Monitoring the aging process in egg-tempera paint films

Tempera painting, a predominant pictorial technique during the Middle Ages, used a variety of aqueous binders, egg yolk being the main one for easel paintings. In an effort to replicate the aging process, multiple mockup paints based on egg yolk tempera, incorporating selected pigments such as lead tin yellow, azurite, and Sienna, were exposed to visible or UVA light for up to 130 days. The chemical transformations within the lipoprotein functional groups of yolk were closely monitored using FTIR spectroscopy throughout the aging process in order to study the reactions leading to the spectra recorded on naturally aged films and historical samples. Lipids and proteins undergo oxidation under complex pathways illustrated by the absorption band evolutions (especially CH₂ and Amide I elongations). Notably, in the presence of pigments, the observed chemical changes display a comparable but accelerated pattern. Decomposition of the amide I band of both unpigmented and pigmented tempera paints IR spectra revealed conformational changes in proteins during the aging process. In addition, thermal analysis suggested chain shortening revealed by lower degradation temperatures. This investigation sheds light on the intricate evolution of tempera binder and its components over time.