A holistic view on the role of egg yolk in Old Masters’ oil paints

Abstract

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.

Tempera and Tempera Grassa─From Wet Paints to Solid Films

“…We can detect the presence of peroxide species and other active radical species with differential scanning calorimetry (DSC). DSC of curing of oil paintings shows an exothermic peak at temperatures below 200 °C, which is due to the overall superposition of two phenomena: the decomposition of peroxides (endothermic) and the recombination of radicals (exothermic) . Note that evaporation of low-molecular-weight compounds also occurs <200 °C, as described above, but in DSC experiments, it is not observed as the pans where measurements are carried out are sealed, impeding evaporation.…”

“…Due to the shear-thinning effect, the paint can be brushed nicely onto the painting. Nevertheless, the paint remains soft with a limited yield stress, thus a brushstroke can be maintained, but no pronounced impasto is possible if we compare it, e.g., to oil paints forming a strong capillary network …”

“…Nevertheless, the paint remains soft with a limited yield stress, thus a brushstroke can be maintained, but no pronounced impasto is possible if we compare it, e.g., to oil paints forming a strong capillary network. 18 All in all, tempera and TG paints exhibit similar flow properties: shear-thinning behavior is observed at intermediate disperse phase volume fraction, and when a critical disperse volume fraction φ c is exceeded, also a yield stress occurs. To achieve similar flow behavior, the total volume fraction of the disperse phase (including pigment, EY, and oil) of a TG must be higher than that of a tempera paint (containing only pigment and EY).…”

“…The kinetics of water loss was monitored gravimetrically, and the change in hardness of the paint surface was tested at regular time intervals pushing a cylindrical piston into the layer at constant speed and depth, imitating the dipping of a finger into the paint. 18 Parallel experiments were carried out on paint layers of the same thickness and composition. Typical results for UB EY-poor TG paint layers are shown in Figure 9.…”

“…In addition to a TG, two oil paints were prepared, the first one called protein-coated pigment (PCP) and the other capillary suspension (CapS). PCP and CapS are oil-based paints prepared as described in Ranquet et al 18 Briefly, in PCP, the pigment particles are uniformly coated by a layer of EY and are subsequently dispersed in the oil, which is the continuous phase. In CapS, the oil paint is added with small amounts of egg, which forms capillary bridges among pigment particles: EY is unevenly distributed in the contact regions between adjacent particles.…”