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Introduction
Forest products have been of service to humans from the very
beginnings of our history (Hill 1952).
The most familiar, and the most important, of these products is
wood. Wood is used in all types of
construction, as a fuel and as a raw material of the paper and rayon
industries. Other products include
rubber, cork, many of our tanning materials and dyes, resins gums, oils,
drugs and even sugar, starch and some chemicals. Additionally, the seeds and fruits of many trees may serve as
food for humans or their livestock. In addition to being of value to
humans, forests themselves have many utilitarian features. They help to regulate climate and
temperature. They aid in the
conservation of the water supply and in flood control by preventing water
runoff. Their roots hold the soil
firmly in place and control erosion.
They may also act as shelter area against drying winds. They afford a range for livestock, a
shelter for wild life and offer many recreational aspects for humans, the
importance of which cannot be underestimated. This section is limited in discussion
to wood and cork following the format of Hill (1952). Other useful materials from trees are
considered in other sections together with similar economic products from
other sources. Wood is a plant secondary tissue
that is formed mainly in the stems of gymnosperms and dicotyledons through
the activity of a growing layer, the Cambium. The cambium is responsible for the growth
of stems in thickness through the formation annually of new layers of both
wood and bark. Wood is a heterogeneous tissue made up of several
different kinds of cells, some that have the function of mechanical support
and others that of conduction. In
softwoods, or gymnosperms, both these functions usually occur in cells called
Tracheids. In
hardwoods, or angiosperms, a division of labor usually exists. Mechanical support is provided by the
several types of wood fibers that make up a greater part of the woody tissue,
while the conduction of water is by tubular cell fusions called Vessels. Tracheids
are also occasionally present. Wood
also functions in the distribution and storage of carbohydrate that is
accomplished in the Parenchyma Cells. These are the only parts of the wood that
is truly living and which contains protoplasm. Parenchyma cells occur in two forms, (1) wood parenchyma and
(2) ray parenchyma. The former are
arranged vertically in the stem while the latter are horizontal. Valuable diagnostic traits are
afforded by the arrangement of the different types of cells in wood. Woods may be distinguished by pores, early
wood and late wood, growth rings, rays, heartwood, sapwood grain and figure. The presence or absence and the
nature and arrangement in cross section of pores that are really vessels,
offer a quick way to classify woods.
Conifers that do not have vessels are in the nonporous wood
category. Hardwoods that do have
vessels may be further divided into those where the pores are arranged in
concentric circles, the outer and inner portions of which differ according to
number and size of pores, and those where the pores are all small and about
the same size and are scattered uniformly through the wood. The first category is called ring-porous
and the second diffuse-porous. . In temperate climates new wood is
formed annually during a limited growing season and definite growth layers
result. These usually have two
distinct areas within each layer. In
springtime when growth resumes, the first wood to be formed contains many
large and thin-walled cells as a response to the greater need for conducting
nutrients. This is the early wood or
spring wood. As the season progresses
a more dense kind of wood is laid down that has smaller, thicker walled
cells, the late wood or summer wood.
This produces a sharp transition between the cells formed at the end
of a growing season and those formed at the beginning of the succeeding one. In cross section this appears as
concentric rings called Growth Rings. The growth ring of one year is called an
annual ring and the number of these indicated the tree’s age. In the tropics where growth may continue
throughout the year growth zones may occur also, but they are due to changes
in weather or other causes rather than to definite growth periods. These are
thin sheers or ribbons that are made up primarily of parenchyma cells
oriented at right angles to the stem’s main axis. They vary in height, width and arrangement. In cross section they are visi8ble as
lines that radiate from the stem’s center.
They are most obvious in radial sections where they can also be used
to identify the tree species by the variety in their form and arrangement. When young all wood cells are
physiologically active. But in time
many of them lose their activity and become skeletons that serve only to
provide strength to the tree.
Eventually two distinct areas develop: (1) a light colored outer region of varying width, the sapwood,
and (2) a darker inner region the heartwood.
Only parenchyma cells in the sapwood remain physiologically
active. The older cells of the
heartwood that have died often attain a color and are very resistant to decay
because of the deposition in them of gums, resins or other waste
substances. This heartwood can be
polished to a high luster and is valuable in making furniture, cabinets and
other woodworking aspects. Although
heartwood is generally distinct from sapwood in durability, appearance, etc.,
it may not always be very clearly differentiated. These along
with texture are terms that are often used indistinctly. Texture usually refers to the relative
size and quality of the various parts of wood, while grain refers to their
structural arrangement. Figure
applies to the design or pattern that appears on the surface of lumber and
may be caused by the kind of grain, the presence of coloring material that have
penetrated the tissues or both. There are many different kinds of grains. In straight-grained wood the various parts
occur parallel to the stem’s main axis.
When they are twisted spirally about the axis they make up
spiral-grained wood. When the
longitudinal course of the different parts undulates slightly, a wavy grain
is the result. Curly grain is due to
various growth irregularities and other causes. The distinctive figures that wood
often shows and that make it valuable for decorative purposes are due mainly
to the different types of grain in combination with the rays, rings, sapwood,
heartwood and many other arrangements of cells. By cutting the wood in different ways these variations may be
stressed in one way or another. In
quartersawing wood is cut parallel to the rays and across the rings, while in
plain sawing the wood is cut at right angles to rays and tangent to the
rings. Sometimes figures in the wood
are caused by masses of coloring material that has penetrated the tissues and
which may occur in zones or streaks.
Snakewood, for example, has streaks on the tangential surface
suggestive of the markings on a snake’s skin. WOOD MECHANICAL PROPERTIES Wood has some
mechanical characteristics that either alone or in combination determine its
utility and fitness for various purposes.
These characteristics differ among species and even in individual
trees. The mechanical properties of
wood are those that enable it to resist various external forces that would
changes its shape and size and produce deformities. Such external forces induce internal resistance, called
stresses, in the wood. When they
exceed the force of cohesion among wood elements, a failure will occur. Some of the more important mechanical
properties are toughness, strength, stiffness, cleavability and hardness. This
designation ought to be restricted to the ability to resist certain forces,
such as pulling, crushing and shearing.
In addition the word should always be modified to indicate the
specific type of resistance involved. Also called compression
strength this is the resistance offered to forces that tend to crush
wood. Such forces may be applied
endwise, and so parallel to the grain, as in a column; or sidewise, where
they are at right angles to the grain, as in the case of railroad ties. The highest crushing strength is endwise. This is the
resistance to forces that would cause the wood to be pulled apart. They also may be applied either parallel
to or at right angles to the grain.
The maximum tensile strength exists when the force is parallel to the
grain and it is 2-4 times grater than the crushing strength. This is resistance
to forces that tend to make the fibers slide past one another. Such forces may be applied parallel to the
grain, at right angles to it or obliquely.
Wood is most resistant to a perpendicular shear. Also known as
bending strength it is usually applied to beams or other parts of timber that
are supported at both ends and loaded between these points. The strength involved is the resistance to
forces that cause the beam to break, and all the other forces mentioned above
are also involved. The upper part of
the beam is under compression; the lower under tension, and shearing also is
involved. Because of a greater
tensile strength, a beam fails first by compression, or buckling, on the
upper side. As the load increases,
the tensile strength gradually fails until the beam snaps across on the
underside. Wood strength is the most
important factor in deciding the value of any particular type for structural
purposes. This varies greatly,
however, and is influenced by the wood density, the moisture content, the
presence of defects and other factors.
The relationship between density and strength is especially close and
density is considered to be the best criterion of strength. Among the strongest woods in North America
are White oak, longleaf pine, larch, hickory and sugar maple. This is the measure of the ability
of wood to resist forces tending to change its shape. It is the capacity to withstand
deformation under a bending strain.
It is often contrasted with flexibility that is the ability to bend
without breaking and it involves toughness and pliability. This is a combination of other
properties that is usually considered to mean the ability of wood to absorb a
large amount of energy and to resist repeated, sudden, sharp blows or
shocks. A tough wood is difficult to
split and although it might rupture it does not break easily. The measure of the power of wood
to resist indentations, abrasions and wear.
It is variable with density and determines the ease with which wood
can be cut or sawed. An expression of the ease with
which wood may be split. This is
desirable for firewood and undesirable when wood must be able to hold nails
or screws. Wood tends to split more
easily along the rays and when its grains are straight. FACTORS INFLUENCING MECHANICAL PROPERTIES There are many factors involved in
the strength, hardness and other properties of wood, with some of the more
important ones being density, moisture and some defects. Density differences are among the
main causes of variations in the mechanical properties of wood. Because of the close relationship between
the two properties, the density of any wood may be the best indication of its
strength. Density is usually
expressed as specific gravity or the relative
density. Density is simply the weight
of a unit volume of any substance that is expressed as weight per cubic food;
but specific gravity is the ratio between the weight of the substances
involved and that of an equal amount of pure water. The density of wood substance is
almost the same in all species and its specific gravity has been estimated at
around 1.55. This shows that wood is
1.55 times more heavy than water and is explains why a piece of wood will
sink as soon as the air in its cavities has been replaced by water. However, the vasriations in density that
are observed in different species, individuals and even parts of the same
individual, are due to differences in the actual amount of wood substance
present. It is the amount of cell
wall material compared to cell cavities.
Woods with thicker walls and smaller cavities are denser and heavier,
while the more porous woods with larger cavities and thinner cell walls are
lighter and less dense. Specific gravity of wood can be
determined in different ways, but usually the weight is calculated from
oven-dry material and the volume is measured with the wood in any desired
condition. Nevertheless, because the
moisture content of green, partly seasoned, and seasoned wood is different,
and since this difference affects the specific gravity, it is obvious that
specific gravity values mean little unless the condition of the wood is
clarified. Because the amount of actual wood
substance has an important bearing on the density of a piece of wood, it can
be assumed that heavier woods are therefore stronger. This is not always the case because the
presence of resins, gums and other infiltrated substances and also the amount
of water may affect the weight without altering the strength. Hickory, Osage Orange, Oak and
Persimmon are some of the heaviest native woods. Tropical woods show great diversity in weight as exampled by
lignum vitae being very heavy and balsa being very light. Wood contains variable amounts of
water that differs according to species and environmental conditions from
40-100 percent of the dry weight. The
water occurs in wood cell cavities or in the cell walls, where it is known as
hygroscopic water. The amount of
hygroscopic water required to saturate the walls is fiber-saturation point
and constitutes 20-35 percent of the dry weight. The variation in the amount of water that is present in wood is
due to a number of things and is made possible by a typical property inherent
in the wood. This property, or
hygroscopicity, is the ability to absorb or give off water under different
conditions, with an accompanying swelling or shrinking. Moisture content of wood
influences its weight, density and frequently its strength. If the amount of water present is above
the fiber-saturation point, the weight is increased but the strength is not
changed. However, if the amount of
water is brought below the fiber saturation point through evaporation, then
the strength and other mechanical properties as well are changed. This means that it is only the loss of
hygroscopic water that is responsible for the increase in strength that
accompanies seasoning, as the drying out of wood is called. This loss of hygroscopic water causes the
wood to shrink, due to changes that take place in the cells. As the water leaves the walls contract,
the cells become more closely compacted, and the fibers become stronger and
stiffer. This tendency of wood to
shrink as it dries is one of the huge drawbacks to its use. The amount of shrinkage varies under
different conditions and is likely to occur unevenly. Wood shrinks very little lengthwise, and
only about half as much radially as tangentially. This result of uneven shrinkage can contribute to warping,
checks, shakes and other defects.
These then counteract any increase in strength. Despite such shortcomings, dried or
seasoned wood is usually stronger, harder, stiffer and more durable than
unseasoned wood. Artificial methods
of seasoning are deployed in order to control the process. Two principal
types of artificial seasoning are kiln drying and air seasoning. In air seasoning, the moisture is removed
by exposure to air without resorting to artificial heat. It is done in the open until the wood ceases
to lose weight. The final moisture
content varies 12-30 percent with the species, the duration of the process
and environmental conditions. The
main objects of air seasoning are to reduce the weight, the amount of
shrinkage and possible defects; to render the wood less subject to decay; to
increase its strength and combustibility; and to prepare it for painting,
preservative treatment and kiln drying. Kiln drying heat is applied to
wood in an enclosed space. Either
seasoned or unseasoned wood can be used.
The moisture is removed more rapidly and completely, the moisture
content of the finished product varies from 4-12 percent. Kiln drying applied to green lumber often
prevents checks, warping and defects due to fungi or insects. Mechanical properties of wood are
often affected by the presence of various types of defects that may be of
some importance. These defects are
due to many causes. Some of them may
be normal characteristics, but ones that limit the usefulness of wood. Wood is dimensionally unstable; it swells,
warps, and checks with humidity and temperature changes. Its strength is unidirectional in that it
is strong with the grain and weak across the grain. Such disadvantages may be surmounted partially by the use of
plywood, wood alloys, or reconstructed wood. Certain defects may develop during
the seasoning of wood. Defects such
as knots and cross grain, may be inherent in the wood structure and others
may be due to external causes. Among
the latter are fungi, insects, marine borers, parasitic seed plants, birds,
lightning, frost and fire. Defects
caused by insects and fungi are of greatest concern. Insect damage may be much greater
than generally realized. All sorts of
wood from standing timber to lumber and wood products may be attacked. Holes produced by wood-boring insects make
up the principal type of injury.
Insects are most destructive as larvae. By the end of the 20th Century, the ravages of
termites have been increasingly serious. Fungi cause
wood to decay. Four conditions are
necessary for the development of these lower plants. Unless a favorable temperature, sufficient
moisture, at least a small amount of oxygen and an adequate food supply are
available, fungus decay cannot occur.
The food is furnished by cellulose and lignin in cell walls, and is
made available by enzymes that are secreted by the fungi. Brown rots remove the cellulose, leaving
behind a brittle brown mass of lignin compounds; the white rots utilize the
lignin and leave the white cellulose behind.
Other fungus species are able to utilize both cellulose and lignin. Woods vary in
their natural resistance to fungi.
This property is known as durability and is so important that it alone
may determine the ultimate use of wood, particularly in the case of poles,
posts and mine timbers that are exposed to moisture. Sapwood decays quicker than heartwood
because the latter usually contains resins, gums, tannins and other
substances that resist fungi.
Seasoned wood is also less apt to decay because of the lower moisture
content. Among native woods that are
naturally very durable are the redwood, cedars, cypress, locust and osage
orange. The least durable woods
include balsam fir and basswood (linden). Frequently
wood is rendered more immune to decay by treating it with preservatives that
are poisonous to fungi. This process
of wood preservation has developed into a considerable industry. Various chemicals are used as
preservatives, mainly creosote and zinc chloride. The methods used are brushing or spraying the surface, dipping
in open tanks and various pressure processes that make possible a deeper
penetration of the preservatives. Wood is used in so many ways that
it is impossible to discuss all in detail.
In many countries there is a great deal more wood cut each year than
is replaced by normal growth, which does not bode well for a continuous
supply in the future. Fuel is an
indispensable necessity of life both in home and industry. Any material that burns readily in air can
be utilized, but this includes a great variety of plant products. The most important of these are wood, peat
and coal, which represent different stages in the carbonization of the
original plant tissue. Farms and rural
communities have accounted for about 90 percent of the total amount of wood
used for fuel. Wood makes an
excellent fuel because it is about 99 percent combustible when dry and so
leaves only a small amount of ash. It
is also flaming fuel and well adapted for heating large surfaces. The value of different kinds of wood for
heating purposes depends on the amount of moisture present. Therefore, seasoned wood is better than
green wood. Hardwoods have the
greatest fuel value, particularly such woods as hickory, eucalyptus, oak,
beech, birch, maple and ash. Longleaf
pine in the southern United States is mainly used while in the western area
Douglas fir, western yellow pine, western hemlock and western larch are used. Peat is made up of deposits of
vegetable matter that have accumulated in swamps and bogs and slowly
decomposed, becoming somewhat carbonized and compacted. The various plant tissues can still be
discerned. The process of peat
formation is continuous, and peat is a valuable fuel in countries where wood
is scarce. It is more bulky to
manipulate and leaves from 5-15 times as much ash. At the lower depths of some peat bogs a soft brown coal called
lignite may be found. This also has
the original plant structures still visible. Coal comprises the fossilized
remains of plants that lived in former geological periods (Hill 1952). The original plant tissue has been more
fully decomposed and converted into carbon.
Coal is much harder and more compact than peat or lignite, and has a
greater heating power. It also yields
a larger amount of smoke and ash.
Anthracite or hard coals are the oldest and contain about 95 percent
carbon. Bituminous or soft coals are
more recent in origin and thus are less completely carbonized. They tend to soften and fuse at temperatures
below the combustion point. Cannel
coal consists of fossilized spores.
It is very compact and oily and burns with a candle like flame. Unlike other coals it does not soil in
one’s hands. Coal is a comparatively
inexpensive source of power and heat and also of many useful chemical
products. Among the latter, which are
obtained by destructive distillation, are oils, such as benzol and naphtha;
coal gas that is used for fuel and illuminating purposes; ammonia; coal tar,
the source of dyes, antiseptics and many other materials; and coke Coke bears the same relationship
to coal that charcoal does to wood.
It is obtained by the smothered combustion of coal in piles or special
ovens, usually as a by-product of the illuminating-gas industry. It is almost pure carbon and burns without
smoke or flame. Coke is an excellent
fuel that is especially used in metallurgy. There are no traces of the
original structure present in petroleum, and it has been generally believed
that petroleum had an organic origin and was formed under pressure from the
minute floating plant and animal life of former shallow seas. Crude petroleum has many uses, but the
substances derived from it by fractional distillation are of much greater
importance. Among these products are
gasoline, kerosene, plastics, petroleum jelly, medicines and paraffin. Lumber from wood has been in use
for building purposes and other construction since early times. In the United States the first sawmill was
established in Maine in 1631.
Thereafter a huge industry has developed. The word “lumber” refers to wood that has been prepared to some
extent for future use. The larger
pieces of lumber that are used in heavy construction are often called
“timber.” The standard unit of
measuring lumber is the board foot, which is the
equivalent of a piece of wood 1 in. thick, 12 in. wide and 1 ft. long. The location of the lumber
industry and species utilized are usually in constant change. In the United States it has always been in
a region where large stands of virgin timber were available. Until 1830 the state of Maine was the main
lumber-producing area, and for the next 40 years New York and Pennsylvania
took the lead. By 1870 the center
shifted to the Lake States, with first Michigan and later Wisconsin. After 1910 the Southern states became the
leading producers, with southern pine replacing the northern hardwoods and
white pine. Then the center moved to
the Pacific Northwest and began utilizing the immense stands of Douglas fir
and other softwoods. The Southern
states continue to produce most lumber, although Washington and Oregon are
still prime providers. There have been over 150 native
American species utilized in the lumber industry. However, the softwoods have furnished about 78 percent of all
the lumber cut. For many years the Eastern white pine, Pinus strobes, was the outstanding
timber tree and this was one of the most valuable trees in the world. The demand for white pine was so great
that the supply soon became diminished and was eventually replaced by other
species. Oak and hemlock have also
had a prominent role. By the end of
the 20th Century the most important tree species were southern
yellow pine, Douglas fir, western yellow pine, oak hemlock, white pine and
red gum. Douglas fir and yellow pine
produce twice as much as all the other combined. Many woods that at one time were considered worthless have
later become important. Examples
include sycamore, beech, red gum and tupelo. Usually about eight percent of the
lumber cut each year is exported; 32 percent is used for structural timbers
and rough lumber for construction; 33 percent goes to the planning mills and
27 percent is used in other woodworking industries. Lumber has been an important North
American foreign trade since the early days of European colonization The demand for American timber has been
especially great in Japan. These are the
large sizes of lumber from sawmills that are used for buildings, bridges and
other types of heavy construction.
They include girders, stringers, beams, joists, rafters, posts, caps,
planks, caps, roofing, boards for sheathing, and flooring. In the early days before steel, immense
quantities of heavy timbers were used in ship building. Structural timbers are obtained mainly
from the softwoods because large sizes are readily available. The timbers are usually sawed from the
heart of the tree, and even though defects may be present, the bulk is so
massive that the strength is not impaired.
Strength is the main requirement of a good structural timber,
especially the resistance to stresses that can be estimated. Numerous timber testing experiments have
made possible the estimation of the working stresses with great
accuracy. Durability, soundness and
ease of working are other desirable qualities. The main species used in the United States for structural
timbers are southern yellow pine, eastern white pine, hemlock, Douglas fir,
western yellow pine, spruce, redwood and larch. These are usually associated with
sawmills and they use a large amount of lumber representing over 60 different
species. Their products are sometimes
classified as “factory lumber,” that is lumber which has been recut to
smaller dimensions and reworked. The
principal products of the planning mills are doors, sashes, window frames,
blinds and interior finishes. The
best wood for millwork has a straight grain and a soft uniform texture. It should not shrink or swell and it
should be easy to work and capable of taking varnishes and paints. The less expensive products are made from
white pine, Douglas fir, yellow pine and other softwoods. Expensive items include birch, oak, red
gum, maple, walnut, mahogany and other hardwoods with a fine figure. Veneers are now being used extensively for
door panels. Interior finishes include
baseboards, columns, cornices, mantels, grills, stair work, posts, balusters,
scrollwork, porch work and trimming.
Most of the North American and imported woods that are decorative and
have good wearing qualities are used for these purposes. A more recent development of the
planning mills has been hardwood flooring.
Only the most durable woods that also have attractive designs are
used. These are kiln dried and
seasoned. The woods used include
beech, maple, oak, tupelo, birch and yellow pine. There are
numerous industries that have many variable requirements and products. Most use only a relatively small amount of
wood, but a few of the more important industries include railroad car, box
and crate, furniture, vehicle, agricultural implement and woodenware
industries. Items such as crates, baskets,
boxes and other containers are used for the transport of canned goods, farm
products and many other articles. For
box-making the wood should be light, east to work, strong, with good
nail-holding power and a surface that can be printed upon. Lower grades of softwood lumber and the
softer hardwoods are mainly used. The
principal species are southern pine, western yellow pine, red gum, hemlock,
white pine and spruce. This requires wood of special
hardness, strength and durability. It
should not shrink or warp, and must be ornamental and capable of a high
polish. Birch, Maple, Oak, Red Gum,
Walnut are the principal species, although chestnut, beech, Elm, tulip and
basswood are also used. A large
number of imported woods that have attractive color or figure are utilized
also. There have been over 60 species
used. Veneers are of increasing
importance for they can be used to cover less expensive and less attractive
woods. There has been a lot of lumber
used annually for the construction of railroad cars with over 40 different
species of wood being used. Oak,
Maple, Hemlock, Cypress and several ornamental species are of some importance
although Yellow Pine and Douglass Fir have been used the most. By the 21st Century railroad
cars have begun to be constructed with plastic walls and that has greatly
diminished the need for wood in this industry. Wood was used extensively in the
manufacture of vehicles in the first half of the 20th Century, but
has been largely substituted by plastic in the 21st Century. Other industries using wood during
the industrial age have been agricultural implements, caskets, coffins,
refrigerators, kitchen cabinets, ship and boat building, matches, woodenware
and novelties, musical instruments, tanks and silos, signs and supplies,
professional and scientific instruments, electrical machinery and apparatus,
machine construction, toys, laundry appliances, handles, supplies for
dairymen, poultrymen and beekeepers, tobacco boxes, patterns and flasks,
sporting and athletic goods, boot and shoe findings, shade and map rollers,
brooms and carpet sweepers, picture frames and moldings, motion-picture and
theatrical scenery, brushes, plumber’s woodwork, shuttles, spools and
bobbins, trunks and valises, sewing machines, pumps, wood pipe and conduits,
airplanes, toothpicks, printing materials, playground equipment, dowels,
clocks, paving materials, saddles and harness, gates and fencing, butcher
blocks and skewers, bungs and faucets, firearms, scales, elevators, whips,
canes and umbrellas, tobacco pipes and artificial limbs. By the 21st Century either
metal or plastics have substituted most of these. Fence posts
are used mainly on farms and along roads and railroad right of ways. The old rail fences have almost
disappeared. Posts are usually cut 7
feet in length and from 4-6 inches in thickness. They are used in the round or are split. Strength, light weight and durability in
the soil and w4eather are the main requirements. Woods that have been utilized for posts are mainly cedar,
redwood, chestnut, oak, tamarack, black locust, ash, osage orange and
cypress. Posts are usually treated
with preservatives before being placed into the ground. There is much wood used in mines
for shafts and for supporting structures, including collars, caps and
props. Of particular importance for
safety are strength and durability, and the wood is used in an environment
that is conducive to decay. Woods
that are normally used for this purpose are pine, oak, tamarack, chestnut,
beech hemlock, maple and Douglas fir.
These timbers rarely enter the trade for most of the supply is
obtained locally from whatever is available. Many poles are used annually to
suspend electricity, telegraph and power-transmission lines. Similar amounts are used for work in
harbors including trestle and bridge construction. These poles are used in the round and because they are prone to
decay at ground level, only those species with durable sapwood are
preferred. These are also treated
with preservatives. Strength, light
weight and accessibility are prime qualities, and shape is also
important. In addition to these
stated requirements, pilings must be able to withstand heavy top loads and
can withstand blows as they are pounded into the ground. Species preferred are lodgepole pine,
Douglas fir, southern pines and cedar.
Sometimes chestnut, cypress, oak, larch, redwood, locust and elm are
also used. The manufacture of wooden
containers bound together with hoops of wood is an old industry that dates to
Biblical and Roman times. However,
production has been in a steady decline through the 20th Century
due to the competition from other types of containers. Two main classes of cooperate are slack
cooperage, to hold dry substances, and tight cooperage, to hold liquids. This includes a great variety of
barrels, casks, tubs, pails, buckets, churns, kegs and other containers. They have been used to transport meat,
fish, tobacco, fruit, flour, vegetables, cement, sugar, glassware, crockery
and etc. A single barrel usually
consists of 15 staves, one set of heading and six hoops. These various parts may be manufactured in
different factories and regions.
There are many grades of slack cooperage that ranges from sugar and
flour barrels, with tongued and grooved staves, to the loose-fitting and less
expensive cement barrels. Wood to make slack cooperage
should be inexpensive, easy to work, light, elastic and free from twisting
and warping. Limbs, tops, defective
logs and other forms of waste lumber may be used. Veneers have been used to some extent. Heads and staves are usually made from
pine, red gum, beech, maple, oak, Douglas Fir and ash. Kegs and barrels to be used for containers for beer, oil, wine
and other liquids require more careful construction. Woods that will impart no taste or odor to
the liquids and which are impermeable are essential. White oak has been the main wood used,
especially when the liquids are to remain the barrels for a long time. Red oak, red gum, yellow birch, white ash,
sugar maple and Douglas fir are frequently used. The finished product is then usually treated on the inside with
paraffin to insure that no leakage will occur. The hoops are mainly of strap steel. Vats and
large tanks constitute heavy cooperage.
They are made with staves and heads of white oak, cypress, Douglas fir
or redwood and bound together with metal straps. The average
life of an untreated railroad tie is only 5-6 years, so that the demand
continues for replacement ties. These
are usually hewn from a seasoned wood but they may be sawed. Strength, durability in the soil and the
ability to resist impact, crushing and spike pulling are important
traits. Treated ties are just as
serviceable as those made from naturally durable woods. Most ties in North America are made from
southern pine, red gum, oak and Douglas fir.
Other species include cedar, chestnut, cypress, maple, beech, tamarack
and hemlock. These are thin slices or sheets of
wood with a uniform thickness.
Although they may be cut as thin as 1/110th inch, the
commercial product is usually about 1/20th in. thick, with 3/8th
in being maximum. Veneers were known
to the early civilizations of Rome and Egypt, but in America their use was
retarded for many years because wood was so abundant and inexpensive. To make veneers the logs or pieces of wood are peeled,
boiled and then cut with a knife. The
rotary process prepares most. This
involves turning a log on a lathe against a stationary knife that produces a
continuous sheet of veneer. The
design in this type of veneer is not particularly striking because the sheet
is cut parallel to the annual rings.
In the slicing process the logs are quarter sawed and thus show a more
attractive grain. The logs are first
quartered and then sliced with a stationary knife to yield separate
sheets. This process is less wasteful
and is used for the more expensive kinds of wood. In the sawing process the quartered logs are cut with a circular
saw. Although the veneers thus
produced are thicker, the most valuable woods are sawed because the fibers do
not tare and the material can be more readily worked. Freshly cut veneers are usually wet and
must be thoroughly dried before adhesives can be applied. Veneers were used primarily only
to cover up inferior woods in the furniture and cabinet industries. They were made primarily from walnut,
mahogany and other woods that had a beautiful color and grain. By the end of the 20th Century
many species were being used and the veneers were utilized in the manufacture
of baskets, boxes, cooperage, door panels, trunks, mirrors, musical
instruments, etc. Veneers make
possible high strength and minimum weight.
Any wood is suitable that comes in large sizes, has a symmetrical
grain and design and few defects and is inexpensive. A variety of both domestic and foreign
woods are used, but more than one-half of the total output in North America
is made from either Douglas fir or red gum.
There are three kinds of hardwood
veneers manufactured. Face veneers
are sliced or sawed from selected logs and are used for only the finest
work. Primary among native species
are black walnut, quartered red and white oak, red gum and sugar maple. Commercial veneers are rotary-cut and are
used for plywood, concealed parts of furniture, etc. Birch, Maple, Beech, basswood, tulip,
tupelo, cottonwood, sycamore and oak have all be used. Container veneers are the least expensive
and are made from any inexpensive wood into barrels, crates, boxes, etc. Softwood veneers, either rotary-cut or
sliced, are made primarily on the Pacific Cost. Over 80 percent are made from Douglas fir, but Sitka spruce,
western yellow pine and Port Orford cedar have been used. Softwood veneers are for structural
plywood or interior paneling. This involves gluing together 3-9
thin veneers. The grain of each
successive layer is at an angle to the next, so the strength is redistributed
and the dimensional instability of one layer is compensated for and reduced
by the others. Thus, the finished
produce is very strong and stable and much less likely to warp or twist than
ordinary wood. Screws and nails may
be driven close to the edge with no danger of splitting the plywood. In the manufacture of a 5-ply
plywood panel, the face, back, cross band and core sheets are prepared. Applying an adhesive and pressing the
glued stock into a panel and finally drying and finishing the product follow
this. Softer woods are usually used
because they can be glued more easily.
One simple kind of plywood has a 3/8ths inch core of poplar
with 1/10th inch birch veneers on each side. Various plant and animal adhesives are
used as well as large quantities of synthetic resins. Modifications of the ordinary process
result in molded or curved plywood and in wood alloys. Plywood is recognized as an
engineering material with its own peculiar properties. It has extensive uses in the home for
doors, flooring, walls, partitions, cabinets, shelves, furniture and interior
trim. Large quantities have been used
also to make concrete forms, prefabricated houses, airplanes, boats, railroad
cars and the bodies of trailers and station wagons. In making wood alloys or densified
wood, such as uralloy, compreg and impreg, from plywood the natural wood
structure is impregnated with synthetic resins and bonded under high
pressure. The resins establish strong
physical and chemical bonds with the wood fibers and create a material with
new properties, which is also very strong, stable, hard and resistant to
decay. This is characterized by a
reorganization of the fibers where they are taken out of their original
unidirectional grain and rearranged in multidirectional patterns. This may be accomplished by chemical or
physical means. Because the use of
chemicals involves breaking down the wood into cellulose and lignin, the
discussion of a chemically reconstructed wood, such as paper, is discussed
under "Sugars,
Starches & Cellulose Products". An example of a wood
that has been reconstructed by physical means is Masonite. Wood chips are subjected to high pressure
in a steam vessel and are then exploded with the abrupt release of the
pressure. This tears the fibers apart
and also reactivates the lignin, a natural plastic, which fixes and binds the
fibers in their new orientation. At
first Masonite was used as an insulating fiberboard because it was an excellent
insulator against heat, sound and electricity. Then it was learned that the application of heat and pressure
converted the boards into a homogeneous grainless synthetic board with
extreme hardness and water resistance. Shingles are thin pieces of wood
used to protect the roofs and sides of buildings from weathering. Handmade shingles rank among the first of
the wood products manufactured by humans.
Single wood must be durable, light in weight, easy to split and able
to hold nails without loosening, and it must not warp. Straight, even-grained woods are
preferred. The durability of shingles
is increased by treatment with a preservative. Red cedar has been the preferred wood in North America. Northern white and southern white cedars,
redwood and cypress are also used. If
available, eastern white pine is an excellent material. By the 21st Century many
substitutes entered the market that were longer lasting and less fire
prone. However, the lighter weight of
shingles make them less expensive to install on existing roofs and aeration
is superior to synthetic products, especially in warmer climates. Shakes are
split shingles and much thicker. They
were important in colonial times, but now are used primarily for special
architectural effects and in more remote regions. In North America red cedar, sugar pine and redwood are widely
used. This consists of thin curled
strands or shreds of wood and is made by placing wood on frames and pressing
it against rapidly moving knives or steel teeth. The material was first known as wood fiber. Excelsior is light and elastic and makes
an excellent material for packing and shipping glassware and other breakable
articles. It is resilient and free
from dust and dirt, and may be used for stuffing upholstery and
mattresses. A very fine grade of
excelsior, wood wool, is used in filters and in the manufacture of matting
and rugs. The use of excelsior has steadily
decreased as synthetic plastics became available as packaging materials. Sawdust is used primarily for
fuel, usually in the form of briquettes or fireplace logs. It is also valuable for many industrial
purposes. It serves as a bedding for
kennels and stables, as a floor covering to absorb moisture, for cleaning,
drying and polishing metal, as a packing medium, as a soil conditioner and an
insulating material. It has played a
part in the making of leather and the conditioning of fur and is an
ingredient of composition flooring, artificial wood, abrasives, wallboard,
floor-sweeping compounds, etc. Shavings find their greatest use
as fuel and as packing material.
White pine shavings are especially desirable. This consists of finely ground
sawdust, shavings and other forms of wood waste that has been used in the
manufacture of linoleum, plastics, nitroglycerin, veneer bonds, composition
flooring, insulating brick, etc. It
is used as filler, an absorbent or a mild abrasive. Light colored woods with low resin content are used. White pine has been the main species. Wood has been heated in order to
convert it into carbon or charcoal since ancient times. It was probably the first chemical process
used by humans. Charcoal is still a
valuable fuel as it has twice the heating power of wood and burns without flame
or smoke. It is widely used in many
European, Asian and Latin American countries, especially where forests are
abundant and is the chief domestic fuel in most tropical countries. Charcoal is also used in medicine, as a
reducing element in the iron and steel industry, and in the manufacture of
chemicals, explosives, gunpowder and some cosmetics. It is the best material for absorbing
impurities and foul odors from both water and the atmosphere. It has been used extensively in gas masks. The best yields of charcoal are obtained
from the denser hardwoods, such as maple, beech, birch, oak, hickory and
mesquite. Willow charcoal is
especially popular for explosives.
The conversion of wood into charcoal was formerly accomplished in
open-air pits by a process of partial combustion. This method is wasteful as all the volatile material contained
in the wood is lost. Beehive kilns
and portable ovens have also been used.
By the 21st Century charcoal burning has been largely
replaced by wood distillation, and the valuable gases and other by-products
are recovered as well as the charcoal. This process is very old and was
known to the ancient Egyptians. It is
today important not only in rendering available the volatile wood elements,
but as a factor in forest conservation.
One of the main sources of wood for destructive distillation is the
waste left by lumbering operations, sawmills, and planning mills. There are two distinct types of wood
distillation. This process uses the denser and
heavier hardwoods and has been in use since the beginning of the 19th
Century. The wood is heated in large
oven retorts. The immediate products
are charcoal, pyroligneous acid, which condenses from some of the gases given
off, tar and oil, and noncondensable wood gases. The tar and oils are allowed to settle out and the pyroligneous
acid is passed through a series of stills where more tar and oils are
removed. Eventually slaked lime is
added and in a final distillation wood alcohol (methanol) passes off and
acetate of lime is left as a residue.
A modification of the process results in the recovery of acetic acid
directly from the pyroligneous acid.
The average yield per cord is 900-1080 lbs of charcoal, 180-200 lbs.
Of acetate of lime or 103-125 lbs. Of acetic acid, 9.5-11 gallons of wood
alcohol, 22-25 gallons of wood tar and oils, and 7,000-11,500 cubic feet of
wood gas. Acetate of lime is used primarily
in the manufacture of acetic acid, which has wide applications in the paint,
textile, leather, film and plastics industries, and acetone, which is
extensively used as a solvent. Wood
alcohol finds it greatest use as a solvent, especially in the varnish and
paint industry; it is the source of a variety of chemical products as well,
such as aniline dyes and formaldehyde.
It has a variety of uses as a fuel, illuminant, denaturant and
ingredients of medical, chemical and other industrial preparations. The wood tar and oils are used as a fuel
or as the source of many industrial oils.
Wood gas also serves as a fuel and may be converted into a substitute
for gasoline. This utilizes resinous woods,
mainly southern yellow pine. The
products are charcoal, wood turpentine, oils, and tar and wood gas. Softwood pyroligneous acid contains only
small amounts of wood alcohol and acetic acid. The wood turpentine is used to some extent in the manufacture
of varnishes, paints and synthetic camphor.
The tar and oils also have industrial uses, one important product
being creosote. Wood is also used as a raw
material for the paper and textile industries and as a source of tanning and
dye materials, food, alcohol and other products, which is discussed in other
sections. This is a forest product of great
antiquity. It is obtained
commercially mainly from the Cork oak, Quercus
suber, a
tree native to the Mediterranean region.
This oak varies from 20-60 ft. tall and about 4 ft. in diameter, with
a short trunk and densely spreading crown.
The evergreen leaves resemble those of holly, but are velvety and
spongy. The acorns are used to feed
pigs. The cork oak ranges from the
Atlantic to Asia Minor and is especially numerous in Spain, Portugal,
Algeria, Tunisia, southern France, Italy, Morocco and Corsica. The tree thrives on rocky siliceous soil
on the lower slopes of mountains. Cork or corkwood consists of the
outer bark of the tree, which can be harvested without injury to the
tree. It is renewed annually. Harvesting consists of making vertical and
horizontal cuts with hatchets or saws, and then prying off large pieces of
the bark. The rich dark-red color of
the exposed areas is one of the typical sights in a cork forest that is being
used for commercial purposes. The
stripping is usually done in midsummer when weather conditions are favorable. The bark of both the trunk and
larger branches is usually used, although in some countries the cutting area
is restricted to the first 6 feet of the trunk. Cork is first removed when the trees are about 20 years
old. This first yield, which is known
as virgin cork, is very rough and coarse and of little value. Subsequent strippings occur every nine years. The second yield is better, but the best
quality of cork is not obtained until the third cutting and thereafter. The trees live for 100-500-years.and give
an average yield of 40-500 lbs per tree.
The best grade of cork consists of inch-thick layers obtained from
young vigorous trees. The stripped pieces of cork are
dried for several days and weighed and then shipped to a processing
area. They are then boiled in large
copper vats. This removes the sap and
tannic acid, increases the volume and elasticity, and flattens the
pieces. It also loosens the outermost
layer, which is scraped off. The
rough edges are trimmed and the flat pieces are sorted and baled. Cork has many properties that make
it valuable in industry. Despite its
bulk, it is very light and exceedingly buoyant due to the fact that it is
composed entirely of dead watertight cells.
It can be readily compressed and is very resilient. Even after 10 years of use cork stoppers
can recover 7s5 percent of the original volume. It is durable, a low heat conductor, and is resistant to the
passage of moisture and liquids. It
also absorbs sounds and vibration and has certain frictional properties. A great variety of products are
manufactured from cork. Sometimes the
natural cork is utilized; in other cases composition cork, made of coarse or
finely ground pieces treated with adhesives and molded. Articles that have been made from
natural cork include bottle stoppers, hats and helmets for use in tropical
areas, tips for cigarettes, carburetor floats, handles for golf clubs,
penholders, fishing rods, mooring buoys, floats, life preservers, life
jackets, surf balls, baseball centers, decoys, mats, tiles, etc. Corkboard, made by heating natural cork,
is used as an insulating material for houses, cold-storage plants, and
refrigerators. It serves as a means
of improving the acoustics of rooms and rendering them soundproof. It is very resilient and thus is a
valuable material for machinery isolation.
Composition cork is used for the
lining of crown caps, the metal tops for sealing bottles, gaskets, toes,
counters, and innersoles for shoes, polishing wheels, friction rolls, and
several types of floor covering. Linoleum is made from cork or wood flour, linseed oil,
resins, such as rosin or kauri gum, pigments, and burlap. The oil is boiled and allowed to solidify
by dripping on pieces of cloth. The
solidified oil is ground up and melted with resins. This mixture is cooled and hardened, and after several days of
curing it is mixed with the work, which has been ground to a fine dust, and
with the dry color pigments. It is
then pressed into burlap cloth with hydraulic presses. The linoleum is then seasoned in ovens and
finished by giving it a protective surface of nitrocellulose lacquer. Linotiles are individual tiles made from
ground cork and linseed oil, but much thicker and denser than linoleum. Many
different species of trees are used for firewood worldwide, and several
species are grown in plantations for this purpose as well as for lumber. Monterey Pine, Pinus radiata, is one of those species especially favored for rapid growth
in the Southern Hemisphere. Several
species of Eucalyptus are also grown for
this purpose. Among the latter the River Red Gum, Eucalyptus camaldulensis Dehnh, is especially favored
for firewood (Cockerham, S. T. 2004) |