Sign up for CNN’s Wonder Theory science newsletter to explore the universe with news on fascinating discoveries and scientific advancements. According to a new study, “Old Masters” like Leonardo da Vinci, Sandro Botticelli, and Rembrandt may have intentionally used proteins, specifically egg yolk, in their oil paintings. Previous detections of protein residue in classic oil paintings were often thought to be contamination, but this study suggests that the inclusion of proteins was deliberate and highlights the technical knowledge of these skilled European painters. Adding egg yolk to their paintings created long-lasting effects that went beyond aesthetics. Compared to the ancient medium called tempera, oil paint offers more intense colors, smoother color transitions, and slower drying time. However, oil paint is more susceptible to color darkening and light damage. As the process of making paint was experimental, it is likely that the Old Masters added familiar ingredients like egg yolk to this new type of paint. The study recreated the paint-making process using egg yolk, distilled water, linseed oil, and pigment to mix lead white and ultramarine blue. The addition of egg yolk altered the properties of the paints, making them oxidize slower and more resistant to humidity. The chemical reactions between the oil, pigment, and proteins in the yolk affected the behavior and viscosity of the paint. Leonardo da Vinci’s “Madonna of the Carnation” exhibited wrinkling, a common issue with oil paint. The study suggests that an insufficient quantity of pigment caused the wrinkling, which could have been avoided with the addition of egg yolk. Other paintings like Botticelli’s “The Lamentation Over the Dead Christ” also showed the presence of proteins, indicating deliberate use rather than contamination. These findings provide new insights into old painting techniques and contribute to a better understanding of art history and preservation.