Damage evaluation of binder materials in traditional architectural polychrome paintings under coupled light–humidity conditions

Architecture polychrome paintings are exposed to fluctuating light and humidity, which may degrade binders and weaken color stability and interlayer adhesion. This study examined bone glue and gelatin using accelerated aging under nine spectral conditions, four relative humidity (RH) levels, and increasing exposure doses. FTIR spectra were analyzed by PCA to derive an integrated damage indicator (|ΔPC 1 | ). Damage showed nonlinear behavior, with 405–439 nm irradiation and RH ≥ 60% producing strong synergistic effects. Bone glue was 1.7–2.5 times more sensitive than gelatin. A nonlinear prediction model (R 2 > 0.92) was established for binder damage under coupled light–humidity exposure, and a humidity-dependent spectral damage model was developed by incorporating spectral power distribution. A case study of heritage buildings showed that 3000 K LEDs caused lower relative damage at moderate RH, whereas halogen lamps performed slightly better at low and high RH. These results support quantitative lighting damage assessment and preventive conservation.