Characterization of degradation and iron deposits of the wood of Nanhai I shipwreck

Abstract Nanhai I shipwreck was a large wooden merchant ship (22.15 m in length and 9.85 m in width) built in the Southern Song Dynasty (1127–1279 A.D.) of China, which was heavily loaded with a large number of porcelain and iron artifacts. In the South China Sea, it was found in 1987 and lifted as a whole in 2007. Its excavation provides a precious opportunity to reveal the preservation status of Nanhai I shipwreck. Iron compounds give rise to challenges in conservation procedures and the long-term stability of Nanhai I shipwreck. In this paper, the degradation of the wood and the iron deposits in the wood structure are investigated from the aspects of microscopic morphology, composition, and distribution to evaluate the preservation state of the waterlogged wood. Physical parameters, chemical composition, and the results of elemental analysis, and FTIR analysis suggest that the cellulose of wood is degraded, and the relative concentration of lignin increases. The ash content varies greatly among different samples, and the element of the highest content in ash is iron. The study on transverse and longitudinal sections from samples indicate that the deposits are unevenly concentrated in the cell lumen, middle lamella, rays, and pits of the cell wall. The cell lumen is filled with deposits in areas close to the surface of the samples. The XRD analysis demonstrates that the deposits in wood are mainly iron deposits, containing compounds pyrite (FeS2), siderite (FeCO3), iron oxyhydroxides (FeOOH), and magnetite (Fe3O4). The micro-X-ray Fluorescence mapping analysis suggests that the content of iron is relatively richer while containing less sulfur on the exterior of the sample. The presence of iron deposits accelerates wood degradation and increases the safety hazards of shipwrecks in the preservation process. We hope that our findings can make a modest contribution to iron removal from waterlogged archeological wood and shipwreck conservation.