Part 1 (2/2)
Pores belong to hardwoods only. The resin ducts in some softwoods present a similar appearance, but are far less numerous. All pores are, of course, situated in the annual rings, but in different species they are differently located as to spring and summerwood. In some woods the largest pores are in the springwood only and therefore run in rings.
Such woods are called ”ring porous,” and the oaks are best examples. In other species the pores are scattered through all parts of the ring in about the same proportion, and such woods are called ”diffuse porous,”
as the birches. Softwoods have no pores proper, and are cla.s.sed ”non-porous.”
_Medullary Rays_--A smoothly-cut cross section of almost any oak, but particularly white oak and red oak, exhibits to the unaided eye narrow, light-colored lines radiating from the center of the tree toward the bark like spokes of a wheel. They are about the breadth of a fine pencil mark, and are generally a sixth of an inch or less apart. They are among the most conspicuous and characteristic features of oak wood, and are known as medullary or pith rays.
Oak is cited as an example because the rays are large and prominent, but they are present in all wood, and const.i.tute a large part of its body.
They vary greatly in size. In some woods a few are visible unmagnified; but even in oak a hundred are invisible to the naked eye to one that can be seen. Some species show none until a gla.s.s is used. Some pines have fifteen thousand to a square inch of cross section, all of which are so small as to elude successfully the closest search of the unaided eye.
The medullary rays influence the appearance of most wood. They determine its character. Oak is quarter-sawed for the purpose of bringing out the bright, flat surfaces of these rays. The prominent flecks, streaks, and patches of silvery wood are the flat sides of medullary rays. In cross section, only the line-like ends are seen, but quarter-sawing exposes their sides to view.
That explains in part why some species are adapted to quarter-sawing and others are not. If no broad rays exist in the wood, as with white pine, red cedar, and cottonwood, quarter-sawing cannot add much to the wood's appearance.
_Grain_--The grain of wood is not a definite quality. The word does not mean the same thing to all who use it. It sometimes refers to rings of yearly growth, and in that sense a narrow-ringed wood is fine grained, and one with wide rings is coa.r.s.e grained. A curly, wavy, smoky, or birdseye wood does not owe its quality to annual rings, yet with some persons, all of these figures are called grain. The term sometimes refers to medullary rays, again to hardness, or to roughness. Some mahogany is called ”woolly grained” because the surface polishes with difficulty. The pattern maker designates white pine as ”even grained”, because it cuts easily in all directions. The handle maker cla.s.ses hickory as ”smooth grained”, because it polishes well and the sole idea of the maker is smoothness to the touch. There are other grains almost as numerous as the trades which use wood. In numerous instances ”figure”
is a better term than ”grain.” Feather mahogany, birdseye birch, burl ash, are figures rather than grains. There is no authority to settle and decide what the real meaning of grain is in wood technology. It has a number of meanings, and one man has as much authority as another to interpret it in accordance with his own ideas, and the usage in his trade. It is a loose term which covers several things in general and nothing in particular.
_Weight_--The weight of wood is calculated from different standpoints.
It has a green weight, an air-dry weight, a kiln-dry weight, and an oven-dry weight. All are different, but the differences are due to the relative amounts of water weighed. Sawlogs generally go by green weight; yard lumber by air-dry or partly air-dry weight; while the wood used in ultimate manufacture, such as furniture, is supposed to be kiln-dry.
The absolute weight of wood, with all air s.p.a.ces, moisture, and other foreign material removed, is about 100 pounds per cubic foot, which is 1.6 times heavier than water; but that is not a natural form of wood. It is known only in the laboratory.
The actual wood substance of one species weighs about the same as another. Dispense with all air s.p.a.ces, all water, and all other foreign substance, and pine and ebony weigh alike. It is apparent that the different weights of woods, as between cedar and oak for example, are due chiefly to porosity. The smaller the aggregate s.p.a.ce occupied by pores and other cavities, the heavier the wood. That accounts for the differences in weights of absolutely dry woods of different kinds, except that a small amount of other foreign material may remain after water has been driven off. Florida black ironwood is rated as the heaviest in the United States, and it weighs 81.14 pounds per cubic foot, oven-dry. The lightest in this country is the golden fig which is a native of Florida also. It weighs 16.3 pounds per cubic foot, oven-dry. When weights of wood are given, the specimen is understood to be oven-dry, unless it is stated to be otherwise: it is a laboratory weight, calculated from small cubes of the wood. Such weights are always a little less than that of the dryest wood of the same kind that can be obtained in the lumber market.
_Moisture in Wood_--The varying weights of the same wood indicate that moisture plays an important part. No man ever saw absolutely dry wood.
If heated sufficiently to drive off all the moisture, the wood is reduced to charcoal and other products of destructive distillation.
The pores and other cavities in green timber are more or less filled with water or sap. This may amount to one-third, one-half, or even more, of the dry weight of the wood. The water is in the hollow vessels and cell walls. A living tree contains about the same quant.i.ty of water in winter as in summer, though the common belief is otherwise. It is misleading to say that the sap is ”down” in one season and ”up” in another, although there is more activity at certain times than in others. Strictly speaking, there is a difference between the water in a tree, and the tree's sap; but in common parlance they are considered identical. What takes place is this: water rises from the tree's roots, through the wood, carrying certain minerals in solution. Some of it reaches the leaves in summer where it mixes with certain gases from the air, and is converted into sap proper. Most of the surplus water, after giving up the mineral substance held in solution, is evaporated through the leaves into the air; but the sap, starting from the leaves which act as laboratories for its manufacture, goes down through the newly-formed (and forming), layer of wood just beneath the bark, and is converted into wood. This newly-formed wood is colorless at first. It builds up the annual ring, first the springwood very rapidly, and then the summerwood more slowly.
The force which causes water to rise through the trunk of a tree is not fully understood. It is one of nature's mysteries which is yet to be solved. Forces known as root pressure, capillary attraction, and osmosis, are believed to be active in the process, but there seems to be something additional, and no man has yet been able to explain what it is.
The seasoning of wood is the process of getting rid of some of the water. As soon as lumber is exposed to air, the water begins to escape.
Long exposure to dry air takes out a large percentage of the moisture which green wood holds, and the lumber is known as air-dry. But some of the original moisture remains, and air at climatic temperature is unable to expel it. The greater heat of a drykiln drives away some more of it, but a quant.i.ty yet remains. The lumber is then kiln-dry. Greater heat than the drykiln's is secured in an oven, and a little more of the wood's moisture is expelled; but the only method of driving all the moisture out is to heat the wood sufficiently to break down its structure, and reduce it to charcoal.
Wood warps in the process of drying unless it seasons equally on all sides. It curls or bends toward the side which dries most rapidly. Dry wood may warp if exposed to dampness, if one side is more exposed and receives more moisture than another. It curls or bends toward the dryer side.
Warping is primarily due to the more rapid contraction or expansion of wood cells on one side of the piece than on the other. Saturated cells are larger than dry ones.
Moisture in wood affects its strength, the dryer the stronger, at least within certain limits. Architects and builders carefully study the seasoning of timber, because it is a most important factor in their business. The moisture which most affects a wood's strength is that absorbed in the cell walls, rather than that contained in the cell cavities themselves.
Some woods check or split badly in seasoning unless attended with constant care. Checking is due chiefly to lack of uniformity in seasoning. One part of the stick dries faster than another, the dryer fibers contract, and the pull splits the wood. The checks may be small, even microscopic, or they may develop yawning cracks such as sometimes appear in the ends of hickory and black walnut logs. Greenwood checks worse in summer than in winter, because the weather is warmer, the wood's surface dries faster, and the strain on the fibers is greater.
Phases of the moon have no influence on the seasoning, checking, warping, or lasting properties of timber.
_Stiffness, Elasticity, and Strength_--Rules for measuring the stiffness of timber are involved in mathematical formulas; but the practical quality of stiffness is not difficult to understand. Wood which does not bend easily is stiff. If it springs back to its original position after the removal of the force which bends it, the wood is elastic. The greatest load it can sustain without breaking, is the measure of its strength. The load required to produce a certain amount of bending is the measure of its stiffness. Flexibility, a term much used by certain cla.s.ses of workers in wood, is the opposite of stiffness. A brittle wood is not necessarily weak. It may sustain a heavy load without breaking, but when it fails, the break is sudden and complete. A tough wood behaves differently, though it may not be as strong as a brittle one.
When a tough wood breaks, the parts are inclined to adhere after they have ceased to sustain the load. Hickory is tough, and in breaking, the wood crushes and splinters. Mesquite is brittle, and a clean snap severs the stick at once.
Builders of houses and bridges, and the manufacturers of articles of wood, study with the greatest care the stiffness, elasticity, strength, toughness, and brittleness of timber. Its chief value may depend upon the presence or absence of one or more of these properties. Take away hickory's toughness and elasticity and it would cease to be a great vehicle and handle material. Reduce the stiffness and strength of longleaf pine and Douglas fir and they would drop at once from the high esteem in which they are held as structural timbers. Destroy the brittleness of red cedar and it would lose one of the chief qualities which make it the leading lead pencil wood of the world.
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