Historical photo curling and fracture mainly caused by the surface performance change of gelatin resulting from alternating dry-wet environment.

• The irreversible shrinkage of gelatin film caused by alternating dry-wet cycles. • Gelatin molecules produce intermolecular aggregation during dry-wet cycles. • Molecular conformation changes during intermolecular aggregation process. • Gelatin protein photos should be stored in a relatively stable humidity environment. The curling of historical photographs seriously endangers the information security and utilization value of image cultural heritage. Previous studies have shown that the shrinkage of emulsion gelatin in alternating dry-wet environment is the main cause of photo curling, while the mechanism of shrinkage is still unclear. In this work, the gelatin film samples with different shrinkage rates were firstly prepared through alternating dry-wet cycles. Then, Fourier-transform infrared spectroscopy (FTIR), UV–Vis-NIR spectroscopy, fluorescence spectrophotometer, static video contact angle meter, X-ray diffraction (XRD), scanning electron microscope (SEM), nuclear magnetic resonance (NMR) analyzer, X-ray photoelectron spectroscopy (XPS), circular dichroism (CD) spectrometer, and other instruments were used to analyze the morphology, hydrophilicity, structure, conformation and brittleness of gelatin films with different shrinkage changes. Finally, through the correlation between the molecular structure, surface characteristics and macroscopic properties of gelatin films were obtained, and the mechanism of the curling and brittle fracture caused by the change of gelatin properties under the dry-wet cycles was revealed. This study provides a scientific basis and experimental data for the preservation and restoration of historical photos based on gelatin. [Display omitted] [ABSTRACT FROM AUTHOR]