Free shrinkage characteristics of <i>Eucalyptus urophylla</i> × <i>E. grandis</i> wood prone to collapse

Abstract Free shrinkage is a natural characteristic of wood during drying, occurring when the moisture content (MC) drops below the fiber saturation point (FSP). However, achieving complete free shrinkage in practical processes is challenging due to drying stresses from varying rates between the surface and core layers. Eucalyptus wood is particularly prone to collapse above the FSP, complicating the free shrinkage process. This study investigates the free shrinkage characteristics of 2-mm thick Eucalyptus urophylla × E . grandis specimens, both sliced along the fiber direction and split tangentially (layered). The findings are as follows: Layered specimens exhibited significant shrinkage ratios (5.7–6.7 %) and equilibrium moisture content (EMC) values (19.0–22.7 %), which can be attributed to the differing anatomical structures across layers. In contrast, sliced specimens did not display a MC gradient during drying, but their shrinkage was somewhat constrained by adjacent layers. Consequently, layered specimens had fewer radial and tangential restrictions, allowing more pronounced unrestricted shrinkage. Also, the slight collapse in smaller sliced specimens had a negligible impact on free shrinkage. To achieve free shrinkage without collapse, it is essential to use small-sized specimens in both fiber and radial directions while maintaining low temperatures and high relative humidity (RH) conditions.