Drying mechanisms in historical egg–oil emulsion binders: a combined ATR-FTIR and unilateral NMR study

In the 20th century, painters adopted emulsion-based paints, associating their distinctive rheological and drying properties with the recipes of the Old Masters. These systems dry through a combination of physical and chemical processes, including water evaporation and oxidative cross-linking of unsaturated triglycerides. In this study, we investigate how the composition and structure of oil-in-water (o/w) and water-in-oil (w/o) emulsions affect the drying kinetics and molecular transformations in the resulting paint films, with lead tin yellow as a pigment. Time-resolved ATR-FTIR spectroscopy and low-field NMR relaxometry are combined to monitor both the evaporation of water and the evolution of molecular mobility during cross-linking. We show that the nature of the continuous phase determines the drying pathway: o/w emulsion-based paints initially dry within minutes due to water evaporation, whereas w/o emulsions paints show extended drying times, governed by autooxidation and polymerization. In emulsions prepared with raw linseed oil, the presence of egg yolk, in the continuous or dispersed phase, significantly delays cross-linking. Conversely, this delay is not observed when lead-treated linseed oil is used suggesting an interaction between lead species and antioxidant compounds in egg yolk. Spatially resolved NMR measurements also reveal the development of molecular mobility gradients across the film thickness when oil is the continuous phase, linked to a gradient in molecular oxygen within the film. • ATR-FTIR and unilateral NMR are complementary to monitor drying paint film processes. • Egg yolk delays the oil cross-linking when raw oil is used in emulsion-based paints. • Lead-treated oil with lead tin yellow prevents the antioxidant effect of egg yolk • Drying pathway depends on the emulsion structure (o/w vs w/o). • Unilateral NMR reveals gradient mobility within the LO and w/o-LO films.