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.

Old Masters frequently used paints containing both egg and oil binders to create their paintings. These two binders can be combined in the paint in many different ways resulting in substantially different behaviors of the wet paint but also affecting drying and curing reactions. This paper focuses on paints bound with egg (tempera paints), and the influence of added oil on microstructure, rheology, drying kinetics, and chemistry is discussed. Such egg tempera paints with oil are called fatty tempera, or tempera grassa (TG), and are sometimes believed to play an intermediate role between oil and tempera paints. Despite their hydrophobic nature, pigments do not enter into the oil droplets emulsified in a TG due to the adsorption of egg proteins on the pigment surface. We further show that the flow behavior of both paint types can be described using classical suspension rheology concepts. When combining the pigment, the egg yolk, and the oil into the calculation of the disperse phase volume fraction φ, minor differences in viscosity and onset of percolating network formation are attributed to the broader particle size distribution for TG. The dry-to-touch is controlled by the evaporation of water in both cases and reached within minutes. The added oil does not change the painting behavior in general; however, different brushability is observed on absorbent substrates when water is lost quickly and the remaining oil in TG keeps the paint soft. After the dry-to-touch state is reached, the oil cross-linking sets in, leading to a second hardening step. Egg components exert an antioxidant effect of the oil, which does affect not only the kinetics of the curing process but also its chemistry and is dependent on the microstructure of the paint film.