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Wood

2007 Schools Wikipedia Selection. Related subjects: Engineering; Organisms

   Trunks
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   Trunks
   A tree trunk as found at the Veluwe, The Netherlands
   Enlarge
   A tree trunk as found at the Veluwe, The Netherlands

   Wood is derived from woody plants, notably trees but also shrubs. Wood
   from the latter is only produced in small sizes, reducing the diversity
   of uses.

   In its most common meaning, "wood" is the secondary xylem of a woody
   plant, but this is an approximation only: in the wider sense, wood may
   refer to other materials and tissues with comparable properties. Wood
   is a heterogeneous, hygroscopic, cellular and anisotropic material.
   Wood is composed of fibers of cellulose (40%–50%) and hemicellulose
   (15%–25%) held together by lignin (15%–30%).

Uses

   Artists can use wood to create delicate sculptures.
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   Artists can use wood to create delicate sculptures.

   Wood has been used for millennia for many purposes, being many things
   to many people. One of its primary uses is as fuel. It is also used as
   a material, for making artworks, boats, buildings, furniture, ships,
   tools, weapons, etc. Wood has been an important construction material
   since humans began building shelters, and remains in plentiful use
   today. Construction wood is commonly known as lumber in North America
   and timber elsewhere. Wood may be broken down and be made into
   chipboard, engineered wood, hardboard, medium-density fibreboard (MDF),
   oriented strand board (OSB), paper or used to make other synthetic
   substances.

Formation

   A tree increases in diameter by the formation, between the old wood and
   the inner bark, of new woody layers which envelop the entire stem,
   living branches, and roots. Where there are clear seasons, this can
   happen in a discrete pattern, leading to what is known as growth rings,
   as can be seen on the end of a log. If these seasons are annual these
   growth rings are annual rings. Where there is no seasonal difference
   growth rings are likely to be absent.

   Within a growth ring it may be possible to see two parts. The part
   nearest the center of the tree is more open textured and almost
   invariably lighter in colour than that near the outer portion of the
   ring. The inner portion is formed early in the season, when growth is
   comparatively rapid; it is known as early wood or spring wood. The
   outer portion is the late wood or summer wood, being produced in the
   summer. In white pines there is not much contrast in the different
   parts of the ring, and as a result the wood is very uniform in texture
   and is easy to work. In hard pines, on the other hand, the late wood is
   very dense and is deep-colored, presenting a very decided contrast to
   the soft, straw-colored early wood. In ring-porous woods each season's
   growth is always well defined, because the large pores of the spring
   abut on the denser tissue of the fall before. In the diffuse-porous
   woods, the demarcation between rings is not always so clear and in some
   cases is almost (if not entirely) invisible to the unaided eye.

Knots

   Knots are portions of branches included in the wood of the stem or
   larger branch. Branches generally originate at or near the pith
   (central axis) of a stem, and the living portion will increase in size
   through the addition of annual woody layers which are a continuation of
   those of the stem. The included portion is irregularly conical in shape
   with the tip at the pith. The fibre direction is at right angles or
   oblique to the grain of the stem, thus producing local cross grain. A
   small knot may also be the result of a dormant bud.

   During the development of a tree the lower limbs die, but may persist
   for a time--often for years. Subsequent layers of growth of the stem
   are no longer intimately joined with the dead limb, but are laid around
   it. Hence dead branches produce knots which are nothing more than pegs
   in a hole, and likely to drop out after the tree has been sawn. In
   grading lumber and structural timber, knots are classified according to
   their form, size, soundness, and the firmness with which they are held
   in place.

   Knots materially affect checking (cracking) and warping, ease in
   working, and cleavability of timber. They are defects which weaken
   timber and depreciate its value for structural purposes where strength
   is an important consideration. The weakening effect is much more
   serious where timber is subjected to bending and tension than where
   under compression. The extent to which knots affect the strength of a
   beam depends upon their position, size, number, direction of fibre, and
   condition. A knot on the upper side is compressed, while one on the
   lower side is subjected to tension. The knot, especially (as is often
   the case) if there is a season check in it, offers little resistance to
   this tensile stress. Small knots, however, may be so located in a beam
   along the neutral plane as actually to increase the strength by tending
   to prevent longitudinal shearing. Knots in a board or plank are least
   injurious when they extend through it at right angles to its broadest
   surface. Knots which occur near the ends of a beam do not weaken it.
   Sound knots which occur in the central portion one-fourth the height of
   the beam from either edge are not serious defects.

   Knots do not necessarily influence the stiffness of structural timber.
   Only defects of the most serious character affect the elastic limit of
   beams. Stiffness and elastic strength are more dependent upon the
   quality of the wood fibre than upon defects in the beam. The effect of
   knots is to reduce the difference between the fibre stress at elastic
   limit and the modulus of rupture of beams. The breaking strength is
   very susceptible to defects. Sound knots do not weaken wood when
   subject to compression parallel to the grain.

   For some purposes, e.g. wall panelling, knots are considered a plus as
   they add visual texture to the wood, giving it a more interesting
   appearance.

   The traditional style of playing the Basque xylophon txalaparta
   involves hitting the right knots to obtain different tones.

Heartwood and sapwood

   A section of a Yew branch showing 27 annual growth rings, pale sapwood
   and dark heartwood, and pith (centre dark spot). The dark radial lines
   are small knots.
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   A section of a Yew branch showing 27 annual growth rings, pale sapwood
   and dark heartwood, and pith (centre dark spot). The dark radial lines
   are small knots.

   Examination of the end of a log of many species reveals a
   darker-colored inner portion, called the heartwood or duramen,
   surrounded by a lighter-colored zone called the sapwood. In some
   instances this distinction in color is very marked; in others, the
   contrast is slight, so that it is not always easy to tell where one
   leaves off and the other begins. The colour of fresh sapwood is always
   light, sometimes nearly white, but more often with a decided tinge of
   yellow or brown.

   Sapwood is comparatively new wood, comprising living cells in the
   growing tree. All wood in a tree is first formed as sapwood. Its
   principal functions are to conduct water from the roots to the leaves
   and to store up and give back according to the season the food prepared
   in the leaves. The more leaves a tree bears and the more vigorous its
   growth, the larger the volume of sapwood required. Hence trees making
   rapid growth in the open have thicker sapwood for their size than trees
   of the same species growing in dense forests. Sometimes trees grown in
   the open may become of considerable size, 30 cm or more in diameter,
   before any heartwood begins to form, for example, in second-growth
   hickory, or open-grown pines.

   As a tree increases in age and diameter an inner portion of the sapwood
   becomes inactive and finally ceases to function, as the cells die. This
   inert or dead portion is called heartwood. Its name derives solely from
   its position and not from any vital importance to the tree. This is
   shown by the fact that a tree can thrive with its heart completely
   decayed. Some species begin to form heartwood very early in life, so
   having only a thin layer of live sapwood, while in others the change
   comes slowly. Thin sapwood is characteristic of such trees as chestnut,
   black locust, mulberry, osage-orange, and sassafras, while in maple,
   ash, hickory, hackberry, beech, and pine, thick sapwood is the rule.

   There is no definite relation between the annual rings of growth and
   the amount of sapwood. Within the same species the cross-sectional area
   of the sapwood is very roughly proportional to the size of the crown of
   the tree. If the rings are narrow, more of them are required than where
   they are wide. As the tree gets larger, the sapwood must necessarily
   become thinner or increase materially in volume. Sapwood is thicker in
   the upper portion of the trunk of a tree than near the base, because
   the age and the diameter of the upper sections are less.

   When a tree is very young it is covered with limbs almost, if not
   entirely, to the ground, but as it grows older some or all of them will
   eventually die and are either broken off or fall off. Subsequent growth
   of wood may completely conceal the stubs which will however remain as
   knots. No matter how smooth and clear a log is on the outside, it is
   more or less knotty near the middle. Consequently the sapwood of an old
   tree, and particularly of a forest-grown tree, will be freer from knots
   than the heartwood. Since in most uses of wood, knots are defects that
   weaken the timber and interfere with its ease of working and other
   properties, it follows that sapwood, because of its position in the
   tree, may have certain advantages over heartwood.

   It is remarkable that the inner heartwood of old trees remains as sound
   as it usually does, since in many cases it is hundreds of years, and in
   a few instances thousands of years, old. Every broken limb or root, or
   deep wound from fire, insects, or falling timber, may afford an
   entrance for decay, which, once started, may penetrate to all parts of
   the trunk. The larvae of many insects bore into the trees and their
   tunnels remain indefinitely as sources of weakness. Whatever
   advantages, however, that sapwood may have in this connection are due
   solely to its relative age and position.

   If a tree grows all its life in the open and the conditions of soil and
   site remain unchanged, it will make its most rapid growth in youth, and
   gradually decline. The annual rings of growth are for many years quite
   wide, but later they become narrower and narrower. Since each
   succeeding ring is laid down on the outside of the wood previously
   formed, it follows that unless a tree materially increases its
   production of wood from year to year, the rings must necessarily become
   thinner as the trunk gets wider. As a tree reaches maturity its crown
   becomes more open and the annual wood production is lessened, thereby
   reducing still more the width of the growth rings. In the case of
   forest-grown trees so much depends upon the competition of the trees in
   their struggle for light and nourishment that periods of rapid and slow
   growth may alternate. Some trees, such as southern oaks, maintain the
   same width of ring for hundreds of years. Upon the whole, however, as a
   tree gets larger in diameter the width of the growth rings decreases.

   There may be decided differences in the grain of heartwood and sapwood
   cut from a large tree, particularly one that is mature. In some trees,
   the wood laid on late in the life of a tree is softer, lighter, weaker,
   and more even-textured than that produced earlier, but in other
   species, the reverse applies. In a large log the sapwood, because of
   the time in the life of the tree when it was grown, may be inferior in
   hardness, strength, and toughness to equally sound heartwood from the
   same log.

Different woods

   There is a strong relationship between the properties of wood and the
   properties of the particular tree that yielded it. For every trees
   species there is a range of density for the wood it yields. There is a
   rough correlation between density of a wood and its strength
   (mechanical properties). For example, while mahogany is a medium-dense
   hardwood which is excellent for fine furniture crafting, balsa is
   light, making it useful for model building. The densest wood may be
   black ironwood.

   Wood is commonly classified as either softwood or hardwood. The wood
   from conifers (e.g. pine) is called softwood, and the wood from
   broad-leaved trees (e.g. oak) is called hardwood. These names are a bit
   misleading, as hardwoods are not necessarily hard, and softwoods are
   not necessarily soft. The well-known balsa (a hardwood) is actually
   softer than any commercial softwood. Conversely, some softwoods (e.g.
   yew) are harder than most hardwoods.

Colour

   In species which show a distinct difference between heartwood and
   sapwood the natural colour of heartwood is usually darker than that of
   the sapwood, and very frequently the contrast is conspicuous. This is
   produced by deposits in the heartwood of various materials resulting
   from the process of growth, increased possibly by oxidation and other
   chemical changes, which usually have little or no appreciable effect on
   the mechanical properties of the wood. Some experiments on very
   resinous Longleaf Pine specimens, however, indicate an increase in
   strength. This is due to the resin which increases the strength when
   dry. Such resin-saturated heartwood is called "fat lighter". Structures
   built of fat lighter are almost impervious to rot and termites; however
   they are very flammable. Stumps of old longleaf pines are often dug,
   split into small pieces and sold as kindling for fires. Stumps thus dug
   may actually remain a century or more since being cut. Spruce
   impregnated with crude resin and dried is also greatly increased in
   strength thereby.
   The wood of Coast Redwood is distinctively red in colour
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   The wood of Coast Redwood is distinctively red in colour

   Since the late wood of a growth ring is usually darker in color than
   the early wood, this fact may be used in judging the density, and
   therefore the hardness and strength of the material. This is
   particularly the case with coniferous woods. In ring-porous woods the
   vessels of the early wood not infrequently appear on a finished surface
   as darker than the denser late wood, though on cross sections of
   heartwood the reverse is commonly true. Except in the manner just
   stated the colour of wood is no indication of strength.

   Abnormal discoloration of wood often denotes a diseased condition,
   indicating unsoundness. The black check in western hemlock is the
   result of insect attacks. The reddish-brown streaks so common in
   hickory and certain other woods are mostly the result of injury by
   birds. The discoloration is merely an indication of an injury, and in
   all probability does not of itself affect the properties of the wood.
   Certain rot-producing fungi impart to wood characteristic colors which
   thus become symptomatic of weakness; however an attractive effect known
   as spalting produced by this process is often considered a desirable
   characteristic. Ordinary sap-staining is due to fungous growth, but
   does not necessarily produce a weakening effect.

Structure

   In coniferous or softwood species the wood cells are mostly of one
   kind, tracheids, and as a result the material is much more uniform in
   structure than that of most hardwoods. There are no vessels ("pores")
   in coniferous wood such as one sees so prominently in oak and ash, for
   example.
   Magnified cross-section of a diffuse-porous hardwood wood (Black
   Walnut), showing the vessels, rays (white lines) and annual rings
   Enlarge
   Magnified cross-section of a diffuse-porous hardwood wood ( Black
   Walnut), showing the vessels, rays (white lines) and annual rings

   The structure of the hardwoods is more complex. They are more or less
   filled with vessels: in some cases ( oak, chestnut, ash) quite large
   and distinct, in others ( buckeye, poplar, willow) too small to be seen
   plainly without a small hand lens. In discussing such woods it is
   customary to divide them into two large classes, ring-porous and
   diffuse-porous. In ring-porous species, such as ash, black locust,
   catalpa, chestnut, elm, hickory, mulberry, and oak, the larger vessels
   or pores (as cross sections of vessels are called) are localized in the
   part of the growth ring formed in spring, thus forming a region of more
   or less open and porous tissue. The rest of the ring, produced in
   summer, is made up of smaller vessels and a much greater proportion of
   wood fibres. These fibres are the elements which give strength and
   toughness to wood, while the vessels are a source of weakness.

   In diffuse-porous woods the pores are scattered throughout the growth
   ring instead of being collected in a band or row. Examples of this kind
   of wood are basswood, birch, buckeye, maple, poplar, and willow. Some
   species, such as walnut and cherry, are on the border between the two
   classes, forming an intermediate group.

   If a heavy piece of pine is compared with a light specimen it will be
   seen at once that the heavier one contains a larger proportion of late
   wood than the other, and is therefore considerably darker. The late
   wood of all species is denser than that formed early in the season,
   hence the greater the proportion of late wood the greater the density
   and strength. When examined under a microscope the cells of the late
   wood are seen to be very thick-walled and with very small cavities,
   while those formed first in the season have thin walls and large
   cavities. The strength is in the walls, not the cavities. In choosing a
   piece of pine where strength or stiffness is the important
   consideration, the principal thing to observe is the comparative
   amounts of early and late wood. The width of ring is not nearly so
   important as the proportion of the late wood in the ring.
   Wood can be cut into straight planks and made into a hardwood floor
   (parquetry).
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   Wood can be cut into straight planks and made into a hardwood floor (
   parquetry).

   It is not only the proportion of late wood, but also its quality, that
   counts. In specimens that show a very large proportion of late wood it
   may be noticeably more porous and weigh considerably less than the late
   wood in pieces that contain but little. One can judge comparative
   density, and therefore to some extent weight and strength, by visual
   inspection.
   The twisty branch of a Lilac tree
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   The twisty branch of a Lilac tree

   No satisfactory explanation can as yet be given for the real causes
   underlying the formation of early and late wood. Several factors may be
   involved. In conifers, at least, rate of growth alone does not
   determine the proportion of the two portions of the ring, for in some
   cases the wood of slow growth is very hard and heavy, while in others
   the opposite is true. The quality of the site where the tree grows
   undoubtedly affects the character of the wood formed, though it is not
   possible to formulate a rule governing it. In general, however, it may
   be said that where strength or ease of working is essential, woods of
   moderate to slow growth should be chosen. But in choosing a particular
   specimen it is not the width of ring, but the proportion and character
   of the late wood which should govern.

   In the case of the ring-porous hardwoods there seems to exist a pretty
   definite relation between the rate of growth of timber and its
   properties. This may be briefly summed up in the general statement that
   the more rapid the growth or the wider the rings of growth, the
   heavier, harder, stronger, and stiffer the wood. This, it must be
   remembered, applies only to ring-porous woods such as oak, ash,
   hickory, and others of the same group, and is, of course, subject to
   some exceptions and limitations.

   In ring-porous woods of good growth it is usually the middle portion of
   the ring in which the thick-walled, strength-giving fibres are most
   abundant. As the breadth of ring diminishes, this middle portion is
   reduced so that very slow growth produces comparatively light, porous
   wood composed of thin-walled vessels and wood parenchyma. In good oak
   these large vessels of the early wood occupy from 6 to 10 per cent of
   the volume of the log, while in inferior material they may make up 25
   per cent or more. The late wood of good oak, except for radial grayish
   patches of small pores, is dark colored and firm, and consists of
   thick-walled fibres which form one-half or more of the wood. In
   inferior oak, such fibre areas are much reduced both in quantity and
   quality. Such variation is very largely the result of rate of growth.

   Wide-ringed wood is often called "second-growth", because the growth of
   the young timber in open stands after the old trees have been removed
   is more rapid than in trees in the forest, and in the manufacture of
   articles where strength is an important consideration such
   "second-growth" hardwood material is preferred. This is particularly
   the case in the choice of hickory for handles and spokes. Here not only
   strength, but toughness and resilience are important. The results of a
   series of tests on hickory by the U.S. Forest Service show that:

          "The work or shock-resisting ability is greatest in wide-ringed
          wood that has from 5 to 14 rings per inch (rings 1.8-5 mm
          thick), is fairly constant from 14 to 38 rings per inch (rings
          0.7-1.8 mm thick), and decreases rapidly from 38 to 47 rings per
          inch (rings 0.5-0.7 mm thick). The strength at maximum load is
          not so great with the most rapid-growing wood; it is maximum
          with from 14 to 20 rings per inch (rings 1.3-1.8 mm thick), and
          again becomes less as the wood becomes more closely ringed. The
          natural deduction is that wood of first-class mechanical value
          shows from 5 to 20 rings per inch (rings 1.3-5 mm thick) and
          that slower growth yields poorer stock. Thus the inspector or
          buyer of hickory should discriminate against timber that has
          more than 20 rings per inch (rings less than 1.3 mm thick).
          Exceptions exist, however, in the case of normal growth upon dry
          situations, in which the slow-growing material may be strong and
          tough."

   The effect of rate of growth on the qualities of chestnut wood is
   summarized by the same authority as follows:

          "When the rings are wide, the transition from spring wood to
          summer wood is gradual, while in the narrow rings the spring
          wood passes into summer wood abruptly. The width of the spring
          wood changes but little with the width of the annual ring, so
          that the narrowing or broadening of the annual ring is always at
          the expense of the summer wood. The narrow vessels of the summer
          wood make it richer in wood substance than the spring wood
          composed of wide vessels. Therefore, rapid-growing specimens
          with wide rings have more wood substance than slow-growing trees
          with narrow rings. Since the more the wood substance the greater
          the weight, and the greater the weight the stronger the wood,
          chestnuts with wide rings must have stronger wood than chestnuts
          with narrow rings. This agrees with the accepted view that
          sprouts (which always have wide rings) yield better and stronger
          wood than seedling chestnuts, which grow more slowly in
          diameter."

   In diffuse-porous woods, as has been stated, the vessels or pores are
   scattered throughout the ring instead of collected in the early wood.
   The effect of rate of growth is, therefore, not the same as in the
   ring-porous woods, approaching more nearly the conditions in the
   conifers. In general it may be stated that such woods of medium growth
   afford stronger material than when very rapidly or very slowly grown.
   In many uses of wood, strength is not the main consideration. If ease
   of working is prized, wood should be chosen with regard to its
   uniformity of texture and straightness of grain, which will in most
   cases occur when there is little contrast between the late wood of one
   season's growth and the early wood of the next.

Water content

   Water occurs in living wood in three conditions, namely: (1) in the
   cell walls, (2) in the protoplasmic contents of the cells, and (3) as
   free water in the cell cavities and spaces. In heartwood it occurs only
   in the first and last forms. Wood that is thoroughly air-dried retains
   from 8-16% of water in the cell walls, and none, or practically none,
   in the other forms. Even oven-dried wood retains a small percentage of
   moisture, but for all except chemical purposes, may be considered
   absolutely dry.

   The general effect of the water content upon the wood substance is to
   render it softer and more pliable. A similar effect of common
   observation is in the softening action of water on paper or cloth.
   Within certain limits the greater the water content the greater its
   softening effect.

   Drying produces a decided increase in the strength of wood,
   particularly in small specimens. An extreme example is the case of a
   completely dry spruce block 5 cm in section, which will sustain a
   permanent load four times as great as that which a green block of the
   same size will support.

   The greatest increase due to drying is in the ultimate crushing
   strength, and strength at elastic limit in endwise compression; these
   are followed by the modulus of rupture, and stress at elastic limit in
   cross-bending, while the modulus of elasticity is least affected.

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