Renaissance artists added egg yolk to their paint. Here's why

“So far, it is difficult to decide where and when egg additives to oil paints have been used because oil paints are often not analyzed for small amounts of proteins to minimize sample size, analytical time, and cost.”

The researchers discovered that the artists added egg yolk to aid their artwork and combat issues with humidity, surface wrinkling, and yellowing. The discovery may now aid in the conservation and preservation of some Old Master artwork.

The researchers found that water uptake from humid environments can be suppressed when the egg proteins form a thin layer around the pigment particles. The substance was also found to provide stiff paints with strong impasto and prevent surface wrinkling during drying.

Finally, the antioxidants found within egg yolk also help prevent yellowing when drying by slowing down the reaction between oxygen and oil components to reduce solid film formation.

“This holistic study combines knowledge from conservation science, rheology, and analytical chemistry to understand in which various ways Old Masters like Botticelli, da Vinci, or Rembrandt might have used proteinaceous binders to modify oil paints to create their artworks,” concluded the authors in their study.

“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 study is published in the journal Nature Communications.

Study abstract:

Old Masters like Botticelli used paints containing mixtures of oils and proteins, but “how” and “why” this was done is still not understood. Here, egg yolk is used in combination with two pigments to evaluate how different repartition of proteinaceous binder can be used to control the flow behavior as well as drying kinetics and chemistry of oil paints. Stiff paints enabling pronounced impasto can be achieved, but paint stiffening due to undesired uptake of humidity from the environment can also be suppressed, depending on proteinaceous binder distribution and colloidal paint microstructure. Brushability at high pigment loading is improved via reduction of high shear viscosity and wrinkling can be suppressed adjusting a high yield stress. Egg acts as antioxidant, slowing down the onset of curing, and promoting the formation of cross-linked networks less prone to oxidative degradation compared to oil alone, which might improve the preservation of invaluable artworks.