Effect of Tension Loading on the Degradation Mechanism of Silk for Ancient Thangka Conservation

The protection of ancient silk Thangkas holds significant importance for the study and preservation of Tibetan Buddhist culture. To investigate the degradation mechanism and evaluate the impact of long-term gravitational loading on silk degradation, additional tensile stress was applied to raw silk and ripe silk threads during the accelerated light-aging process. Morphology, mechanical properties, structural characteristics, and amino acid composition of the silk samples were analyzed by SEM, Instron Universal testing machine, FTIR, amino acid analysis, and XRD. The results demonstrated that the degradation mechanism of silk fibers aged with tension loading aligned with those aged without tension loading. Nevertheless, the tension loading during the light-aging significantly accelerated degradation rates, particularly in sericin-free ripe silks. Both tensile stress and breaking elongation of the fibers exhibited accelerated decreases under tension loading. Besides, the maximum secant modulus at ε_m was achieved at earlier aging stages for the tension-loaded silk fibers compared to non-loaded controls. The silk fibers with tension loading during the light-aging process presented larger decline of β-sheet structure and crystallinity. Both β-sheet content and crystallinity of the silk had a good correlation with the modulus of silk fiber, and the Tyr content indicated the conservation state of raw silk light-aged with tension loading. This study provides insights into assessing aging in historical silk subjected to prolonged gravitational loading and offers scientific evidence for conserving ancient silk-based Thangka artifacts.