Five centuries later, science reveals why the great Renaissance masters such as Leonardo and Botticelli added egg yolk to their oil paints: laboratory experiments show that proteins used as additives protect paintings from yellowing, from moisture and the formation of cracks during drying.
The discovery, which will help the conservation of the works, is published in the journal Nature Communications by an international group of experts which sees the University of Pisa at the forefront, with the Institute of Chemistry of OrganoMetallic Compounds of the CNR and the National Interuniversity Consortium for the Science and Technology of Materials (Instm) of Florence.
"So far, scientific investigations on paintings have mostly been aimed at identifying the materials used by painters, but this is not enough to understand the motivations behind the artistic practice", Ilaria Bonaduce, associate professor in the Department of Chemistry, told ANSA and Industrial Chemistry of the University of Pisa, where a research group dedicated to cultural heritage has been working for more than 20 years.
This time, one practice in particular has ended up under the scrutiny of the researchers, that of adding egg yolk in oil paint, which was used by many famous painters: not only the great masters of the Italian Renaissance, but also artists of the caliber by Albrecht Durer, Johannes Vermeer and Rembrandt.
"In the laboratory we prepared some paints with the addition of the yolk and we spread them to study their chemical and physical behavior - says Bonaduce - in an attempt to understand the reasons that motivated this choice by the artists".
The results of the analyzes (conducted with techniques of rheology, differential scanning calorimetry, thermogravimetry and analytical pyrolysis coupled with mass spectrometry) have shown that egg proteins form a thin layer which coats the pigment particles and prevents the absorption of ambient humidity.
Furthermore, the yolk makes the color mixture more consistent, prevents the formation of small cracks during drying and, with its antioxidant substances, prevents the yellowing of the pigments.
"Now we are continuing to work using other analytical techniques to investigate the microstructure, in order to then move on to the study of famous paintings," adds Bonaduce.
"Samples of Sandro Botticelli's 'Lamentation over the Dead Christ with Saints Jerome, Paul and Peter', preserved in Munich, are already available, but we will also work on other works by Titian and Ghirlandaio."