0 | 6.2 |
| red | 31 | .396 | | | | |
| white | 41 | .318 | | | | |
| Tamarack | 52 | .491 | .558 | 13.6 | 3.7 | 7.4 |
|----------------------------------------------------------------------------|

This weight divided by 62.43 gives the specific gravity per
green volume. It is purely a fictitious quantity. To convert
this figure into actual density or specific gravity of the dry
wood, it is necessary to know the amount of shrinkage in volume.
If S is the percentage of shrinkage from the green to the
oven-dry condition, based on the green volume; D, the density of
the dry wood per cubic foot while green; and d the actual
D
density of oven-dry wood, then ---------- = d.
1 - .0 S

This relation becomes clearer from the following analysis:
Taking V and W as the volume and weight, respectively, when
green, and v and w as the corresponding volume and weight when
w W V - v
oven-dry, then, d = --- ; D = --- ; S = ------- X 100, and
v V V
V - v
s = ------- X 100, in which S is the percentage of shrinkage
v
from the green to the oven-dry condition, based on the green
volume, and s the same based on the oven-dry volume.

In tables of specific gravity or density of wood it should
always be stated whether the dry weight per unit of volume when
green or the dry weight per unit of volume when dry is intended,
since the shrinkage in volume may vary from 6 to 50 per cent,
though in conifers it is usually about 10 per cent, and in
hardwoods nearer 15 per cent. (See Table XIV.)

COLOR

In species which show a distinct difference between heartwood
and sapwood the natural color of heartwood is invariably darker
than that of the sapwood, and very frequently the contrast is
conspicuous. This is produced by deposits in the heartwood of
various materials resulting from the process of growth,
increased possibly by oxidation and other chemical changes,
which usually have little or no appreciable effect on the
mechanical properties of the wood. (See HEARTWOOD AND SAPWOOD,
above.) Some experiments[28] on very resinous longleaf pine
specimens, however, indicate an increase in strength. This is
due to the resin which increases the strength when dry.