Characterization of the Influence of the Particle Size of Pigments on Proteinaceous Binders Used in Ancient Wall Paintings Following Photo and Thermal Ageing

This study evaluated the effects of photothermal aging and the particle sizes of pigments upon the proteinaceous binder used in ancient wall paintings. Changes in the morphological characteristics, thermal properties, spectral characteristics, and protein secondary structure for pure and mixed samples after aging were studied by laser particle size analysis, scanning electron microscopy (SEM), thermogravimetric analysis with differential scanning calorimetry (TGA-DSC), and attenuated total reflection – Fourier transform infrared spectroscopy (ATR-FTIR). The morphology was flatter for the mixed samples with smaller particle sizes, while those with larger particle sizes were filled with interstices, and severe cracking and flaking appeared after aging. By comparison with pure rabbit skin glue, TGA-DSC showed that the thermal stability decreased for the mixed samples as the particle size increased. ATR-FTIR demonstrated that the intensities of the absorbance peaks for the glue increased as the size of the azurite pigment particles decreased. The spectra for the mixed samples after aging were blue-shifted for amide I and red-shifted for amide II, where the shifts were higher as the particle size decreased. In addition, the infrared peak fitting confirmed that the protein secondary structure contents significantly changed after aging and the changes were correlated with the sizes of the particles.