2 BULLETIN 552, U. S. DEPARTMENT OF AGRICULTURE. 
woods that are difficult to dry, were the rule. Sometimes as much 
as 20 or 25 per cent of the seasoned lumber was rendered unfit for the 
use intended by defects which had their origin in the drying process. 
Since the quality of the finished product can be impaired seriously by 
wrong methods, the importance of right methods becomes apparent. 
FIBER SATURATION POINT AND SHRINKAGE. 
Water exists in wood in two conditions: 1 (a) as free water con- 
tained in the cell cavities, and (b) as water absorbed in the cell walls. 
When wood contains just enough water to saturate the cell walls, it 
is said to be at the " fiber saturation point." Any water in excess of 
this which the wood may contain is in the form of free water in the 
i 
Fig. 1. — Shrinkage as affected by direction of annual rings; approximately twice as great tangentially 
as radially. 
cell cavities. Removal of the free water has no apparent effect upon 
the properties of the- wood except to reduce its weight, but as soon 
as any of the absorbed water is removed the wood begins to shrink. 
Since the free water is the first to be removed, shrinkage does not 
begin, as a general rule, until the fiber saturation point is reached. 
In the case of eucalyptus and some of the oaks, however, shrinkage 
begins above this point. For most woods the fiber saturation point 
corresponds with a moisture content of from 25 to 30 per cent of the 
dry weight of the wood. Figure 1 shows graphically the difference 
between tangential and radial shrinkage. 
Shrinkage is due to the contraction of the cell walls, and sets up 
stresses which tend to cause the wood to check. As observed in a 
cross section of a piece of lumber, shrinkage in the tangential direc- 
tion is about twice as great as in the radial direction; lengthwise of 
i The term "sap " sometimes is used wrongly to mean the moisture in wood, and at other times to mean 
the sapwood. Sap is formed, mainly in the early spring, in the leaves from water rising from the roots 
through the sapwood. In the leaves this water is converted into true sap, which contains sugar and soluble 
gums. The sap descends through the bark and feeds the tissues in process of formation between the bark 
and the sapwood. The heartwood contains no sap. 
