FIGURE 245.—Rapid changes in the dryness of lumber may be largely prevented by compact piling after seasoning
WOOD Checking and Weathering Can Be Prevented by Paint
The checking and weathering of wood can be prevented to a large degree by keeping the wood well painted or by storing it under cover, or by both. Such care of wood prevents rapid changes in moisture content because wood is hygroscopic; that is, when wood dries it shrinks; when it absorbs moisture it swells. Shrinkage of a flat-sawed board is considerably more in the direction of the width than in the direction of thickness. The stresses and strains set up by this unequal shrinkage cause a lengthwise separation of the wood known as seasoning checks. By retarding rapid-surface drying the tendency to checking, especially at the ends of thick stock, is greatly reduced.
Wood exposed to damp weather or rain will absorb moisture on the surface and the surface layers will try to swell. This swelling is resisted by the dry interior of the wood, with the result that a squeezing action, which tends to give the surface layers a permanent distortion, is set up. Later, when the surface dries out again, and attempts to shrink, a pulling action takes place, which may result in surface checks. If this process is repeated often enough weathering of the piece may take place. The injurious effects of changes in moisture content may be minimized, however, by retarding the rate of change. As already stated, this may be done by the use of paint or other moisture-resistant coatings, or by storing the wood under cover, or by both.