Tempera and Tempera Grassa─From Wet Paints to Solid Films

Abstract

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.