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 



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 Avails, 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 



1 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. 



