Elasticity is of great importance in some uses of wood, as in long tool handles used in agricultural implements, such as rakes, hoes, scythes, and in axes, in archery bows, in golf sticks, etc., in all of which, hickory, our most elastic wood, is used.[8]
HARDNESS OF WOOD.
Hardness is the ability of wood to resist indentations, and depends primarily upon the thickness of the cell walls and the smallness of the cell cavities, or, in general, upon the density of the wood structure. Summer wood, as we have seen, is much harder than spring wood, hence it is important in using such wood as yellow pine on floors to use comb-grain boards, so as to present the softer spring wood in as narrow surfaces as possible. See _Handwork in Wood_, p. 41, and Fig. 55. In slash-grain boards, broad surfaces of both spring and summer wood appear. Maple which is uniformly hard makes the best floors, even better than oak, parts of which are comparatively soft.
The hardness of wood is of much consequence in gluing pieces together.
Soft woods, like pine, can be glued easily, because the fibers can be forced close together. As a matter of fact, the joint when dry is stronger than the rest of the board. In gluing hard woods, however, it is necessary to scratch the surfaces to be glued in order to insure a strong joint. It is for the same reason that a joint made with liquid glue is safe on soft wood when it would be weak on hard wood.[9]
TOUGHNESS OF WOOD.
Toughness may be defined as the ability to resist sudden shocks and blows. This requires a combination of various qualities, strength, hardness, elasticity and pliability. The tough woods, _par excellence_, are hickory, rock elm and ash. They can be pounded, pulled, compressed and sheared. It is because of this quality that hickory is used for wheel spokes and for handles, elm for hubs, etc.
In the selection of wood for particular purposes, it is sometimes one, sometimes another, and more often still, a combination of qualities that makes it fit for use.[10]
It will be remembered that it was knowledge of the special values of different woods that made "the one horse shay," "The Deacon"s Masterpiece."
"So the Deacon inquired of the village folk Where he could find the strongest oak, That couldn"t be split nor bent nor broke,-- That was for spokes and floor and sills; He sent for lancewood to make the thills; The cross bars were ash, from the straightest trees, The panels of whitewood, that cuts like cheese, But lasts like iron for things like these.
The hubs of logs from the "Settler"s Ellum,"-- Last of its timber,--they couldn"t sell "em.
Never an ax had seen their chips, And the wedges flew from between their lips, Their blunt ends frizzled like celery tips; Step and prop-iron, bolt and screw, Spring, tire, axle and linch pin too, Steel of the finest, bright and blue; Thorough brace, bison skin, thick and wide; Boot, top dasher from tough old hide, Found in the pit when the tanner died.
That was the way to "put her through."
"There!" said the Deacon, "naow she"ll dew!""
[Footnote 1: Hygroscopicity, "the property possessed by vegetable tissues of absorbing or discharging moisture and expanding or shrinking accordingly."--_Century Dictionary._]
[Footnote 2: This is shown by the following table, from Forestry Bulletin No. 10, p. 31, _Timber_, by Filibert Roth:
POUNDS OF WATER LOST IN DRYING 100 POUNDS OF GREEN WOOD IN THE KILN.
Sap-wood or Heart-wood outer part. or interior.
1. Pines, cedars, spruces, and firs 45-65 16-25 2. Cypress, extremely variable 50-65 18-60 3. Poplar, cottonwood, ba.s.swood 60-65 40-60 4. Oak, beech, ash, elm, maple, birch, hickory, chestnut, walnut, and sycamore 40-50 30-40 ]
[Footnote 3: The following table from Roth, p. 37, gives the approximate shrinkage of a board, or set of boards, 100 inches wide, drying in the open air:
Shrinkage Inches.
1. All light conifers (soft pine, spruce, cedar, cypress) 3
2. Heavy conifers (hard pine, tamarack, yew, honey locust, box elder, wood of old oaks) 4
3. Ash, elm, walnut, poplar, maple, beech, sycamore, cherry, black locust 5
4. Ba.s.swood, birch, chestnut, horse chestnut, blue beech, young locust 6
5. Hickory, young oak, especially red oak Up to 10
The figures are the average of radial and tangential shrinkages.]
[Footnote 4: How much different woods vary may be seen by the following table, taken from Filibert Roth, _Timber_, Forest Service Bulletin No. 10, p. 28:
WEIGHT OF KILN-DRIED WOOD OF DIFFERENT SPECIES.
------------------------------------+---------------------------------Approximate.
+-----------+---------------------Weight of+---------+-----------Specific1 cubic1,000 feetweight.foot.of lumber.
------------------------------------+-----------+---------+-----------PoundsPounds (a) Very heavy woods:Hickory, oak, persimmon,osage, orange, blacklocust, hackberry, bluebeech, best of elm, and ash0.70-0.8042-483,700 (b) Heavy woods:Ash, elm, cherry, birch,maple, beech, walnut, sourgum, coffee tree, honeylocust, best of southernpine, and tamarack.60-.7036-423,200 (c) Woods of medium weight:Southern pine, pitch pine,tamarack, Douglas spruce,western hemlock, sweet gum,soft maple, sycamore, lightsa.s.safras, mulberry,grades of birch and cherry.50-.6030-362,700 (d) Light woods:Norway and bull pine, redcedar, cypress, hemlock,the heavier spruce and fir,redwood, ba.s.swood, chestnut,b.u.t.ternut, tulip, catalpa,buckeye, heavier grades ofpoplar.40-.5024-302,200 (e) Very light woods:White pine, spruce, fir, whitecedar, poplar.30-.4018-241,800 ------------------------------------+-----------+---------+-----------
[Footnote 5: For table of weights of different woods see Sargent, _Jesup Collection,_ pp. 153-157.]
[Footnote 6: See Forestry Bulletin No. 70, pp. 11, 12, and Forestry Circular No. 108.]
[Footnote 7: For table of strengths of different woods, see Sargent, _Jesup Collection_, pp. 166 ff.]
[Footnote 8: For table of elasticity of different woods, see Sargent, _Jesup Collection_, pp. 163 ff.]
[Footnote 9: For table of hardnesses of different woods, see Sargent, _Jesup Collection_, pp. 173 ff.]
[Footnote 10: For detailed characteristics of different woods see Chapter III.]
THE PROPERTIES OF WOOD.
REFERENCES[A]
Moisture and Shrinkage.
Roth, _For. Bull._, No. 10, pp. 25-37.
Busbridge, _Sci. Am. Sup._ No. 1500. Oct. 1, "04.
Weight, Strength, Cleavability, Elasticity and Toughness.
Roth, _For. Bull._, 10, p. 37-50.
Boulger, pp. 89-108, 129-140.
Roth, _First Book_, pp. 229-233.
Sargent, _Jesup Collection_, pp. 153-176.
Forest Circulars Nos. 108 and 139.
[Footnote A: For general bibliography, see p. 4.]
CHAPTER III.
THE PRINc.i.p.aL SPECIES OF AMERICAN WOODS.