Some Variables Affecting the Shrinkage of Western Hemlock

The shrinkage of western hemlock during drying was measured by a specially constructed instrument that kept a continuous record of the change. The instrument was so constructed that shrinkage and moisture content could be determined without removing the samples from the drying chamber. Three drying schedules were used: 120?F. – 45 percent RH, 160?F. – 55 percent RH, and 200?F – 66 percent RH. Both high (ave. 151 percent) and low (ave. 63 percent) moisture content boards were subjected to the three schedules. The study showed that high moisture content boards shrank rapidly at elevated temperatures at the beginning of the drying period. However, this accelerated shrinkage did not apparently cause an abnormal amount of degrade. Approximately 25 percent of the total shrinkage occurred above the fiber saturation point under the above conditions. The amount of shrinkage occurring above the fiber saturation point varied with the initial moisture content. The shrinkage rate at all three drying schedules was more uniform for the low moisture content boards. Other data tabulated indicated that volumetric shrinkage increased with weight of water per cubic foot of wood, that total shrinkage increased with increases in specific gravity, and that radial shrinkage increased as ray volume increased.

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