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Wood
Wood
is a solid material derived from
woody plants, notably
trees but also
shrubs.
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, biodegradable, combustibl and
anisotropic material.
Wood is
composed of fibers of
cellulose (40%–50%) and
hemicellulose (15%–25%) held together by
lignin (15%–30%).[1]
Wood is an organic material found as the primary content of
the stems of woody plants, especially trees, but also shrubs. These perennial
plants are characterized by stems that grow outward year after year. Dry wood is
composed of fibers of cellulose (40%-50%) and hemi cellulose (20%-30%) held
together by lignin (25%-30%). Plants that do not produce wood are called
'herbaceous'
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Uses
Artists can use wood to create delicate
sculptures.
Wood has been used for millennia
for many purposes. One of its primary uses is as
fuel. It is also used as for making artworks, furniture, tools, and weapons,
and as a
construction material.
Wood has been an important
construction material since humans began building shelters, houses, boats.
It remains in common use today for wooden boats and wooden houses. In buildings
made of other materials, wood will still be found as a supporting material
(notably in roof construction) or exterior decoration. Wood to be used for
construction work is commonly known as lumber in
North America. Elsewhere,
lumber will usually refer to felled trees, and the word for sawn planks
(etc) ready for use will be
timber.
Wood which in its native form is
unsuitable for construction may be broken down mechanically (into fibres or
chips) or chemically (into cellulose) and used as a raw material for other
building materials such as
chipboard,
engineered wood,
hardboard,
medium-density fibreboard (MDF),
oriented strand board (OSB). Also, wood fibres are an important component of
most
paper, and cellulose is used as a component of some
synthetic materials.
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 indistinct or 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 color 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.[2]
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
A knot on a tree
at the
Garden of the Gods public park in
Colorado Springs,
Colorado (October 2006).
A knot is a particular type of
imperfection in a piece of timber, which reduces its strength, but which may be
exploited for artistic effect. In a longitudinally-sawn plank, a knot will
appear as a roughly circular "solid" (usually darker) piece of wood around which
the roughly parallel fibres (grain)
of the rest of the "flows" (parts and rejoins).
A knot is actually a portion of a
side
branch (or a dormant bud) included in the wood of the stem or larger branch.
The included portion is irregularly conical in shape (hence the roughly circular
cross-section) with the tip at the point in stem diameter at which the plant's
cambium was located when the branch formed as a bud. Within a knot, the
fibre direction (grain)
is up to 90 degrees different from the fibres of the stem, thus producing local
cross grain.
During the development of a tree,
the lower limbs often die, but may persist for a time, sometimes years.
Subsequent layers of growth of the attaching stem are no longer intimately
joined with the dead limb, but are grown around it. Hence, dead branches produce
knots which are not attached, and likely to drop out after the tree has been
sawn into boards.
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. This firmness is affected by,
among other factors, the length of time for which the branch was dead while the
attaching stem continued to grow.
Knots materially affect cracking
(known in the industry as checking) and warping, ease in working, and
cleavability of timber. They are defects which weaken timber and lower its value
for structural purposes where strength is an important consideration. The
weakening effect is much more serious when timber is subjected to forces
perpendicular to the grain and/or
tension than where under load along the grain and/or
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 purposes for which appearance
is more important than strength, such as wall panelling, knots are considered a
benefit, 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.
Heartwood is wood that, as a
result of genetically programmed processes, has died and become resistant to
decay. It appears in a cross-section as a discolored circle, following annual
rings in shape. Heartwood is usually much darker than still living wood, and
forms with age. Many woody plants do not form heartwood, but other processes,
such as decay, can discolor wood in similar ways, leading to confusion. Some
uncertainty still exists as to whether heartwood is truly dead, as it can still
chemically react to decay organisms, but only once (Shigo 1986, 54).
Sapwood is living wood 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.
Wood products as
plywood are typically classified as
engineered wood and not considered raw wood.
Color
In species which show a distinct
difference between heartwood and sapwood the natural color 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
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 color 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
The structure of the hardwoods is
more complex.[3]
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).
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
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."[4]
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."[4]
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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