Medium- and Long-Term Drying Effects in Tempera Paint Films Explored Through NMR Relaxometry Analysis

The drying of egg yolk tempera paint in the dark has been investigated from 24 h up to 2 years by nuclear magnetic resonance (NMR) relaxometry and spectroscopy. Variations around a reference recipe have been carried out to investigate several aspects: the amount of added water to the binder, the nature of the minerals or the pigments and the drying time. The ¹H signal measured here corresponds to the ¹H contained in the fatty acids of the egg yolk (EY). The $R_1$ NMR dispersion (NMRD) profile increases when the added water in the preparation of the tempera is above 50 wt% linked to a destructuration of egg yolk high- and low-density lipoproteins (HDL and LDL). In addition to the chemical nature of the minerals, the shape and the surface area also play an important role in the destructuration of egg yolk HDL and LDL. The most striking result is the discontinuous evolution of the NMRD with time with the appearance of a step around 1 or 2 months. Depending on the water content in the initial tempera, a steep change in $R_1$ is observed while before and after only a weak evolution occurs.