Deterioration thresholds and pathways of cinnabar and red lead pigment layers in salt-damaged wall paintings

Soluble salts are a major threat to historical wall paintings, yet the response of pigment layers to salt exposure remains unclear. In this study, cinnabar (HgS) and red lead (Pb 3 O 4 ) mock-ups were treated with graded salt concentrations, artificially aged, and characterized by complementary techniques. The pigment layers showed distinct deterioration thresholds and pathways. Cinnabar discolored at 0.5% salt content and deteriorated severely above 3%, mainly through binder degradation, while the pigment remained largely stable. Red lead was more salt-sensitive, showing surface instability, oxidation, binder degradation, and visible alteration products from 0.5%, with pronounced chromatic and microstructural damage above 2%. Pb (II)-containing carbonate products were detected to at least 100 μm depth. Overall, salts mainly affected the binder in cinnabar, but altered both binder and pigment in red lead, suggesting that levels below 0.5% are critical for limiting deterioration. These findings support conservation risk assessment and desalination planning for salt-damaged wall paintings.