint-making process by combining egg yolk, distilled water, linseed oil, and pigment to produce lead white and ultramarine blue colors. The addition of egg yolk dramatically affected these paints, providing additional benefits like extended aging time and resistance to humidity. The chemical reactions between the oil, pigment, and proteins in the yolk directly influenced the behavior and viscosity of the paint. By using less pigment and adding egg yolk, artists could create high impasto paintings, where the brushstrokes remained visible. The presence of egg yolk can be observed in Leonardo da Vinci’s “Madonna of the Carnation,” which shows wrinkling due to the drying process. This effect can be avoided with the inclusion of egg yolk. Another painting examined in the study, Botticelli’s “The Lamentation Over the Dead Christ,” includes brushstrokes typical of oil paintings and also shows the presence of proteins. These initial findings might encourage further research into this understudied topic, providing a better understanding of old painting techniques and contributing to the preservation of art history.