Weathering behavior of cinnabar-based tempera paints upon natural and accelerated aging

The darkening process of cinnabar and vermilion-based paint surfaces as well as pigment-binder interactions upon weathering have been studied by subjecting egg-yolk tempera dosimeters to UV-aging, RH cycling, and 2-year outdoor weathering with and without direct exposure to rain and sunlight. X-ray photoelectron spectroscopy proved that photooxidation resulted in Hg enrichment in the cinnabar pigment surface of weathered paints exposed to direct sunlight and rain, which caused important binder loss as well as thermal-induced crack formation and severe darkening of the unprotected pigments. Remarkably, darkening occurred in the absence of halogens and all pigments, independent of their preparation process (ground minerals or synthesis by a wet-process), were affected. After UV-aging, RH-cycling, and outdoor weathering in the absence of direct sunlight and rain, in contrast, pigments were still covered by egg yolk and did not undergo significant alteration. The binder, however, suffered important conformational and physical changes, including crack formation. These changes were significantly influenced by the size of the pigment. Additionally, XRD results indicate the presence of metacinnabar as an impurity in the original cinnabar and vermilion pigments, which might have been mistakenly identified as an alteration product in previous studies.