Effect of pressure drying and pressure steaming on warp and stiffness of radiata pine lumber

Matched 100- by 40-mm sawn lumber was dried in three kiln runs: run 1: dry 160/100 degrees C, steam 100/100 degrees C (high-temperature (HT) control); run 2: dry 160/100 degrees C, pressure steam 130/130 degrees C; run 3: pre ssure dry 160/130 degrees C, pressure steam 130/130 degrees C. Times for pr essure drying and normal HT schedules were similar but pressure steaming re quired only 1 hour compared to 3 hours at normal settings. The pressure tre atments caused noticeable darkening but less residual stresses. Compared to the HT control, normal HT drying/pressure steaming gave 25 percent lower t wist and approximately one-third less rejection for twist in excess of pre- set limits. Combined pressure drying/pressure steaming gave no improvement in warp as compared to normal HT drying/pressure steaming. Machine stress g rading indicated that the material subjected to both pressure drying and pr essure steaming was approximately 12 percent stiffer compared to the HT con trol material. However, small-clear results indicated that the material sub jected to both pressure drying and pressure steaming was approximately 8 pe rcent less stiff. Mean bending strength and mean compression strength were not significantly affected. In conclusion, pressure treatments are unlikely to have a noticeable effect on the strength of structural timber. The vari ation in the structural timber properties due to defects such as knots, slo ping grain, etc, should mask any effect caused by the pressure treatments. It is concluded that pressure steaming after normal HT drying will signific antly improve drying quality and the process could be easily implemented co mmercially.