Thermal expansion of wood at different equilibrium moisture contents

The measurement of wood thermal expansion at fixed values of moisture content (MC) between the dry state and the cell wall saturation point is a very difficult task, as MC varies with temperature. Being hygroscopic deformations much larger than thermal ones, in order to evaluate the latter, when changing temperature also relative humidity should be adapted. To achieve this goal a special apparatus was designed to vary relative humidity as a function of temperature to keep MC constant, and thus to measure the thermal expansion coefficient without hygroscopic effects. The moisture content was assessed to remain constant by testing the heating cycle of each specimen, keeping the specimen itself on a precision balance before the thermal expansion measurement. The radial coefficients of thermal expansion of ten specimens of Norway spruce were measured at 7.0% and 11.4% MC. No statistically significant differences were found between the two data sets, indicating a negligible effect of moisture content on wood thermal expansion at MC values typical of indoor environments. However, the thermal expansion coefficients measured at 7.0% and 11.4% MC seem to be higher than the values for dry state reported in literature, indicating an effect of the moisture content.