[Ill.u.s.tration: Fig. 29. Redwood Burl (full size).]
[Ill.u.s.tration: Fig. 30. Bird"s-eye Maple (full size.)]
[Ill.u.s.tration: Fig. 31. Burl on White Oak.]
Irregularity of grain is often caused by the presence of advent.i.tious and dormant buds, which may be plainly seen as little k.n.o.bs on the surface of some trees under the bark. In most trees, these irregularities are soon buried and smoothed over by the successive annual layers of wood, but in some woods there is a tendency to preserve the irregularities. On slash (tangent) boards of such wood, a great number of little circlets appear, giving a beautiful grain, as in "Bird"s-eye maple," Fig. 30. These markings are found to predominate in the inner part of the tree. This is not at all a distinct variety of maple, as is sometimes supposed, but the common variety, in which the phenomenon frequently appears. Logs of great value, having bird"s-eyes, have often unsuspectingly been chopped up for fire wood.
The term "grain" may also mean the "figure" formed by the presence of pith rays, as in oak, Fig. 32, or beech, or the word "grain" may refer simply to the uneven deposit of coloring matter as is common in sweet gum, Fig. 33, black ash, or Circa.s.sian walnut.
[Ill.u.s.tration: Fig. 32. Figure Formed by Pith Rays in Oak (full size).]
[Ill.u.s.tration: Fig. 33. Sweet Gum, Showing Uneven Deposit of Coloring Matter (full size.)]
The presence of a limb const.i.tutes a knot and makes great irregularity in the grain of wood, Fig. 34. In the first place, the fibers on the upper and lower sides of the limb behave differently, those on the lower side running uninterruptedly from the stem into the limb, while on the upper side the fibers bend aside making an imperfect connection. Consequently to split a knot it is always necessary to start the split from the lower side. On the other hand it is easier to split around a knot than thru it. The texture as well as the grain of wood is modified by the presence of a branch. The wood in and around a knot is much harder than the main body of the trunk on account of the crowding together of the elements. Knots are the remnants of branches left in the trunk. These once had all the parts of the trunk itself, namely bark, cambium, wood, and pith. Normally, branches grow from the pith, tho some trees, as Jack pine and redwood, among the conifers, and most of the broad-leaf trees have the power of putting out at any time advent.i.tious buds which may develop into branches. When a branch dies, the annual layer of wood no longer grows upon it, but the successive layers of wood on the trunk itself close tighter and tighter around it, until it is broken off. Then, unless it has begun to decay, it is successively overgrown by annual layers, so that no sign of it appears until the trunk is cut open. A large trunk perfectly clean of branches on the outside may have many knots around its center, remnants of branches which grew there in its youth, as in Fig. 34, and Fig. 8, p. 18. The general effect of the presence of a knot is, that the fibers that grow around and over it are bent, and this, of course, produces crooked grain.
Following are the designations given to different knots by lumbermen: A _sound_ knot is one which is solid across its face and is as hard as the wood surrounding it and fixed in position. A _pin_ knot is sound, but not over 1/4" in diameter. A _standard_ knot is sound, but not over 1-1/2" in diameter. A _large_ knot is sound, and over 1-1/2"
in diameter. A _spike_ knot is one sawn in a lengthwise position. A _dead_, or, _loose_ knot is one not firmly held in place by growth or position.
(4) _Pith._ At the center or axis of the tree is the pith or _medulla_, Fig. 34. In every bud, that is, at the apex of every stem and branch, the pith is the growing part; but as the stem lengthens and becomes overgrown by successive layers of wood the pith loses its vital function. It does not grow with the plant except at the buds.
It varies in thickness, being very small,--hardly more than 1/16", in cedar and larch,--and so small in oak as to be hardly discernible; and what there is of it turns hard and dark. In herbs and shoots it is relatively large, Fig. 5, p. 15, in a three-year old shoot of elder, for example, being as wide as the wood. In elder, moreover, it dies early and pulverizes, leaving the stem hollow. Its function is one of only temporary value to the plant.
[Ill.u.s.tration: Fig. 34. Section Thru the Trunk of a Seven Year Old Tree, Showing Relation of Branches to Main Stem. A, B, two branches which were killed after a few years" growth by shading, and which have been overgrown by the annual rings of wood; C, a limb which lived four years, then died and broke off near the stem, leaving the part to the left of XY a "sound" knot, and the part to the right a "dead" knot, which unless rotting sets in, would in time be entirely covered by the growing trunk; D, a branch that has remained alive and has increased in size like the main stem; P, P, pith of both stem and limb.]
THE STRUCTURE OF WOOD.
REFERENCES:[A]
Roth, _Forest Bull._ No. 10, pp. 11-23.
Boulger, pp. 1-39.
Sickles, pp. 11-20.
Pinchot, _Forest Bull._ No. 24, I, pp. 11-24.
Keeler, pp. 514-517.
Curtis, pp. 62-85.
Woodcraft, 15: 3, p. 90.
Bitting, _Wood Craft_, 5: 76, 106, 144, 172, (June-Sept. 1906).
Ward, pp. 1-38.
_Encyc. Brit._, 11th Ed., "Plants," p. 741.
Strasburger, pp. 120-144 and Part II, Sec. II.
Snow, pp. 7-9, 183.
[Footnote A: For general bibliography, see p. 4.]
CHAPTER II.
PROPERTIES OF WOOD.
There are many properties of wood,--some predominant in one species, some in another,--that make it suitable for a great variety of uses.
Sometimes it is a combination of properties that gives value to a wood. Among these properties are hygroscopicity, shrinkage, weight, strength, cleavability, elasticity, hardness, and toughness.
THE HYGROSCOPICITY[1] OF WOOD.
It is evident that water plays a large part in the economy of the tree. It occurs in wood in three different ways: In the sap which fills or partly fills the cavities of the wood cells, in the cell walls which it saturates, and in the live protoplasm, of which it const.i.tutes 90 per cent. The younger the wood, the more water it contains, hence the sap-wood contains much more than the heart-wood, at times even twice as much.
In fresh sap-wood, 60 per cent. of the water is in the cell cavities, 35 per cent. in the cell walls, and only 5 per cent. in the protoplasm. There is so much water in green wood that a sappy pole will soon sink when set afloat. The reason why there is much less water in heart-wood is because its cells are dead and inactive, and hence without sap and without protoplasm. There is only what saturates the cell walls. Even so, there is considerable water in heart-wood.[2]
The lighter kinds have the most water in the sap-wood, thus sycamore has more than hickory.
Curiously enough, a tree contains about as much water in winter as in summer. The water is held there, it is supposed, by capillary attraction, since the cells are inactive, so that at all times the water in wood keeps the cell walls distended.
THE SHRINKAGE OF WOOD.
When a tree is cut down, its water at once begins to evaporate. This process is called "seasoning."[A] In drying, the free water within the cells keeps the cell walls saturated; but when all the free water has been removed, the cell walls begin to yield up their moisture. Water will not flow out of wood unless it is forced out by heat, as when green wood is put on a fire. Ordinarily it evaporates slowly.
[Footnote A: See _Handwork in Wood_, Chapter III.]
The water evaporates faster from some kinds of wood than from other kinds, _e.g._, from white pine than from oak, from small pieces than from large, and from end grain than from a longitudinal section; and it also evaporates faster in high than in low temperatures.
Evaporation affects wood in three respects, weight, strength, and size. The weight is reduced, the strength is increased, and shrinkage takes place. The reduction in weight and increase in strength, important as they are, are of less importance than the shrinkage, which often involves warping and other distortions. The water in wood affects its size by keeping the cell walls distended.
If all the cells of a piece of wood were the same size, and had walls the same thickness, and all ran in the same direction, then the shrinkage would be uniform. But, as we have seen, the structure of wood is not h.o.m.ogeneous. Some cellular elements are large, some small, some have thick walls, some thin walls, some run longitudinally and some (the pith rays) run radially. The effects will be various in differently shaped pieces of wood but they can easily be accounted for if one bears in mind these three facts: (1) that the shrinkage is in the cell wall, and therefore (2) that the thick-walled cells shrink more than thin-walled cells and (3) that the cells do not shrink much, if any, lengthwise.
(1) The shrinkage of wood takes place in the walls of the cells that compose it, that is, the cell walls become thinner, as indicated by the dotted lines in Fig. 35, which is a cross-section of a single cell. The diameter of the whole cell becomes less, and the opening, or lumen, of the cell becomes larger.
[Ill.u.s.tration: Fig. 35. How Cell Walls Shrink.]
(2) Thick-walled cells shrink more than thin-walled cells, that is, summer cells more than spring cells. This is due to the fact that they contain more shrinkable substance. The thicker the wall, the more the shrinkage.
Consider the effects of these changes; ordinarily a log when drying begins to "check" at the end. This is to be explained thus: Inasmuch as evaporation takes place faster from a cross than from a longitudinal section, because at the cross-section all the cells are cut open, it is to be expected that the end of a piece of timber, Fig.
36, A, will shrink first. This would tend to make the end fibers bend toward the center of the piece as in B, Fig. 36. But the fibers are stiff and resist this bending with the result that the end splits or "checks" as in C, Fig. 36. But later, as the rest of the timber dries out and shrinks, it becomes of equal thickness again and the "checks"
tend to close.
[Ill.u.s.tration: Fig. 36. The Shrinkage and Checking at the End of a Beam.]
(3) For some reason, which has not been discovered, the cells or fibers of wood do not shrink in length to any appreciable extent. This is as true of the cells of pith rays, which run radially in the log, as of the ordinary cells, which run longitudinally in it.
In addition to "checking" at the end, logs ordinarily show the effect of shrinkage by splitting open radially, as in Fig. 37. This is to be explained by two factors, (1) the disposition of the pith (or medullary) rays, and (2) the arrangement of the wood in annual rings.
[Ill.u.s.tration: Fig. 37. The Shrinkage and Splitting of a Log.]
(1) The cells of the pith rays, as we have seen in Chapter I, run at right angles to the direction of the ma.s.s of wood fibers, and since they shrink according to the same laws that other cells do, viz., by the cell wall becoming thinner but not shorter, the strain of their shrinkage is contrary to that of the main cells. The pith rays, which consist of a number of cells one above the other, tend to shrink parallel to the length of the wood, and whatever little longitudinal shrinkage there is in a board is probably due mostly to the shrinkage of the pith rays. But because the cells of pith rays do not appreciably shrink in their length, this fact tends to prevent the main body of wood from shrinking radially, and the result is that wood shrinks less radially than tangentially. Tangentially is the only way left for it to shrink. The pith rays may be compared to the ribs of a folding fan, which keep the radius of unaltered length while permitting comparative freedom for circ.u.mferential contraction.
(2) It is evident that since summer wood shrinks more than spring wood, this fact will interfere with the even shrinkage of the log.