Changes in vibrational properties and colour of spruce wood by hygrothermally accelerated ageing at 95–140 °C and different relative humidity levels

Spruce wood specimens were hygrothermally treated at different temperatures (T, 95–140 °C) and relative humidity during heating (RHh, 0–92%). Their loss in dry mass (ML), specific dynamic Young’s modulus (E′/ρ), mechanical loss tangent (tan δ), and CIELAB colour parameters were measured at 25 °C and 60% relative humidity (RH) before and after the hygrothermal treatment. The changes in physical properties by the hygrothermal treatment were formulated as functions of ML. Those ML dependencies were combined with the ML–time curve at 20 °C previously predicted by using a time–T–RHh superposition, and the changes in physical properties during long-term ageing were predicted. The predicted vibrational properties were stable at 0–80% RH, whereas a significant decrease in E′/ρ and increase in tan δ were predicted at 92% RH, suggesting serious degradation in acoustic quality in humid conditions. The predicted changes in colour reasonably agreed with those during natural ageing. The combination of hygrothermal treatment and time–T–RHh superposition is a useful tool to predict the effects of ageing in ambient conditions, particularly when the target property simply depends on the chemical changes in wood constituents.