Traditional understanding of Old Masters' oil paints often focuses solely on oil as the binding medium, perhaps with resins. However, recent analyses have revealed the presence of proteins in paintings by artists such as Botticelli, da Vinci, and others, spanning various centuries and geographic locations. The exact role and reasons for incorporating these proteins remain unclear, as the technical knowledge of Old Masters' paint preparation is largely lost. The transition from egg tempera to oil painting in the 15th century Italy, exemplified by wrinkling in some of Leonardo da Vinci's early works, highlights the challenges faced by artists adapting to oil paints. While oil paints had been known for over a century, their use in monochrome applications on precious metals differed greatly from the complex blending and layering techniques of the Renaissance. This study hypothesizes that protein additives, like egg yolk, played a crucial role in modifying oil paints' properties, enabling the sophisticated handling and techniques characteristic of the Old Masters' work. The paper aims to investigate the impact of egg yolk on paint rheology (flow behavior), drying/curing, and long-term stability, providing insights into the Old Masters' techniques and contributing to the preservation of artworks.

Existing literature shows evidence of protein additives in Old Master oil paintings, but detailed information regarding the preparation techniques and effects of these additives remains scarce. The authors cite previous studies detecting proteins in paintings from different periods and regions, highlighting the widespread but poorly understood use of protein binders. While historical treatises like Cennini's provide recipes for pigment preparation sometimes involving materials like egg, glue, or gums, these are often associated with aqueous binders, not oil paints. One exception is a recipe from the *Liber diversarum arcium*, which explicitly links protein addition to oil painting. However, this recipe is specific to blue pigments and the broader implications for other pigments remain unexplored. The authors emphasize the lack of systematic investigation into the effects of protein additions on oil paint properties, necessitating this present study.

This study systematically investigates the effect of egg yolk on the properties of oil paints. The researchers used linseed oil as the binding medium and two pigments—lead white (LW) and ultramarine blue (UB)—known for their historical use and contrasting properties. Three model paint systems were prepared: (1) oil paint (pure pigment dispersed in oil); (2) capillary suspension (CapS), where a small amount of egg yolk was directly added to oil paint; and (3) protein-coated pigment (PCP), where the pigment was pre-coated with a diluted egg yolk solution before dispersion in oil. These systems were chosen to represent different potential approaches employed by Old Masters. The authors used a combination of rheological measurements (yield stress and high shear viscosity) to analyze the wet paint's flow properties and evaluate brush ability and impasto. They also employed gravimetric analysis, differential scanning calorimetry (DSC), thermogravimetry (TG), and pyrolysis-gas chromatography-mass spectrometry (Py/GC/MS) to investigate the drying/curing kinetics, oxidation, and cross-linking processes in the paints over time. Furthermore, contact angle and interfacial tension measurements were conducted to study interactions between oil, egg yolk, and pigments. Finally, analysis of paint film roughness was performed using an automated brush application and image analysis. The researchers performed experiments on paints at various pigment volume concentrations and egg yolk additions to investigate the influence of these parameters on paint properties. The impact of humidity was systematically studied to assess its influence on paint yield stress.

The study revealed significant effects of egg yolk on oil paint properties depending on the preparation method: 1. **Impasto and Yield Stress:** CapS paints, prepared by adding a small amount of egg yolk to oil paint, exhibited significantly higher yield stress than oil paints or PCP paints. This increased yield stress enables the creation of pronounced impasto, as seen in preserved brushstrokes. The high shear viscosity, however, remained comparable to binary suspensions, ensuring good brushability. 2. **Humidity Effects:** Lead white (LW) oil paints showed a dramatic increase in yield stress upon water uptake from the environment, potentially leading to undesirable paint stiffening. However, this stiffening effect was suppressed in LW-PCP paints, where the egg yolk coating on the pigment particles prevented the formation of capillary suspensions. Ultramarine blue (UB) paints were not affected by humidity. 3. **Drying Kinetics:** Egg yolk acted as an antioxidant, significantly delaying the drying time of both UB- and LW-based paints, particularly in PCP paints. The antioxidant effect was attributed to components of egg yolk, such as phospholipids, proteins, and carotenoids, which inhibit lipid oxidation. 4. **Oxidative Degradation:** Analysis of cured paint films suggested that the presence of egg yolk, especially in PCP paints, may lead to the formation of a more stable, cross-linked oil network less prone to oxidative degradation. The delay in curing due to antioxidant activity may promote this. 5. **Wrinkling:** Wrinkling in paint films is closely related to the yield stress and pigment loading. Adding egg yolk increased the yield stress, thereby suppressing wrinkling, even at lower pigment loadings.

The findings of this study directly address the research question by demonstrating the multifaceted effects of egg yolk in Old Masters' oil paints. The ability of egg yolk to influence yield stress, humidity resistance, drying time, and oxidative stability offers compelling explanations for its inclusion in historical oil paints. The ability to modify paint properties allows for specific artistic effects, including impasto and the prevention of wrinkling. The antioxidant properties of egg yolk offer a mechanism to improve the long-term preservation of artworks. These results have significant implications for conservation science and art history, providing a deeper understanding of the technical choices made by Old Masters and informing future conservation efforts. The research highlights the complex interplay between pigment type, egg yolk distribution, and environmental factors in determining the final paint properties.

This multidisciplinary study provides a holistic understanding of the role of egg yolk in Old Masters' oil paints. The ability of egg yolk to tailor the rheological properties, drying kinetics, and long-term stability of oil paints provides a plausible explanation for its use. Further research could explore other proteinaceous materials, other pigment types, and the impact of variations in egg yolk composition on paint properties. The study's insights contribute to both art historical understanding and conservation science.

The study focuses on a limited number of pigments and preparation methods. The findings may not be fully generalizable to all pigments and historical practices. The model paint systems, while aiming to replicate historical techniques, are simplifications of complex historical processes. The precise historical preparation methods of Old Masters remain uncertain, and the study's conclusions should be considered within this context.