WEIGHT, DENSITY, AND SPECIFIC GRAVITY

From data obtained from a large number of tests on the strength of different woods it appears that, other things being equal, the crushing strength parallel to the grain, fibre stress at elastic limit in bending, and shearing strength along the grain of wood vary in direct proportion to the weight of dry wood per unit of volume when green. Other strength values follow different laws. The hardness varies in a slightly greater ratio than the square of the density. The work to the breaking point increases even more rapidly than the cube of density. The modulus of rupture in bending lies between the first power and the square of the density. This, of course, is true only in case the greater weight is due to increase in the amount of wood substance. A wood heavy with resin or other infiltrated substance is not necessarily stronger than a similar specimen free from such materials. If differences in weight are due to degree of seasoning, in other words, to the relative amounts of water contained, the rules given above will of course not hold, since strength increases with dryness. But of given specimens of pine or of oak, for example, in the green condition, the comparative strength may be inferred from the weight. It is not permissible, however, to compare such widely different woods as oak and pine on a basis of their weights.[27]

The weight of wood substance, that is, the material which composes the walls of the fibres and other cells, is practically the same in all species, whether pine, hickory, or cottonwood, being a little greater than half again as heavy as water. It varies slightly from beech sapwood, 1.50, to Douglas fir heartwood, 1.57, averaging about 1.55 at 30° to 35° C., in terms of water at its greatest density 4° C. The reason any wood floats is that the air imprisoned in its cavities buoys it up. When this is displaced by water the wood becomes water-logged and sinks. Leaving out of consideration infiltrated substances, the reason a cubic foot of one kind of dry wood is heavier than that of another is because it contains a greater amount of wood substance. Density is merely the weight of a unit of volume, as 35 pounds per cubic foot, or 0.56 grams per cubic centimetre. Specific gravity or relative density is the ratio of the density of any material to the density of distilled water at 4° C. (39.2° F.). A cubic foot of distilled water at 4° C. weighs 62.43 pounds. Hence the specific gravity of a piece of wood with a density of 35 pounds is

35



-------
=
0.561
.
62.43



To find the weight per cubic foot when the specific gravity is given, simply multiply by 62.43. Thus, 0.561 × 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°C. (212°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)
COMMON NAME OF SPECIES Moisture content Specific gravity oven-dry, based on Shrinkage from green to oven-dry condition
Volume when green Volume when oven-dry In volume Radial Tangential
Per cent

Per cent Per cent Per cent
Hardwoods





Ash, black 77 0.466



white 38 .550 0.640 12.6 4.3 6.4
" 47 .516 .590 11.7

Basswood 110 .315 .374 14.5 6.2 8.4
Beech 61 .556 .669 16.5 4.6 10.5
Birch, yellow 72 .545 .661 17.0 7.9 9.0
Elm, rock 46 .578



slippery 57 .541 .639 15.5 5.1 9.9
white 66 .430



Gum, red 71 .434



Hackberry 50 .504 .576 14.0 4.2 8.9
Hickory, big shellbark 64 .601
17.6 7.4 11.2
" 55 .666
20.9 7.9 14.2
bitternut 65 .624



mockernut 64 .606
16.5 6.9 10.4
" 57 .662
18.9 8.4 11.4
" 48 .666



nutmeg 76 .558



pignut 59 .627
15.0 5.6 9.8
" 54 .667
15.3 6.3 9.5
" 55 .667
16.9 6.8 10.9
" 52 .667
21.2 8.5 13.8
shagbark 65 .608
16.0 6.5 10.2
" 58 .646
18.4 7.9 11.4
" 64 .617



" 60 .653
15.5 6.5 9.7
water 74 .630



Locust, honey 53 .695 .759 8.6

Maple, red 69 .512



sugar 57 .546 .643 14.3 4.9 9.1
" 56 .577



Oak, post 64 .590 .732 16.0 5.7 10.6
red 80 .568 .660 13.1 3.7 8.3
swamp white 74 .637 .792 17.7 5.5 10.6
tanbark 88 .585



white 58 .594 .704 15.8 6.2 8.3
" 62 .603 .696 14.3 4.9 9.0
" 78 .600 .708 16.0 4.8 9.2
yellow 77 .573 .669 14.2 4.5 9.7
" 80 .550



Osage orange 31 .761 .838 8.9

Sycamore 81 .454 .526 13.5 5.0 7.3
Tupelo 121 .475 .545 12.4 4.4 7.9
TABLE XIV (CONT.)
SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS
(Forest Service Cir. 213)
COMMON NAME OF SPECIES Moisture content Specific gravity oven-dry, based on Shrinkage from green to oven-dry condition
Volume when green Volume when oven-dry In volume Radial Tangential
Per cent

Per cent Per cent Per cent
Conifers





Arborvitæ 55 .293 .315 7.0 2.1 4.9
Cedar, incense 80 .363



Cypress, bald 79 .452 .513 11.5 3.8 6.0
Fir, alpine 47 .306 .321 9.0 2.5 7.1
amabilis 117 .383



Douglas 32 .418 .458 10.9 3.7 6.6
white 156 .350 .437 10.2 3.4 7.0
Hemlock (east.) 129 .340 .394 9.2 2.3 5.0
Pine, lodgepole 44 .370 .415 11.3 4.2 7.1
lodgepole 58 .371 .407 10.1 3.6 5.9
longleaf 63 .528 .599 12.8 6.0 7.6
red or Nor 54 .440 .507 11.5 4.5 7.2
shortleaf 52 .447



sugar 123 .360 .386 8.4 2.9 5.6
west yellow 98 .353 .395 9.2 4.1 6.4
" 125 .377 .433 11.5 4.3 7.3
" 93 .391 .435 9.9 3.8 5.8
white 74 .363 .391 7.8 2.2 5.9
Redwood 81 .334



" 69 .366



Spruce, Engelmann 45 .325 .359 10.5 3.7 6.9
" 156 .299 .335 10.3 3.0 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 density of oven-dry wood, then

D

----------=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 oven-dry, then,



w


W


V - v




V - v



d=--- ; D=---;S=-------×100, ands= -------×100,


v


V


V




v



in which S is the percentage of shrinkage 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].)