How do water and acid in marine archaeological wood affect its mechanical properties?

The prolonged burial of marine archaeological wood in seawater mudflats results in the internal saturation of the wood with water, acidification, and salt deposition. Despite the application of desalination, deacidification, and drainage treatments to archaeological wood following excavation, the effects of such environments on wood are irreversible. To understand the effects of moisture content, acidic environments, and insoluble salts (Fe₂S) on the mechanical properties of marine archaeological wood, this study employed molecular dynamics simulations to analyze the tensile performance of wood cell walls under various conditions. The results indicate that, compared to completely dry wood, water molecules can significantly enhance the tensile strength of wood cell walls. However, in systems with a 12 % moisture content, the tensile strength is lower than in systems with a 4 % moisture content. The worst tensile performance of wood cell walls occurs in acidic and salt-deposition conditions. This is mainly due to the fact that an appropriate amount of moisture enhances the interconnections between wood fibers, whereas acidic and salt deposition conditions disrupt these connections. Additionally, the impact of varying quantities of moisture molecules on the mechanical properties of wood also varies. Further exploring the optimal moisture content for the preservation of waterlogged wooden artifacts, using archaeological fir wood as an example, we constructed models of amorphous wood fibers and matrices with moisture content varying from 0 % to 36 %. The results indicate that the system with 8 % moisture content exhibits optimal mechanical performance. Specifically, the shear modulus and chain-direction elastic modulus reach 4.52 GPa and 12.93 GPa, respectively, representing an increase of 25.2 % and 45.1 % compared to completely dry systems. The molecular mechanism by which moisture content influence the mechanical properties of wood was analyzed through parameters such as mean square displacement, diffusion coefficient, and free volume. It is hypothesised that an appropriate amount of water molecules can fill the gaps between fibers, enhancing the inter-fiber bonding and stiffness, and thereby improving the mechanical properties of wood. However, as water molecules continue to be added, this positive effect diminishes. The objective of our study is to gain insight into the mechanical behavior of archaeological wood cell walls from a molecular perspective. This paves the way for multiscale studies to determine the optimal moisture content for preserving wooden artifacts and to identify environmental conditions that can maximize the mechanical performance of archaeological wooden objects.