Relative Density, Equilibrium Moisture Content, and Dimensional Stability of Western Hemlock Bark
Keywords:Inner bark, outer bark, specific gravity, shrinkage, western hemlock
The measurement of western hemlock bark samples from three coastal sites in British Columbia revealed that inner bark relative density (0.382) is less than that of the adjacent sapwood (0.413) and markedly less than that of outer bark (0.463). The equilibrium moisture content of the inner and outer bark are equivalent at both 70 and 30% relative humidity, and slightly higher than that of the sapwood.
The generally higher shrinkage of bark compared with wood is the result of bark cell collapse during drying. In the outer bark, some collapse or crushing takes place in the standing tree. This compacting of tissue reduces the shrinkage of outer bark relative to the inner bark. The actual shrinkage per unit change in moisture content of the inner bark is the same as that for the sapwood. The outer bark appears to be more dimensionally stable. The longitudinal shrinkage of both inner (2.9%) and outer (2.2%) bark is markedly greater than that of the sapwood (0.1-0.2%).
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