The kiln drying of lumber; a practical and theoretical treatise . n) when the disk was cut. The explanationof the behavior of these tongues as described above issignificant. In Chapter VI I have stated that drying shouldtake place uniformly from the opposite faces and thatif drying took place from one face only the board orstick would tend to cup—^not with the first dried faceas the concave surface, as might at first be supposed,but just the reverse. The surface which dried firstwill become the convex surface. This phenomenon is exactly what takes place in thetwo parallel tongues of the pronge
The kiln drying of lumber; a practical and theoretical treatise . n) when the disk was cut. The explanationof the behavior of these tongues as described above issignificant. In Chapter VI I have stated that drying shouldtake place uniformly from the opposite faces and thatif drying took place from one face only the board orstick would tend to cup—^not with the first dried faceas the concave surface, as might at first be supposed,but just the reverse. The surface which dried firstwill become the convex surface. This phenomenon is exactly what takes place in thetwo parallel tongues of the pronged disk, and the 120 THE KILN DRYING OF LUMBER explanation is as follows: The outer surfaces, beingexposed to the air, dry first below their fiber saturationpoint, and these surfaces dry in a stretched condition,being held by the resistance of the rest of the tongueto bending. That this is so is shown by the slightbending of the tongues outward (Fig. 23-C). The dried surface hardens in this stretched con-dition. The rest of the tongue subsequently dries more A. Fig. 24.—Test disks for removal of casehardening. In A the stresses have beenentirely neutralized and casehardening eliminated. In B, the process has been carriedtoo far and the condition has been reversed. slowly and shrinks more than the outer surface; con-sequently the curvature ultimately becomes reversedand the tongues bend inwardly as shown in Figure rapidly dried surface of a board or stick (driedfrom the green condition) therefore tends to be convexand not concave. This action is often of considerable consequence inthe drying of wood, especially of material sawed intoblanks for manufacture after drying. Removal of Casehardening.—^A clear understand- HOW WOOD DRIES 121 ing of the nature of casehardening will, with a littlethought, suggest a means by which it may be fact, it is not only possible, but entirely practicable,on a large scale, to eliminate casehardening, providedit has not gone
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Keywords: ., bookcentury1900, bookdecade1910, bookidkilndry, booksubjectlumber
