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Tempera paint solidification followed by single-sided NMR: An egg-cellent article that will dry you up

 
The drying of an aqueous paint is an important criterion, since it determines not only the aesthetic aspect of the pictorial layer, but also its homogeneity and therefore the paint properties (especially its mechanical properties) and its durability over time. This article focuses on the physical drying of egg tempera paints, the most widely used technique in the Middle Ages. The egg yolk binder forms a film as it dries, which holds the pigment to the substrate. In the present study, the evaporation of water is monitored temporally and spatially using single-sided nuclear magnetic resonance (NMR), a non-invasive and non-destructive technique. Relaxometry is used to describe changes in the mobility of the different species in the yolk (mainly water and lipids) during the drying process. Inhomogeneities in the binder and paint films based on different pigments (azurite, lead‑tin yellow, sienna) are highlighted. Other changes in molecular mobility are recorded on longer time scales as a result of the chemical drying of egg yolk. This study provides the first characterization of the microstructure of tempera paints during the drying process and contributes to a better understanding of this artistic technique.