Wood–Water Relations of Thermally Treated Pleated Beech and Oak Wood

Pleating (20% compression) is a thermo-hydromechanical modification that improves wood pliability but markedly increases longitudinal dimensional instability. This study evaluates the combined effects of pleating, fixation (maintaining the compressed state for 5 h), and subsequent thermal treatment (160 and 200 °C) on the wood–water relations of beech (Fagus sylvatica L.) and sessile oak (Quercus petraea (Matt.) Liebl.). Density, equilibrium moisture content, swelling, shrinking, anisotropy, and anti-swelling efficiency were determined over two soaking–drying cycles. Thermal modification substantially reduced equilibrium moisture content from approximately 10.3%–10.4% in untreated wood to 7.8%–8.0% at 160 °C and 4.9%–5.0% at 200 °C. In contrast, mechanical modification largely mitigated the density loss caused by thermal treatment. Although pleating and fixation greatly increased longitudinal dimensional changes, thermal treatment effectively compensated for this effect. Treatment at 200 °C provided the greatest dimensional stability but was accompanied by more extensive chemical degradation. The results demonstrate that combining pleating with moderate thermal treatment may offer an effective, chemical-free approach to improving the dimensional stability of compressed hardwoods.