Uses
Wood has been used by man 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 timber
in International
English, and lumber
in American
English. 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 more or less
well-defined parts. The part nearest the centre 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.
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. Note that 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.
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.
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 color 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 be broken 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.
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.
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. 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.
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.
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."
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.
See also
