﻿24 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 



dries beyond the fiber-saturation point il begins to shrink, and it 

 will continue to shrink as long as it loses moisture. In fact, this 

 shrinkage is very Dearly proportional to the amount of drying below 

 the fiber-saturation point. Shrinkage is not uniform in all direc- 

 tions, however. The longitudinal shrinkage, parallel to the length 

 of a board or vertical in a standing tree, is practically nothing, and 

 may be neglected here. The tangential shrinkage, parallel to the 

 circumference or rings or in a horizontal direction in the standing 

 tree, is usually from one and one-half to three times as great as the 

 radial shrinkage (horizontal in the standing tree, from the pith to 

 the circumference, perpendicular to the rings and to the tangential 

 direction). Shrinkage is more or less proportional to densitj^ or 

 weight of wood; the heavier woods, as a rule, shrink more than the 

 lighter ones. 



Shrinkage is accompanied by a hardening of the wood, a reduc- 

 tion in its plasticity, and a reduction in the rate at which the mois- 

 ture transfuses through it. There are also important changes in the 

 mechanical properties. The wood becomes stronger under stresses, 

 such as bending, tension, and compression, and also gains in stiff- 

 ness. The increase in these properties as the wood is dried from the 

 fiber-saturation point to zero moisture may be as much as several 

 hundred per cent of the values in the green wood. 



DRYING DEFECTS DUE TO UNEVEN SHRINKAGE. 



Most of the defects ordinarily classed as drying defects would not 

 exist if it were not for uneven shrinkage and the attendant stresses 

 set up by it. Take the simplest case, a hypothetical one, in which a 

 board dries without moisture gradient and with uniform radial 

 shrinkage and uniform tangential shrinkage. If the board be radial 

 (quarter-sawed or edge grain) or tangential (plain-sawed or flat 

 grain), it will remain flat in drying, but after drying the radial 

 board will be thinner and wider than the tangential one if they 

 were both of the same width and thickness when green. If, however, 

 the board is neither radial nor tangential, but has the grain running 

 uniformly at an angle to the sides and edges, the difference between 

 radial and tangential shrinkage will cause " diamonding," the sides 

 and edges no longer being at right angles to each other. In a board 

 partly quartered and partly slash grained the difference between 

 radial and tangential shrinkage will cause the board to cup, the 

 edges turning away from the heart. 



CASEHARDENING. 



As the outer surfaces of the board reach and pass the fiber-satu- 

 ration point they begin to shrink. In order to shrink, however, 

 they must squeeze together all of the green wood inside, since it has 

 not yet reached the fiber-saturation point and is therefore not ready 

 to shrink of its own accord. The first result is that the surface 

 layers, in trying to squeeze the inside or core, create in it a state 

 of compression and in themselves a corresponding state of tension, 

 or pull. Imagine a rubber band stretched across a book or bundle 

 of papers. The band is stretched and the book compressed or 

 squeezed. The only difference is that the tension is put into the 



