24 BULLETIN 1136, U. S. DEPARTMENT OF -AGRICULTURE. 
dries beyond the fiber-saturation point it begins to shrink, and it 
will continue to shrink as long as it loses moisture. In fact, this 
shrinkage is very nearly 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 density 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 
