Characterization of Freeze-Thaw Treatments Upon Binders in Ancient Chinese Wall Paintings by X-Ray Diffraction (XRD) and Attenuated Total Reflection – Fourier Transform Infrared (ATR-FTIR) Spectroscopy

Abstract

This study investigated the physico-chemical properties of the binders used in ancient Chinese wall paintings under repeated freeze–thaw cycles. Gum Arabic, cow bone, and fish bladder glue samples were prepared before and after three, six, and nine freeze–thaw cycles. The surface morphology, crystallinity, and spectral characteristics were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. The secondary structures of the protein binders were also analyzed before and after thaw–freeze cycles. The results show that the surface morphologies of the gum Arabic, cow bone, and fish bladder glue gradually changed from undulating to smooth as the number of freeze–thaw cycles increased. The intensities of the XRD peaks and crystallinity of the binders gradually decreased after six freeze–thaw cycles. ATR-FTIR spectroscopy indicated that no significant changes occurred in the original chemical compositions of the binders after multiple freeze–thaw cycles. The wavenumbers of the characteristic peaks remained unchanged but the intensities of the peaks changed significantly. Analysis of the secondary protein structures indicated that the α-helix contents of the cow bone and fish bladder glue samples decreased significantly, whereas the parallel β-sheet and random coil contents increased.