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WOOD STRUCTURE Nature of
Wood Elements Diagnostic Features Porous & Nonporous Woods Early & Late
Wood Rays
Sapwood & Heartwood Grain & Figure WOOD MECHANICAL PROPERTIES Strength Stiffness Toughness Hardness
Cleavability INFLUENCING ECHANICAL PROPERTIES Density Moisture Defects WOOD USES
FOREST
PRODUCTS AS FUEL
Wood Peat Coal Petroleum LUMBER Lumber Industry
Lumber Utilization Export Lumber Structural
Timbers
Planing-Mill
Products Other Woodworking Industries Boxes & Crates Furniture
& Fixtures Railroad Cars
Vehicles
Misc. Industries POSTS
MINE TIMBERS POLES & PILING COOPERAGE RAILROAD TIES VENEERS PLYWOOD
WOOD ALLOYS
RECONSTRUCTED
WOOD SHINGLES & SHAKES EXCELSIOR
SAWDUST &
SHAVINGS WOOD FLOUR
CHARCOAL
& WOOD DISTILLATION MISC. USES FOR WOOD CORK
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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 pressu |