| 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.  Spruce impregnated with crude resin and dried is greatly increased in strength thereby.

[Footnote 28:  Bul. 70, U.S.  Forest Service, p. 92; also p. 126, appendix.]

Since the late wood of a growth ring is usually darker in color than the early wood, this fact may be used in judging the density, and therefore the hardness and strength of the material.  This is particularly the case with coniferous woods.  In ring-porous woods the vessels of the early wood not infrequently appear on a finished surface as darker than the denser late wood, though on cross sections of heartwood the reverse is commonly true.  Except in the manner just stated the color of wood is no indication of strength.