Multi-scale structural deterioration analysis of archaeological wood from the Shahe Ancient Bridge site in Xi'an, Shaanxi, China

Due to the unstable variations in moisture content, the wood piles near the ground surface at Shahe Ancient Bridge site have undergone more severe degradation. In this study, two representative wood piles from the site were selected as samples to analyze their deterioration from a multi-scale perspective. The findings suggested that the primary microorganisms of the wood piles contained not only white rot and brown rot fungi commonly found in dry archaeological wood, but also soft rot fungi and erosive bacteria commonly found in waterlogged archaeological wood. The hemicellulose and non-crystalline cellulose were almost completely degraded, and even the cellulose in the crystalline zone underwent some degradation. The microfibrils were fragmented and disoriented. The beta-O-4 and beta-beta bonds in lignin were partially broken, resulting in a decrease in the molecular weight of the lignin. Compared to G-type lignin, S-type lignin demonstrated a higher susceptibility to degradation, potentially explaining why Phoebe sp. wood underwent more extensive degradation than Abies sp. wood. Archaeological wood was not rich in mesopores compared to the reference wood. The results of this study contribute to the understanding of the deterioration state of the Shahe Ancient Bridge site and provide a fundamental understanding for its subsequent protection.