Swelling of Springwood and Summerwood in Softwoods

Specimens of southern yellow pine, Douglas-fir, and redwood were investigated for differential movement of surfaces with changing moisture content. Dry boundaries were marked with datum lines to avoid the effects of optical illusion in wet wood. In the tangential direction, summerwood swelling averaged 5.3 percent and springwood averaged 4.8 percent. In the radial direction, summerwood swelling averaged 10.l percent while springwood averaged only 2.4 percent. Raised grain can occur both as a result of differential shrinkage and swelling and as a result of the recovery of mechanically compressed springwood fibers. Paint coatings crack, not from differential movement in the wood, but from their own internal stresses as they become embrittled by weathering and lose their initial specific adhesion for wood. Adhesion is less in the small cavities of summerwood than in the larger cavities of springwood, so flaking occurs first over wide summerwood bands.

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