CAUSES OF SURFACE CHECKS IN WOOD IN VARNISH-DRYING ROOMS 240054
Peculiarly complex in its cellular structure, wood is subject to a deformation that accompanies changes in its moisture content that is neither uniform nor isometric. Deformation is generally about 50 times as great in the radial direction of the log as longitudinally and about twice as great circumferentially as radially; so, when moisture changes occur due to changes in the degree of humidity of the surrounding air, the behavior of wood is very uncertain. Conditions are complicated further by the manner in which drying takes place.
A description is given of how water is contained in wood, including details of wood structure, and the action of moisture in causing swelling and subsequent shrinking is discussed. The fiber-saturation point marks the limit of the amount of moisture that can enter between the fibrils, at which limit swelling ceases. It is determined by making endwise compression tests on a series of small blocks of the wood, as its drying proceeds. Water is, therefore, contained in wood as hygroscopic moisture adsorbed in the cell walls and as free-water occurring in the capillary spaces, with the fiber-saturation point as the dividing line between the two conditions.
In describing how wood dries, “set” and “casehardening” are explained, since the problem of the dry-kiln engineer is to control these drying stresses so that the wood shall suffer no serious injury. When drying a green stick of wood in air or in the kiln, surface checks will at first tend to open up as a result of the tensile stress in the outer surface. But when the reversal of stress takes place as the drying proceeds, these surface checks will close again and may even pinch tightly together by reason of the compression of the “set” surface. They may become invisible, even when the lumber is planed. What happens when dry wood having these incipient but invisible surface checks is placed in a varnish-drying room that has a high relative humidity and a high temperature is set forth, and three specific causes of varnish-drying trouble are stated.