SUPPORTS 263
water that is free in the cell cavities. Water in chemical combination is a constant
thing and has no effect on the swelling and shrinking of wood. Also, that which is
free in the cell cavities, like sap in growing wood, or which is taken up in resoaking,
has no direct effect. It is the water absorbed by the cell walls themselves that has
a pronounced effect upon the thickness of the walls of the wood cells. It is the
water absorbed by the cell walls that is the important factor bearing upon the
shrinking and swelling of wood. When green wood has become thoroughly dry,
it still contains a certain amount of moisture, but the amount varies with that
in the surrounding air. In an atmosphere saturated with water vapor the cell
walls can take up 20 to 30 per cent of moisture (based on the dry weight of the
wood) while the fibre cavities are still empty. This percentage varies with the
species. The condition when the cell walls are saturated with water vapor is
called the * fibre saturation point/ Under normal atmospheric conditions (60-70
per cent relative humidity) air-dried wood contains about 12 per cent by weight of
water. Data on the relationship of average moisture content of wood to relative
humidity of the atmosphere are given in Technical Note No. F-sjy Forest Products
Laboratory (Madison, Wis.), as follows:
Relative Humidity Average Moisture
of Atmosphere Content of Wood
(Per Cent) (Per Cent)
20 5.0
25 5.8
30 6.6
35 7-4
40 8.2
45 9-1
50 i o.o
55 10.9
60 11.9
65 13-0
?o 14-3
75 15.7
80 17.5
85 19.5
90 22.2
95 25.6
Wood' does not shrink equally in all directions. There is practically no longitudinal
shrinkage or expansion. Change in dimension, caused by change in moisture
content, is all transverse and, in general, the tangential decrease is about twice as
great as the radial. This explains why logs of wood crack radially on drying.
Forsaith (p. 114) says that, in general, a loss of I per cent by weight in air-dried
timber will cause a radial shrinkage of about 0.2 of I per cent and a tangential
shrinkage of 0.3 of I per cent.