INFLUENCE OF SAWING ORIENTATION ON MOISTURE MOVEMENT THROUGH SOFTWOOD BOARDS

Abstract

Earlywood, formed at the beginning of the growth period each year in temperate regions, has tracheids that are thinner-walled and larger in diameter than those laid down later in the season, the latewood. As a consequence, the permeability to moisture movement differs, particularly as some of the pits in the latewood tracheids may close or aspirate. Boards sawn from a log will have the orientation of the growth rings (representing seasonal accumulation of woody material) at various angles to the long dimension of the boards. The extreme cases, known as flat-sawn and quarter-sawn, respectively, represent the situations where the growth rings lie parallel to the long dimension or at right angles to it. Tests were undertaken on the drying behaviour of both flat-sawn and quarter- sawn, 50mm-thick sapwood boards of Pinus radiata dried under so-called high-temperature conditions, 120/70°C, with an air velocity over the boards of 5m s-1 in a single-board drying tunnel. Moisture-content profiles were determined by slicing samples after various stages of drying. In the wet core, the moisture distribution was uniform with quarter-board boards, but showed peakiness corresponding to the latewood zones in flat-sawn boards. With either orientation, there is a thin dry layer below fibre saturation at the surface, which in the case of flat-sawn boards is one growth-ring-wide. Quarter-sawn boards dry more slowly than flat-sawn boards due to different drying mechanisms, and these differences in drying behaviour is also reflected in differences in strain development as the wood tries to shrink in conformity with the local moisture content