DIFFERENT GRAINS OF WOOD
The terms “fine-grained,” “coarse-grained,” “straight-grained,” and “cross-grained” are frequently applied in the trade. In common usage, wood is coarse-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 coarse-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 fibers 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 fibers is spiral or twisted around the tree (as shown in Fig. 15), and sometimes commonly in the butts of gum and cypress, the fibers 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 illustrated 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.
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Fig. 15. Spiral Grain. Season checks, after removal of bark, indicate the direction of the fibers or grain of the wood. |
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 pass 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.
In the wood of many broad-leaved trees the wood fibers 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 fibers above and below, and leads to an irregular interlacement of these fibers, which adds to the toughness, but reduces the cleavability of the wood. At the juncture of the limb and stem the fibers 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 fibers bend aside, are not continuous into the limb, and hence the connection is not perfect (see Fig. 18). Owing to this arrangement of the fibers, 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.
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 luster 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 space, i.e., there is more colored matter per square inch. Wood is translucent, a thin disk of pine permitting light to pass through quite freely. This translucency affects the luster 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 pronounced 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.
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 fibers of the dark summer-wood bands in pines, and to the compact fibers of oak or hickory, and many, especially tropical woods, have such thick-walled cells and so little empty or air space that they never float.
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.
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 butt 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.
The bases of limbs (knots) are usually heavy, very heavy in conifers, and also the wood which surrounds them, but generally the wood of the limbs is lighter than that of the stem, and the wood of the roots is the lightest.
In general, it may be said that none of the native woods in common use in this country are when dry as heavy as water, i.e., sixty-two pounds to the cubic foot. Few exceed fifty pounds, while most of them fall below forty pounds, and much of the pine and other coniferous wood weigh less than thirty pounds per cubic foot. The weight of the wood is in itself an important quality. Weight assists in distinguishing maple from poplar. Lightness coupled with great strength and stiffness recommends wood for a thousand different uses. To a large extent weight predicates the strength of the wood, at least in the same species, so that a heavy piece of oak will exceed in strength a light piece of the same species, and in pine it appears probable that, weight for weight, the strength of the wood of various pines is nearly equal.
Weight of Kiln-dried Wood of Different Species
| Species | Approximate | ||
| Specific Weight |
Weight of | ||
| 1 Cubic Foot |
1,000 Feet Lumber |
||
| (a) Very Heavy Woods: Hickory, Oak, Persimmon, Osage Orange, Black Locust, Hackberry, Blue Beech, best of Elm and Ash | 0.70-0.80 | 42-48 | 3,700 |
| (b) Heavy Woods Ash, Elm, Cherry, Birch, Maple, Beech, Walnut, Sour Gum, Coffee Tree, Honey Locust, best of Southern Pine and Tamarack | 0.60-0.70 | 36-42 | 3,200 |
| (c) Woods of Medium Weight: Southern Pine, Pitch Pine, Tamarack, Douglas Spruce, Western Hemlock, Sweet Gum, Soft Maple, Sycamore, Sassafras, Mulberry, light grades of Birch and Cherry | 0.50-0.60 | 30-36 | 2,700 |
| (d) Light Woods: Norway and Bull Pine, Red Cedar, Cypress, Hemlock, the Heavier Spruces and Firs, Redwood, Basswood, Chestnut, Butternut, Tulip, Catalpa, Buckeye, heavier grades of Poplar | 0.40-0.50 | 24-30 | 2,200 |
| (e) Very Light Woods: White Pine, Spruce, Fir, White Cedar, Poplar | 0.30-0.40 | 18-24 | 1,800 |
“FIGURE” IN WOOD
Many theories have been propounded as to the cause of “figure” in timber; while it is true that all timber possesses “figure” in some degree, which is more noticeable if it be cut in certain ways, yet there are some woods in which it is more conspicuous than in others, and which for cabinet or furniture work are much appreciated, as it adds to the value of the work produced.
The characteristic “figure” of oak is due to the broad and deep medullary rays so conspicuous in this timber, and the same applies to honeysuckle. Figure due to the same cause is found in sycamore and beech, but is not so pronounced. The beautiful figure in “bird’s eye maple” is supposed to be due to the boring action of insects in the early growth of the tree, causing pits or grooves, which in time become filled up by being overlain by fresh layers of wood growth; these peculiar and unique markings are found only in the older and inner portion of the tree.
Pitch pine has sometimes a very beautiful “figure,” but it generally does not go deep into the timber; walnut has quite a variety of “figures,” and so has the elm. It is in mahogany, however, that we find the greatest variety of “figure,” and as this timber is only used for furniture and fancy work, a good “figure” greatly enhances its value, as firmly figured logs bring fancy prices.
Mahogany, unlike the oak, never draws its “figure” from its small and almost unnoticeable medullary rays, but from the twisted condition of its fibers; the natural growth of mahogany produces a straight wood; what is called “figured” is unnatural and exceptional, and thus adds to its value as an ornamental wood. These peculiarities are rarely found in the earlier portion of the tree that is near the center, being in this respect quite different from maple; they appear when the tree is more fully developed, and consist of bundles of woody fibers which, instead of being laid in straight lines, behave in an erratic manner and are deposited in a twisted form; sometimes it may be caused by the intersection of branches, or possibly by the crackling of the bark pressing on the wood, and thus moving it out of its natural straight course, causing a wavy line which in time becomes accentuated.
It will have been observed by most people that the outer portion of a tree is often indented by the bark, and the outer rings often follow a sinuous course which corresponds to this indention, but in most trees, after a few years, this is evened up and the annual rings assume their nearly circular form; it is supposed by some that in the case of mahogany this is not the case, and that the indentations are even accentuated.
The best figured logs of timber are secured from trees which grow in firm rocky soil; those growing on low-lying or swampy ground are seldom figured. To the practical woodworker the figure in mahogany causes some difficulty in planing the wood to a smooth surface; some portions plane smooth, others are the “wrong way of the grain.”
Figure in wood is effected by the way light is thrown upon it, showing light if seen from one direction, and dark if viewed from another, as may easily be observed by holding a piece of figured mahogany under artificial light and looking at it from opposite directions. The characteristic markings on mahogany are “mottle,” which is also found in sycamore, and is conspicuous on the backs of fiddles and violins, and is not in itself valuable; it runs the transverse way of the fibers and is probably the effect of the wind upon the tree in its early stages of growth. “Roe,” which is said to be caused by the contortion of the woody fibers, and takes a wavy line parallel to them, is also found in the hollow of bent stems and in the root structure, and when combined with “mottle” is very valuable. “Dapple” is an exaggerated form of mottle. “Thunder shake,” “wind shake,” or “tornado shake” is a rupture of the fibers across the grain, which in mahogany does not always break them; the tree swaying in the wind only strains its fibers, and thus produces mottle in the wood.
INDEX: Seasoning of Wood