Old Masters" such as Leonardo da Vinci, Sandro Botticelli and Rembrandt may have used proteins, especially egg yolk, in their oil paintings, according to a new study. Trace quantities of protein residue have long been detected in classic oil paintings, though they were often ascribed to contamination.

A new study published Tuesday in the journal Nature Communications found the inclusion was likely intentional — and sheds light on the technical knowledge of the Old Masters, the most skilled European painters of the 16th, 17th, or early 18th century, and the way they prepared their paints.