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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.
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)