Variation in the sorption properties of historic parchment evaluated by dynamic water vapour sorption

Understanding the hydration of the collagen in the historic parchment structure is of great importance to the conservation and restoration processes. In this study, modern and historic (dated 1817 and 1769) parchments were investigated using dynamic water vapour adsorption/desorption experiments.The relationship between the equilibrium moisture content against the relative humidity at constant temperature for two consecutive sorption cycles, as well as the hysteresis and kinetic properties were analysed in terms of the structure-water interactions and the influence of the ageing. It was found that historical parchment samples exhibited higher equilibrium moisture content levels throughout most of the hygroscopic range, except for the adsorption curve above 90%, and the hysteresis was greater than that of the new sample. Also, the hysteresis recorded in the first sorption cycle consists of two components, while in the second cycle the higher RH component is no longer seen. By applying the parallel exponential kinetic model for describing the sorption kinetics, it was observed that the difference in the hysteresis is apparently mostly due to the parchment polymer matrix relaxation.