4 DEPARTMENT BULLETIN 1425, U. S. DEPT. OF AGRICULTURE 
Moisture is held in green or wet wood in two ways. It is con- 
tained within the otherwise practically empty cell cavities, and it is 
absorbed in the cell walls. The cell water is called “ free” water; 
that in the cell walls may be termed “imbibed” water. Free water 
is found in the cell cavities only when the cell walls are fully 
saturated. 
Shrinkage of wood takes place only with a loss of moisture and 
swelling with the absorption of moisture. But all loss of moisture 
is not accompanied by shrinkage. As wood dries, it first gives up 
its free water. After the cell cavities become empty, the moisture 
in the saturated cell walls is drawn off. Wood does not start to 
shrink until the cell walls begin to lose moisture. 
The point at which the cell cavities are empty but the cell walls 
are still saturated is thus an important one in drying. It is known 
as the fiber-saturation point. The moisture content at this point 
varies from 20 to 35 per cent, but for most woods is between 25 and 
30 per cent. In actual practice, of course, the cells near the surface 
fall below this point before those on the interior have reached it, 
and the outer wood tends to shrink before the inner. Such a state 
is often the cause of serious drying troubles. 
Free water is present in both the heartwood and sapwood of most 
living trees but in greatly differing quantity. Sapwood usually con- 
tains more moisture than heartwood. Butt logs ordinarily have a 
higher moisture content than top logs. Contrary to common belief, 
the quantity of moisture in green wood has little seasonal variation. 
Species and locality of growth, however, have an important bearing 
upon it. 
Variation of moisture content was very marked in the many de- 
terminations of green wood made in the air-seasoning investigations 
upon which this study is based. Differences between species were, 
of course, large, but in all species the select grades contained more 
moisture than the common grades, owing to the greater proportion 
of sapwood in the better class of stock. The moisture content of 
western white pine averaged about 84 per cent for selects and 75 
per cent for common; that of sugar pine 190 and 75 per cent; that 
of white fir 200 and 90 per cent; and that of redwood 200 and 70 
per cent. The moisture content of coast Douglas fir probably 
ranges from 53 to 32 per cent and that of western hemlock from 
120 to 28 per cent. Variation resulting from locality of growth is 
well illustrated by the moisture content of western yellow pine. The 
moisture content of stock from California ranged from 185 to 100 
per cent, whereas that in stock from the Inland Empire ranged from 
115 to 80 per cent. 
MOVEMENT OF MOISTURE IN WOOD 
As already stated, wood upon drying loses first its free water 
and then that which is absorbed in the cell walls. The pores them- 
selves have very little to do with drying or the movement of moisture 
in wood. The moisture does not flow out of the pores of wood to 
the surface, but comes to the surface only along the cell walls. Thus, 
because of the nature of wood structure, the end grain of wood loses 
moisture more rapidly than does the side grain, and flat or plain- 
