Light-Induced Degradation of Cinnabar and Red Lead Paint Layers in Historical Wall Paintings: A Quantitative Evaluation of Light-Exposure Limits

The degradation of cinnabar and red lead paint layers under light exposure poses a critical challenge to the long-term preservation of historical wall paintings. Understanding the light-induced degradation processes is essential for regulating light environments and mitigating risks to cultural heritage. However, the intrinsic relationship between the degradation processes of cinnabar and red lead paint layers and light intensity remains unclear. To address this issue, this study employed multiple analytical techniques (scanning electron microscopy-energy dispersive spectrometry, colorimetry, X-ray photoelectron spectroscopy, Raman mapping, and Fourier transform infrared spectroscopy) to quantitatively examine the physical and chemical changes in paint layers (pigments mixed with rabbit skin glue binders) of mock-up wall paintings, revealing for the first time the degradation processes under varying light intensities. The findings indicate that maintaining light intensity below 2000 lux serves as a critical threshold to effectively minimize color differences, micro-morphological changes, oxidation, and degradation in cinnabar paint layers, thereby preserving their surface and internal chemical stability. In contrast, red lead paint layers exhibit extreme sensitivity to light exposure, with significant surface color differences and micro-morphological changes occurring even at a light intensity as low as 1000 lux. These changes are accompanied by the gradual disintegration of the chemical structure, severe oxidation and degradation, and the formation of altered products on the surface, which progressively penetrate into the interior of the paint layers. These findings could provide an evidence-based reference for optimizing light conditions in the preservation of wall paintings, highlighting the need to preserve red lead paint layers under low illuminance environments.