Sandro Botticelli, "The Lamentation of Christ", c. 1490/95. The grass foreground and the stone tomb background are painted with oil paints containing proteins. Credit: Bavarian State Painting Collections, Munich/ Nat. Comm. 14, 1534 (2023).
We know that the Old Masters like Botticelli, da Vinci or Rembrandt used to add animal proteins, such as egg yolk, to their paints, to modify their properties and allow more sophisticated paint handling. But chemists and art historians were not sure of the science behind it.
Now an international team comprising researchers from the University of Pisa has investigated how different distributions of protein binder derived from egg yolk affected the properties of oil paints, combining historical research with experimental reconstructions of ancient oil paints and modern analysis techniques borrowed from physics and chemistry1. “We tried to produce an overall explanation of how different preparation procedures, and thus different repartition of the ingredients in the paint microstructure, can affect the paint properties, and how this could have been useful to the Old Masters” says Ilaria Bonaduce from the University of Pisa, one of the authors.
The scientists started from historical sources such as the Liber diversarum arcium, a codex of traditional medieval painting. There they found the oldest recipe that explicitly mentions the use of egg yolk in oil paint. Oil paints with different proportions of protein binder were then produced, to study how the changes in composition resulted in different paint properties. Subsequently, the scientists analysed the newly produced samples focusing on the physics of the fresh paints and the chemistry of the paint film formation.
The results show that adding egg yolk results in an increase in the strength of the network of pigment particles in the paint. This strength directly determines how easily the tempera can be brushed. According to the researchers, a layer of egg proteins minimizes the contact between pigment molecules and oil particles, promoting paint plasticity and stiffness. It also has a beneficial effect on colours. Humidity plays a crucial role during paint preparation, and a common solution for the Old Masters was to add oil to prevent excessive stiffness.
But oil can also cause loss of colour and darkening over time, due to the ageing of fatty acid. A coating layer of egg yolk proteins on pigment particles might have solved this problem allowing the artists to use higher pigment content and achieve more stable paints.
