32 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
The lower dotted line, C, presents a typical moisture gradient for 
almost-oven-dry conditions in the interior-of the block and for a 
surface equilibrium value lower than that of A. The nearly uniform 
moisture content makes curve C much flatter than the steep A, that 
is, the slope of C is much less than that of the steep part of A. 
The intermediate dotted line, B, follows moisture conditions in- 
termediate to those of A and C. | 
Besides changing with the temperature and the humidity of the | 
surrounding air, the moisture gradient is affected by a number of | 
oS) 
30 
45 
40 
SiS) 
—_— i ee ee ee ee ee 
30 
25 
20 
Molstudre Contrent-Fer Centr 
Fiber Saturation FPorwnr 
K-———— Thickness +1 
FIGURE 6.—Typical moisture gradients, across the thickness of a board, at three stages 
of drying. Curve A is intended to represent the moisture gradient just after the 
drying has commenced. The moisture content in the center of the piece then is 
still very high, but that at the surface has reached equilibrium with the surround- 
ing atmosphere. Curve B illustrates an intermediate stage, when the center of the 
piece has just reached the fiber-saturation point. Curve C shows an extremely dry 
condition, in which the surface has reached zero moisture content 
internal factors, such as the amount and the distribution of the mois- | 
ture in the wood, the size of the piece, and its structural character- 
istics. Moisture gradients, therefore, are likely to be constantly 
changing. In fact, if dry wood is placed in an extremely moist at- 
mosphere, the zero-thickness end of the curve will become the highest 
point on it, the reverse of the situation shown in the figure. 
SHRINKAGE 
In discussing the shrinkage of wood during drying and the 
resulting internal stresses, together with the moisture gradient, 
it is convenient to think of a piece of wood, especially a stress sec- 
