Time through colors: A kinetic model of red vermilion darkening from Raman spectra

Darkening of Red Vermilion (cinnabar) is a well-known phenomenon which needs, at the moment, a full comprehension of its nature. Some paintings reveal a disfiguring process of blackening degradation of Red Vermilion, albeit other relics preserve the original color during the time. The presence of halide elements like Cl− ions, in addition to the light exposure, represents the most alleged cause to explain the darkening phenomenon. However, the real effect of chlorine impurities is still not completely understood. In this study, starting from a multi-technique characterization, we propose a kinetic model of mutual composition of alpha/beta cinnabar derived by the experimental darkening of pure alpha synthetic cinnabar intentionally doped with Cl− ions and treated under prolonged UV light exposure. The model was further applied to ancient samples by analyzing the Raman spectra of antique cinnabar pigments belonging to manuscripts of XIII-XVII centuries.