Rheological and structural characterization of emulsion-based paints

Paints obtained by dispersing pigments in emulsions of egg yolk and vegetable oil have been used by 20th century artists, likely drawing on techniques dating back to the 15th century. Understanding their flow behavior is key to shedding light on artists' material choices, gestures, and the resulting visual effects. In this study, we investigate two modern recipes that yield either oil-in-water (o/w) or water-in-oil (w/o) emulsions, formulated with egg yolk and linseed oil-raw or cooked with lead oxide following a historical process designed to accelerate drying. Stable, homogeneous paints were successfully prepared by dispersing lead-tin yellow pigment into these emulsions. The volume fraction of the dispersed phase, which includes solid pigment particles and emulsion droplets, emerged as a critical parameter for stable shear flow. Higher volume fractions resulted in increased yield stress, consistency, and viscosity across a wide range of shear rates. Dynamic rheological measurements further revealed the significant influence of the nature of the continuous phase on the viscoelastic properties of the paints. Finally, confocal microscopy on model systems (using glass microspheres as pigment analogs) showed preferential localization of particles within one emulsion phase. These observations offer new insights into how preparation protocols influence the microstructure-and subsequently the working properties-of historical emulsion paints.