g.
C. (39.2 deg. F.). A cubic foot of distilled water at 4 deg. C. weighs
62.43 pounds. Hence the specific gravity of a piece of wood with
a density of 35 pounds is 35 / 62.43 = 0.561. To find the weight
per cubic foot when the specific gravity is given, simply
multiply by 62.43. Thus, 0.561 X 62.43 = 35. In the metric
system, since the weight of a cubic centimetre of pure water is
one gram, the density in grams per cubic centimetre has the same
numerical value as the specific gravity.

Since the amount of water in wood is extremely variable it
usually is not satisfactory to refer to the density of green
wood. For scientific purposes the density of "oven-dry" wood is
used; that is, the wood is dried in an oven at a temperature of
100 deg.C. (212 deg.F.) until a constant weight is attained. For
commercial purposes the weight or density of air-dry or
"shipping-dry" wood is used. This is usually expressed in pounds
per thousand board feet, a board foot being considered as
one-twelfth of a cubic foot.

Wood shrinks greatly in drying from the green to the oven-dry
condition. (See Table XIV.) Consequently a block of wood
measuring a cubic foot when green will measure considerably less
when oven-dry. It follows that the density of oven-dry wood does
not represent the weight of the dry wood substance in a cubic
foot of green wood. In other words, it is not the weight of a
cubic foot of green wood minus the weight of the water which it
contains. Since the latter is often a more convenient figure to
use and much easier to obtain than the weight of oven-dry wood,
it is commonly expressed in tables of "specific gravity or
density of dry wood."

|----------------------------------------------------------------------------|
| TABLE XIV |
|----------------------------------------------------------------------------|
| SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS |
| (Forest Service Cir. 213) |
|----------------------------------------------------------------------------|
| | | Specific gravity | Shrinkage from green to |
| | Mois- | oven-dry, based on | oven-dry condition |
| COMMON NAME | ture |--------------------+---------------------------|
| OF SPECIES | content | Volume | Volume | In | | Tangen- |
|