The Eggstraordinary Mystery of Proteins in Renaissance Paintings

An interdisciplinary team of researchers propose that Italian Renaissance masters – artists such as Sandro Botticelli and Leonardo da Vinci – added protein to their oil paintings to prevent wrinkling and issues with humidity. The study is published in Nature Communications. 

An “eggcellent” mystery

Proteins, such as those found in egg yolk, have been detected in oil painting works of famous Renaissance artists. Why proteins were added to the oils has been difficult to decipher, as Old Masters’ technical knowledge was often communicated orally through workshops, meaning little documentation exists. 

Dr. Ophélie Ranquet, formerly of the Karlsruhe Institute of Technology (KIT), and colleagues now report the first systematic study on the effects of adding protein materials – egg yolk, in this instance – to oil paints.

The research team prepared model paints comprising linseed oil – a drying oil commonly used by artists – egg yolk and either lead white (LW) or synthetic ultramarine blue (UB) pigments. “These pigments were selected for their extensive use in the artistic field through history and because they differently affect the curing and ageing of an oil paint layer,” the researchers say.

Synthetic blue pigment was tested rather than a natural blue pigment that would have been used by Renaissance artists. Ranquet and colleagues state that this decision reflected the need to ensure a constant pigment composition for the preparation of large amounts of paint throughout the four-year study.

Two model systems were prepared: a capillary suspension (CapS), which is where a few drops of egg yolk are added to the oil paint and mixed with a palette knife, and a protein-coated pigment (PCP), which is prepared by grinding the pigment with a solution of diluted egg yolk. The former produces a stiff paint. These model systems were prepared using the two different pigments – LW and UB. 

“As the focus of this work is on the effect of adding egg yolk to an oil paint, we selected two chemically very different, but readily available pigments to perform the comprehensive series of experiments and varied paint preparation only to achieve different repartition of egg yolk,” the researchers describe.

Egg yolk aids impasto technique and protects against humidity

The effects of adding egg yolk on the rheological properties of the paints were assessed. These properties included yield stress, which is the amount of stress required to break the paint structure and enable its flow. “The presence of egg yolk (in PCP paints), has distinctly different effects on yield stress, depending on the pigment type. The yield stress increases in UB oil paints when the pigment particles are coated with egg, while it decreases in LW oil paints,” the researchers say. Renaissance artists may have added protein materials to their oil to aid the impasto technique, which is where paint is laid on the surface area thickly so that it holds the imprint of the brush or knife used to apply it.

The researchers stress that Old Masters would have likely struggled to control the humidity of their work space, and the amount of humidity taken up by their pigments. By adding egg to the oils, the formation of a thin layer around the paint’s pigment particles could have prevented water uptake.

Ranquet and colleagues also investigated how the addition of egg impacted air-drying of the oil paints, finding that it delayed curing reactions and increased overall drying time. Based on their comprehensive experiments, they suggest that the antioxidants within the egg’s yolk may slow down the reactions that occur between the oil components and oxygen, preventing a “yellowing” of the paint.

“It is shown how artists might have used proteinaceous materials to influence impasto of their fresh oil paints, to overcome unexpected problems with humidity, produce paint layers stable against wrinkling and oxidative degradation, giving us the opportunity to admire their masterpieces still today,” the researchers conclude.

Reference: Ranquet O, Duce C, Bramanti E, Dietemann P, Bonaduce I, Willenbacher N. A holistic view on the role of egg yolk in Old Masters’ oil paints. Nat Comms. 2023;14(1):1534. doi: 10.1038/s41467-023-36859-5.