A comparative study on moisture sorption thermodynamics of ancient and recent cypress

  Objective  This research was carried out to explore the differences and reasons of moisture sorption thermodynamics between archaeological wood and recent wood, and provide a theoretical basis for the protection and research of excavated archaeological wood, and be conducive to improving the dimensional stability of the unearthed moisture-saturated wooden relics.

  Method  Ancient and recent cypress (Cupressus sp.) were used as research objects, and the microscopic morphology and chemical groups of the wood samples were characterized by scanning electron microscope and Fourier transform infrared spectroscopy, respectively. The adsorption and desorption isotherms for both wood at 25 and 50 ℃ were measured by dynamic vapor sorption, and fitted based on the Hailwood-Horrobin moisture sorption theory. The differential adsorption heat Q_S, Gibbs free energy change ΔG and differential adsorption entropy ΔS of the wood samples were calculated by Clausius-Clapeyron formula to analyze the discrepancy of sorption thermodynamics between archaeological and recent wood.

  Result  Compared with recent wood, the cell wall of archaeological wood was decayed obviously. A large number of cell wall pores and mycelia were found. The cellulose and hemicellulose were degraded seriously with increased relative content for lignin. The content of polar group −OH and −COOH decreased. Under certain temperature conditions, the equilibrium moisture content of archaeological wood was higher than that of recent wood, while the values of Q_S and ΔS were lower than that of recent wood, and an inflection point occurred at the moisture content of 5%, corresponding to a saturation of the monolayer water. On the other hand, there was little difference in ΔG between archaeological wood and recent wood. The values of Q_S, ΔG and ΔS of archaeological wood in hygroscopic equilibrium state were lower than those in desorption equilibrium state.

  Conclusion  In this study, it was found that the cell wall structure of archaeological wood decayed after a long time of corrosion. Compared with recent wood, the degradation degree of cellulose and hemicellulose in archaeological wood was the greatest. And the moisture sorption of archaeological wood reduced. Under certain temperature conditions, the hygroscopicity of archaeological wood was higher than that of recent wood, but its thermodynamic values were lower. There were thermodynamic sorption hysteresis of archaeological wood.