Moisture gradients in wood subjected to relative humidity and temperatures simulating indoor climate variations as found in museums and historic buildings

Climate-induced mechanical damage to cultural heritage objects of hygroscopic materials is not yet fully understood. This is particularly true of objects in historic buildings with less climate-controlled indoor environments. Research aiming at clarifying the response of hygroscopic materials to changes of the ambient relative humidity and temperature is scarce. The objective of this study was to use a method to monitor relative humidity and temperature at three different depths inside samples of Scots pine (Pinus sylvestris L.), subjected to relative humidity and temperature fluctuations in a climate chamber. This approach is important because mechanical stress is related to the moisture content of the material. However, the knowledge on how moisture gradients in wood progress before reaching equilibrium has not been studied in depth in cultural heritage science. The monitored relative humidity and temperature data in the wooden samples were converted to moisture content using a method that took both temperature and the hysteresis effect into account. The samples were subjected to step changes and daily relative humidity fluctuations at different temperatures. Moreover, museum climate, a non-heated historic building climate, and intermittent heating of a typical church were simulated in the experiments. Low temperatures reduced the moisture diffusion rate, resulting in moisture content fluctuations of smaller amplitudes. A response delay was noted which gradually increased with depth in the wood and with low temperatures. A combination of a step-change and daily fluctuations increased the time to reach equilibrium due to the slower desorption process compared to adsorption process. Occasionally, the moisture content could also exceed equilibrium at some depths. The moisture content levels in wood during intermittent heating stayed stable overall. The conclusion is that moisture transport in wood is complex and is influenced by the moisture history of wood, as well as duration and amplitude of the combined fluctuations in relative humidity and temperature.