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Yolk proteins in oil paints make art egg-cellent

Artists like Leonardo da Vinci and Rembrandt may have used oil paint mixed with egg yolk for more refined artistic expression, with a bonus effect of preserving artwork for modern admiration.

(CN) — The egg, one of our most versatile foodstuffs: it can be scrambled, poached, boiled, fried — and as it turns out, used to make painted art better.

A study, published Tuesday in the journal Nature Communications and conducted by lead author Ophélie Ranquet of the Karlsruhe Institute of Technology in Germany, investigated the documented presence of egg yolk in works of the time.

The research into oil paints used by painters from the Renaissance era through to the Baroque and Romanticism periods revealed that artists may have used egg yolk as a protein additive in their oils for a more workable material that could resolve issues with humidity, wrinkling and yellowing of their masterpieces.

Oil painting, having been accomplished in some form since antiquity, reached what many consider its zenith in Europe during the 15th and 17th centuries in the hands of the Old Masters — artists like Sandro Botticelli, Leonardo da Vinci, Johannes Vermeer and Rembrandt. They favored oil paint as its slow-drying quality allowed them to adjust elements of the painting before it fully dried, layering and blending paint to create more depth and color in the work, sometimes contributing to greater realism.

Oil paint, composed in its simplest form with just a pigment suspended in a drying oil binder, allows for greater manipulation of the material as the paints’ composition can be varied or added to. The study investigates the possible role of protein additives to oil paint.

“They can strongly influence the flow behavior of paints, i.e., their brushability and impasto, and hence the initial painting process," the study authors wrote. "Egg yolk also affects the paints’ drying, i.e., the complex oxidation and chemical cross-linking process.”

Researchers investigated two separate ways to add the yolk to their paints. In one, they simply added a couple drops of egg yolk to oil paint and mixed with a palette knife, resulting in a stiff paint they termed a “capillary suspension.” They also created a protein-coated pigment by grinding just the pigment with a solution of diluted egg yolk, which they allowed to dry before grinding it a second time, this time with oil.

Ranquet and the team used oil paints in lead white and ultramarine blue for the study, colors that aligns with what the Old Masters would have had available.

They found the differing interaction between the egg yolk and the oil for each color and for each method altered the fundamental chemical structure of the paints, changing how the paint would apply and dry to a canvas, and affecting how the color could be used while painting.

“The addition of egg thus seems to favor the formation of a well cross-linked network. This might be due to the known reactivity between lipid oxidation products and proteins, which may cause polymeric network modifications in the paint and copolymerization,” the study authors wrote.

Unlike other paints, oil paint does not dry by evaporation — it goes through a curing process where oxidation can take days to months to fully set the paint. The addition of egg yolk added even more drying time as it suppressed water intake, possibly allowing artists to adjust their paintings with greater freedom.

The team compared also each of the paints’ yield stress — the capability of a paint to flow — determining that a capillary suspension oil paint had a higher yield stress and therefore made a rougher paint with which artists could have employed impasto, a technique of applying a thick layer of paint for a dimensional effect.

The protein-coated paint presented differently, which researchers attributed to the hydrophilic layer of egg yolk on the pigment particles.

Researchers posited that this might have allowed artists to retain the chemical benefits of the egg yolk additive without compromising the coloring of the paint, “Adding some proteinaceous material during pigment preparation, resulting in a coating layer, might have solved the problem of unintentional formation of capillary suspensions, resulting in better, more stable paints with higher pigment content," they wrote.

The altered composition of the yolk-added oil paint also affects "the chemical and physical stability of aging paints, possibly reducing wrinkling and crack formation, yellowing, and darkening,” the study authors wrote.

Egg yolk’s antioxidant properties have led to less degradation of the paint in work of the Old Masters, which researchers say may have helped preserve the artworks to the present day, where we are still able enjoy the works as done hundreds of years ago.

Categories:Arts, Science

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