~ o> SSS 
KILN DRYING HANDBOOK 33 
tion, as made up of a number of layers, like the leaves of a book. 
The outside layers, that is, the covers, may be termed a shell, but in 
the following discussion all of them will be called layers. 
As the drying of green wood progresses the amount of free water 
in the cells gradually diminishes, and soon those near the surface lose 
all their free water, that is, they reach the fiber-saturation point. 
It is at this point, which is a very definite one for most species, 
usually between 25 and 30 per cent moisture content, that the changes 
in the properties of the wood caused by drying begin to take place. 
As wood dries beyond its fiber-saturation point it starts to shrink 
and it will continue to shrink as long as it loses moisture. In fact, 
the amount of shrinkage is very nearly proportional to the degree of 
drying below the fiber-saturation point. When the moisture gradient 
is steep, however, the surface layers may be well below the fiber- 
saturation point even though the average moisture content may still 
be far above it, and the consequent tendency of the inclosing outer 
layers to contract may cause some shrinkage of the entire piece to 
take place. So it often happens, on account of such a moisture 
condition, that shrinkage throughout the piece commences while the 
average moisture content is still above the fiber-saturation point. 
Wood is not a homogenous material, and many of its properties 
are different along different axes (in different structural directions). 
Shrinkage is one of these properties. Longitudinal shrinkage (par- 
allel to the length of a board) is practically nothing for most species 
and can be neglected in most drying problems, although cross- 
grained stock often shrinks appreciably in a lengthwise direction on 
account of the longitudinal components of the tangential and the 
radial shrinkages. The shrinkage is more or less proportional to the 
density (the unit weight) of the wood; the heavier woods, as a rule, 
shrink more than the lighter ones. 
Below the fiber-saturation point, drying is accompanied by a 
hardening of the wood and a reduction in its plasticity. There are 
also important changes in its mechanical properties; the wood be- 
comes stronger under such stresses as bending, tension, and compres- 
sion, and also gains in stiffness. The increase in these properties 
as the wood is dried from the fiber-saturation point to zero moisture 
content varies somewhat; for compression it may be more than 
100 per cent of the values in the green wood. On the other hand, 
a few of the mechanical properties, especially those having to do 
with resistance to suddenly applied loads, remain practically con- 
stant as the wood dries, 
DRYING DEFECTS CAUSED BY UNEVEN SHRINKAGE 
Most of the defects ordinarily classed as drying defects would 
not exist if uneven shrinkage and the attendant stresses set up by 
it could be eliminated. Take, for example, the simplest case, a hypo- 
thetical one, in which boards dry without moisture gradient and 
with uniform radial and tangential shrinkage. If the boards are 
truly radial (quarter-sawed or edge-grain) or truly tangential 
(plain-sawed or flat-grain), they will remain flat in drying but, 
assuming that all were of the same width and thickness when green, 
after drying the radial boards will be both thinner and wider than 
32006°—29——3 
