Reduction of growth stress in logs by direct heat treatment: assessment of a commercial-scale operation

Reducing the growth stresses in wood is important in increasing yield during sawing. Many methods have been attempted to reduce these stresses, but few practical procedures have been found. In this study, log air heating by smoking was performed to reduce the residual stresses in fresh logs of larch, fir, Japanese cedar, oak, and ash. The logs were directly heated for 70 hours with high-temperature smoke in a special furnace. The surface released stains and the distribution of released strain across the diameter of the log were measured. Other changes in wood quality were verified as well to evaluate the effectiveness of the treatment. The residual stresses evaluated by the released strain were reduced by half or even 100 percent of the original values. The residual stresses were reduced in the normal wood and the reaction wood as well. It was determined that the moisture content was reduced, especially in sapwood, but that it should not be reduced below the fiber saturation point, to avoid surface checks. Within suitable treatment conditions, the bending strength and the modulus of elasticity are not reducer appreciably. The degree of cellulose crystallinity of the cell wall was increased slightly and the EMC at 20?C and 65 percent relative humidity lowered. These changes improved the dimensional stability of the wood, especially in the case of fir, but its color became darker after the treatment. Finding the best schedule for individual species will make the treatment effective for pre-drying and the improvement of lower quality wood.

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