Seasoning of Wood Part 9

=111. Long-Leaf Willow= (_Salix fluviatilis_) (Sand Bar Willow). A small-sized tree. Ranges from the Arctic Circle to Northern Mexico.

=112. Bebb Willow= (_Salix bebbiana_ var. _rostrata_). A small-sized tree. More abundant in British America than in the United States, where it ranges southward to Pennsylvania and westward to Minnesota.

=113. Glaucous Willow= (_Salix discolor_) (p.u.s.s.y Willow). A small-sized tree. Common along the banks of streams, and ranges from Nova Scotia to Manitoba, and south to Delaware; west to Indiana and northwestern Missouri.

=114. Crack Willow= (_Salix fragilis_). A medium to large-sized tree.

Wood is very soft, light, very flexible and fairly strong, is fairly durable in contact with the soil, works well and stands well. Used princ.i.p.ally for basket making, hoops, etc., and to produce charcoal for gunpowder. Very common, and widely distributed in the United States.

=115. Weeping Willow= (_Salix babylonica_). Medium- to large-sized tree.

Wood similiar to _Salix nigra_, but not so valuable. Mostly an ornamental tree. Originally came from China. Widely planted in the United States.

YELLOW WOOD

=116. Yellow Wood= (_Cladrastis lutea_) (Virgilia). A small to medium-sized tree. Wood yellow to pale brown, heavy, hard, close-grained and strong. Not used to much extent in manufacturing.

Not common. Found princ.i.p.ally on the limestone cliffs of Kentucky, Tennessee, and North Carolina.

SECTION IV

GRAIN, COLOR, ODOR, WEIGHT, AND FIGURE IN WOOD

DIFFERENT GRAINS OF WOOD

The terms "fine-grained," "coa.r.s.e-grained," "straight-grained," and "cross-grained" are frequently applied in the trade. In common usage, wood is coa.r.s.e-grained if its annual rings are wide; fine-grained if they are narrow. In the finer wood industries a fine-grained wood is capable of high polish, while a coa.r.s.e-grained wood is not, so that in this latter case the distinction depends chiefly on hardness, and in the former on an accidental case of slow or rapid growth. Generally if the direction of the wood fibres is parallel to the axis of the stem or limb in which they occur, the wood is straight-grained; but in many cases the course of the fibres is spiral or twisted around the tree (as shown in Fig. 15), and sometimes commonly in the b.u.t.ts of gum and cypress, the fibres of several layers are oblique in one direction, and those of the next series of layers are oblique in the opposite direction. (As shown in Fig. 16 the wood is cross or twisted grain.) Wavy-grain in a tangential plane as seen on the radial section is ill.u.s.trated in Fig. 17, which represents an extreme case observed in beech. This same form also occurs on the radial plane, causing the tangential section to appear wavy or in transverse folds.

When wavy grain is fine (_i.e._, the folds or ridges small but numerous) it gives rise to the "curly" structure frequently seen in maple. Ordinarily, neither wavy, spiral, nor alternate grain is visible on the cross-section; its existence often escapes the eye even on smooth, longitudinal faces in the sawed material, so that the only guide to their discovery lies in splitting the wood in two, in the two normal plains.

[Ill.u.s.tration: Fig. 15. Spiral Grain. Season checks, after removal of bark, indicate the direction of the fibres or grain of the wood.]

[Ill.u.s.tration: Fig. 16. Alternating Spiral Grain in Cypress.

Side and end view of same piece. When the bark was at _o_, the grain of this piece was straight. From that time, each year it grew more oblique in one direction, reaching a climax at _a_, and then turned back in the opposite direction. These alternations were repeated periodically, the bark sharing in these changes.]

Generally the surface of the wood under the bark, and therefore also that of any layer in the interior, is not uniform and smooth, but is channelled and pitted by numerous depressions, which differ greatly in size and form. Usually, any one depression or elevation is restricted to one or few annual layers (_i.e._, seen only in one or few rings) and is then lost, being compensated (the surface at the particular spot evened up) by growth. In some woods, however, any depression or elevation once attained grows from year to year and reaches a maximum size, which is maintained for many years, sometimes throughout life.

In maple, where this tendency to preserve any particular contour is very great, the depressions and elevations are usually small (commonly less than one-eighth inch) but very numerous.

On tangent boards of such wood, the sections, pits, and prominences appear as circlets, and give rise to the beautiful "bird's eye" or "landscape" structure. Similiar structures in the burls of black ash, maple, etc., are frequently due to the presence of dormant buds, which cause the surface of all the layers through which they pa.s.s to be covered by small conical elevations, whose cross-sections on the sawed board appear as irregular circlets or islets, each with a dark speck, the section of the pith or "trace" of the dormant bud in the center.

[Ill.u.s.tration: Fig. 17. Wavy Grain in Beech (_after Nordlinger_).]

In the wood of many broad-leaved trees the wood fibres are much longer when full grown than when they are first formed in the cambium or growing zone. This causes the tips of each fibre to crowd in between the fibres above and below, and leads to an irregular interlacement of these fibres, which adds to the toughness, but reduces the cleavability of the wood. At the juncture of the limb and stem the fibres on the upper and lower sides of the limb behave differently.

On the lower side they run from the stem into the limb, forming an uninterrupted strand or tissue and a perfect union. On the upper side the fibres bend aside, are not continuous into the limb, and hence the connection is not perfect (see Fig. 18). Owing to this arrangement of the fibres, the cleft made in splitting never runs into the knot if started on the side above the limb, but is apt to enter the knot if started below, a fact well understood in woodcraft. When limbs die, decay, and break off, the remaining stubs are surrounded, and may finally be covered by the growth of the trunk and thus give rise to the annoying "dead" or "loose" knots.

[Ill.u.s.tration: Fig. 18. Section of Wood showing Position of the Grain at Base of a Limb. P, pith of both stem and limb; 1-7, seven yearly layers of wood; _a_, _b_, knot or basal part of a limb which lived for four years, then died and broke off near the stem, leaving the part to the left of _a_, _b_, a "sound" knot, the part to the right a "dead" knot, which would soon be entirely covered by the growing stem.]

COLOR AND ODOR OF WOOD

Color, like structure, lends beauty to the wood, aids in its identification, and is of great value in the determination of its quality. If we consider only the heartwood, the black color of the persimmon, the dark brown of the walnut, the light brown of the white oaks, the reddish brown of the red oaks, the yellowish white of the tulip and poplars, the brownish red of the redwood and cedars, the yellow of the papaw and sumac, are all reliable marks of distinction and color. Together with l.u.s.ter and weight, they are only too often the only features depended upon in practice. Newly formed wood, like that of the outer few rings, has but little color. The sapwood generally is light, and the wood of trees which form no heartwood changes but little, except when stained by forerunners of disease.

The different tints of colors, whether the brown of oak, the orange brown of pine, the blackish tint of walnut, or the reddish cast of cedar, are due to pigments, while the deeper shade of the summer-wood bands in pine, cedar, oak, or walnut is due to the fact that the wood being denser, more of the colored wood substance occurs on a given s.p.a.ce, _i.e._, there is more colored matter per square inch. Wood is translucent, a thin disk of pine permitting light to pa.s.s through quite freely. This translucency affects the l.u.s.ter and brightness of lumber.

When lumber is attacked by fungi, it becomes more opaque, loses its brightness, and in practice is designated "dead," in distinction to "live" or bright timber. Exposure to air darkens all wood; direct sunlight and occasional moistening hasten this change, and cause it to penetrate deeper. Prolonged immersion has the same effect, pine wood becoming a dark gray, while oak changes to a blackish brown.

Odor, like color, depends on chemical compounds, forming no part of the wood substance itself. Exposure to weather reduces and often changes the odor, but a piece of long-leaf pine, cedar, or camphor wood exhales apparently as much odor as ever when a new surface is exposed. Heartwood is more odoriferous than sapwood. Many kinds of wood are distinguished by strong and peculiar odors. This is especially the case with camphor, cedar, pine, oak, and mahogany, and the list would comprise every kind of wood in use were our sense of smell developed in keeping with its importance.

Decomposition is usually accompanied by p.r.o.nounced odors. Decaying poplar emits a disagreeable odor, while red oak often becomes fragrant, its smell resembling that of heliotrope.

WEIGHT OF WOOD

A small cross-section of wood (as in Fig. 19) dropped into water sinks, showing that the substance of which wood fibre or wood is built up is heavier than water. By immersing the wood successively in heavier liquids, until we find a liquid in which it does not sink, and comparing the weight of the same with water, we find that wood substance is about 1.6 times as heavy as water, and that this is as true of poplar as of oak or pine.

[Ill.u.s.tration: Fig. 19. Cross-section of a Group of Wood Fibres (Highly Magnified.)]

Separating a single cell (as shown in Fig. 20, _a_), drying and then dropping it into water, it floats. The air-filled cell cavity or interior reduces its weight, and, like an empty corked bottle, it weighs less than the water. Soon, however, water soaks into the cell, when it fills up and sinks. Many such cells grown together, as in a block of wood, when all or most of them are filled with water, will float as long as the majority of them are empty or only partially filled. This is why a green, sappy pine pole soon sinks in "driving"

(floating down stream). Its cells are largely filled before it is thrown in, and but little additional water suffices to make its weight greater than that of the water. In a good-sized white pine log, composed chiefly of empty cells (heartwood), the water requires a very long time to fill up the cells (five years would not suffice to fill them all), and therefore the log may float for many months. When the wall of the wood fibre is very thick (five eighths or more of the volume, as in Fig. 20, _b_), the fibre sinks whether empty or filled.

This applies to most of the fibres of the dark summer-wood bands in pines, and to the compact fibres of oak or hickory, and many, especially tropical woods, have such thick-walled cells and so little empty or air s.p.a.ce that they never float.

[Ill.u.s.tration: Fig. 20. Isolated Fibres of Wood.]

Here, then, are the two main factors of weight in wood; the amount of cell wall or wood substance constant for any given piece, and the amount of water contained in the wood, variable even in the standing tree, and only in part eliminated in drying.

The weight of the green wood of any species varies chiefly as a second factor, and is entirely misleading, if the relative weight of different kinds is sought. Thus some green sticks of the otherwise lighter cypress and gum sink more readily than fresh oak.

The weight of sapwood or the sappy, peripheral part of our common lumber woods is always great, whether cut in winter or summer. It rarely falls much below forty-five pounds, and commonly exceeds fifty-five pounds to the cubic foot, even in our lighter wooded species. It follows that the green wood of a sapling is heavier than that of an old tree, the fresh wood from a disk of the upper part of a tree is often heavier than that of the lower part, and the wood near the bark heavier than that nearer the pith; and also that the advantage of drying the wood before shipping is most important in sappy and light kinds.

When kiln-dried, the misleading moisture factor of weight is uniformly reduced, and a fair comparison possible. For the sake of convenience in comparison, the weight of wood is expressed either as the weight per cubic foot, or, what is still more convenient, as specific weight or density. If an old long-leaf pine is cut up (as shown in Fig. 21) the wood of disk No. 1 is heavier than that of disk No. 2, the latter heavier than that of disk No. 3, and the wood of the top disk is found to be only about three fourths as heavy as that of disk No. 1.

Similiarly, if disk No. 2 is cut up, as in the figure, the specific weight of the different parts is:

_a_, about 0.52 _b_, about 0.64 _c_, about 0.67 _d_, _e_, _f_, about 0.65

showing that in this disk at least the wood formed during the many years' growth, represented in piece _a_, is much lighter than that of former years. It also shows that the best wood is the middle part, with its large proportion of dark summer bands.

[Ill.u.s.tration: Fig. 21. Orientation of Wood Samples.]

Cutting up all disks in the same way, it will be found that the piece _a_ of the first disk is heavier than the piece _a_ of the fifth, and that piece _c_ of the first disk excels the piece _c_ of all the other disks. This shows that the wood grown during the same number of years is lighter in the upper parts of the stem; and if the disks are smoothed on the radial surfaces and set up one on top of the other in their regular order, for the sake of comparison, this decrease in weight will be seen to be accompanied by a decrease in the amount of summer-wood. The color effect of the upper disks is conspicuously lighter. If our old pine had been cut one hundred and fifty years ago, before the outer, lighter wood was laid on, it is evident that the weight of the wood of any one disk would have been found to increase from the center outward, and no subsequent decrease could have been observed.

In a thrifty young pine, then, the wood is heavier from the center outward, and lighter from below upward; only the wood laid on in old age falls in weight below the average. The number of brownish bands of summer-wood are a direct indication of these differences. If an old oak is cut up in the same manner, the b.u.t.t cut is also found heaviest and the top lightest, but, unlike the disk of pine, the disk of oak has its firmest wood at the center, and each successive piece from the center outward is lighter than its neighbor.

Examining the pieces, this difference is not as readily explained by the appearance of each piece as in the case of pine wood.

Nevertheless, one conspicuous point appears at once. The pores, so very distinct in oak, are very minute in the wood near the center, and thus the wood is far less porous.

Studying different trees, it is found that in the pines, wood with narrow rings is just as heavy as and often heavier than the wood with wider rings; but if the rings are unusually narrow in any part of the disk, the wood has a lighter color; that is, there is less summer-wood and therefore less weight.

In oak, ash, or elm trees of thrifty growth, the rings, fairly wide (not less than one-twelfth inch), always form the heaviest wood, while any piece with very narrow rings is light. On the other hand, the weight of a piece of hard maple or birch is quite independent of the width of its rings.