Egg yolks have been widely used as a binding medium in oil paints since the 15th century, particularly by Old Masters such as Leonardo da Vinci, Rembrandt, and Johannes Vermeer. This traditional technique, known as egg tempera, involves mixing pigments with egg yolks and water to create a smooth, opaque paint.

There are several reasons why egg yolk was preferred as a binding medium for oil paints by the Old Masters:

1. Fast Drying: Egg yolk emulsions dry rapidly, allowing artists to work quickly and efficiently. This property was particularly beneficial during the creation of multi-layered paintings, where each layer needed to dry thoroughly before applying the next.

2. Good Adhesion: Egg yolk, when combined with oil, creates a stable and adhesive medium that bonds well with various surfaces, including canvas, wood panels, and frescoes. This adhesion ensures the longevity of the paintings, preventing cracking and peeling over time.

3. Optical Properties: Egg yolk contains lecithin, a phospholipid that acts as an emulsifier, allowing the smooth blending of oil and water. This results in a paint with excellent clarity, vibrancy, and luminosity, enhancing the visual appeal of the artworks.

4. Transparency and Opacity: Egg yolk can be used to achieve both transparent and opaque effects depending on the desired outcome. By varying the ratio of egg yolk to oil, artists could create layers with different levels of transparency, adding depth and complexity to their paintings.

5. Versatility: Egg yolk can be combined with various oils and pigments to create a wide range of artistic effects. It allows for the creation of both smooth, even layers as well as impasto techniques with visible brushstrokes, providing artists with versatility in their artistic expression.

6. Historical Precedence: Egg yolk was a well-established binding medium used in tempera painting long before the introduction of oil paints. This familiarity with the material made it a natural choice for early oil painters, who sought to combine the advantages of oil paints with the traditional techniques they were accustomed to.

7. Natural and Organic: Egg yolk is a natural and organic material, readily available and biodegradable, aligning with the sustainable and environmentally conscious practices prevalent during the Renaissance and Baroque periods.

While egg tempera has been partially replaced by more modern binding mediums over time, its significance in the works of Old Masters cannot be overstated. The egg yolk medium continues to be appreciated and studied for its unique characteristics, contributing to the enduring beauty and historical value of these masterpieces.