3 Old Masters Linked to Secret Egg Yolk in Oil Paintings

Egg yolk, long dismissed as a contaminant in centuries-old oil paintings, may have been added on purpose by some of Europe's most celebrated painters, according to a study published March 28, 2023, in the journal Nature Communications, CNN reported.

What ingredient did researchers detect in Old Master oil paintings?

The study found trace quantities of protein residue, particularly from egg yolk, embedded in oil paintings attributed to the "Old Masters" — a term for the most skilled European painters working in the 16th, 17th and early 18th centuries, according to CNN's account of the research. Protein traces of this kind have turned up in classic oil paintings for years, but conservators and researchers typically chalked the residue up to accidental contamination rather than a deliberate choice by the artist. The new paper argues the opposite: that painters folded protein into their oil paint on purpose, as part of a technical process for controlling how the paint behaved on canvas or panel.

Which artists' paintings showed egg yolk traces?

CNN's report names three painters whose works were examined as part of or in connection with the findings: Leonardo da Vinci, Sandro Botticelli and Rembrandt. Those three span roughly two centuries of European painting, from the Italian Renaissance workshops of da Vinci and Botticelli in the 15th and early 16th centuries through the Dutch Golden Age output of Rembrandt in the 17th century. The presence of the same protein marker across painters working in different countries and different eras is what pushed the research team toward intentional use rather than a one-off accident in a single studio.

Why was the protein previously dismissed as contamination?

Oil paint, as the term implies, is built primarily from pigment suspended in drying oil — not egg, which is the binder used in a separate, older medium called tempera. Because the two techniques were historically understood as distinct, protein found inside an oil painting was easy to explain away: a stray fleck from a nearby tempera panel, dust, or residue picked up during handling and conservation over hundreds of years. The Nature Communications paper challenges that default explanation by finding the protein pattern consistent enough, across different paintings, to argue for intentional formulation rather than incidental cross-contamination.

Where and when was the research published?

The findings appeared in Nature Communications, a peer-reviewed journal, with CNN publishing its summary of the study on March 28, 2023, and updating the piece on March 30, 2023. CNN's article does not detail the specific laboratory technique used to identify the protein, nor does it name the paper's lead authors or their institutional affiliation in the portion of the report available. Readers seeking the underlying methodology — how samples were taken from paintings centuries old without damaging them, and how researchers distinguished egg yolk from other possible contaminants — will need the full Nature Communications paper, which CNN cited as the source of the findings but did not excerpt in detail.

What does this mean for how Old Masters' technique is understood?

If painters were mixing egg yolk into oil paint on purpose, it suggests a level of technical sophistication in Renaissance and Baroque workshops that goes beyond simply applying pigment and oil. Egg yolk is the binding agent in tempera paint, a medium that predates widespread oil painting in Europe. Finding it deliberately folded into oil-based works raises the possibility that some Old Masters treated paint formulation as its own craft, blending mediums to get specific working properties — though the CNN report stops short of specifying what those properties were or how they changed a painting's appearance or longevity.

What to watch next

• Whether the full Nature Communications paper names additional painters or paintings beyond da Vinci, Botticelli and Rembrandt tested for the same protein marker.
• Whether museums and conservation labs revisit existing chemical analyses of other Old Master works using the same detection method.
• Whether the finding changes how conservators clean or restore oil paintings, given that egg-based binders can respond differently to solvents than pure oil.
• Whether art historians connect the practice to specific workshop traditions in Florence, Milan or Amsterdam rather than treating it as a universal technique.

CNN's report frames the discovery as evidence that egg yolk's role in Old Master paintings was "likely intentional," a shift from the long-standing assumption that any protein found in an oil painting got there by accident. The paper itself, published in a peer-reviewed journal, is the primary document behind that claim, even though the full study was not reproduced in the portion of CNN's coverage reviewed for this report.