Wood is one of the most significant structural materials used throughout human history. As documented by the earliest artifacts of human activity, wood has been associated with activities of hunting and gathering, early development of agriculture, and the foundations of civilization, as well as its obvious use as a fuel for fire. Archaeological studies of virtually every known civilization confirm the use of wood for a wide range of items and attest to wood's intimate involvement with human evolution and its progress through time.
As a structural material, wood has some rather remarkable properties. Despite its relatively low density (most woods float), it has physical characteristics that make it highly suitable for the building of structures that require resistance to bending, limited compressibility, relative ease of forming the members, and reliable means of attaching the structures together. In addition, properly prepared wood also possess aesthetic and physical beauty due to its color, strength, and grain characteristics that are highly valued in the fabrication of fine furniture and architectural components.
Among the many uses of wood products, the production of dimensional lumber ranks as one of the most significant, particularly in the construction of residential dwellings. The majority of timber species used for dimensional lumber are conifers (gymnosperms ), which are typically evergreen (non-deciduous) trees, and come from areas where these plants are the major component of the flora in their habitats (e.g., coniferous forests). The wood produced from them is called softwood despite the fact that for many species it is quite hard and durable. This term is used in contrast to hardwood, a term applied to the wood obtained from angiosperm trees, which are typically (but not always) deciduous, and have slightly different cellular characteristics of their xylem. In North America, three major groups of conifers are used for the majority of dimensional lumber: spruce (Picea spp.), pine (Pinus spp.), and fir (Abies spp. or Pseudotsuga spp.). The woods of these species are very similar in appearance and have similar construction properties. The lumber industry identifies wood produced from them as SPF lumber (spruce-pine-fir), and this forms the bulk of the wood used for the construction of houses.
In the building trades, dimensional lumber is typically referred to as "2-by" material, alluding to the thickness dimension (in inches) of the rough lumber as it is sawn at a mill. A 2 × 4 when rough-sawn is nominally 2 inches (5 centimeters) in thickness by 4 inches (10 centimeters) in width and is supplied in standard lengths of 8, 10, 12, 14, 16, and 20 feet, depending on the length of the felled log before sawing. After drying the wood and surfacing the faces, a 2 × 4 actually measures 1.5 × 3.5 inches (3.7 × 8.8 centimeters) in width. Other commonly used dimensions are 2 × 3, 2 × 6, 2 × 8, 2 × 10, and 2 × 12, each of which are actually one-half to three-quarters of an inch thinner than the given dimensions. Dimensional lumber forms the major structural elements of floors, walls, joists, and rafters in home construction. Recently, the use of factory-made roof trusses built from dimensional wood materials provides for quick and easy construction of roof systems that are structurally strong and provide for free spans without the need for additional structural support from within the building. Construction workers or carpenters who erect the wooden frame of the building are called framers, and dimensional lumber is the material these people rely on for their livelihood.
Woods that are used for building furniture, cabinetry, millwork, or other architectural features are typically hardwoods, although some conifer woods are also used for these items. Woods that are valued for furniture and cabinetry typically have aesthetically pleasing characteristics of uniform color, interesting patterns of earlywood and latewood in cut and surfaced lumber (this characteristic is termed grain or figure), and also possess desirable properties of hardness and durability. There are literally thousands of different species of hardwood (angiosperm) trees from around the world that have the potential for use in fine furniture and cabinetry; however, there are only relatively few species that are used commercially for this purpose. This is due to the requirement of having a reliable local source for adequate amounts of lumber (which differs in various parts of the world), the wood's machineability and finishing properties to provide a pleasing end-product, and consumers who favor certain wood species over others. In North America, the three major species used for furniture production are oak (Quercus spp., in particular, red oak), walnut (Juglans spp.), and cherry (Prunus spp.), which are more or less commonly available throughout the region. Many other species of hardwoods have similarly desirable properties, and furniture made from them is also valued; their use may reflect regional availability. Certain species of woods are also best suited to specific applications given their physical attributes. The strength and shock resistance of white ash (Fraxinus americana ) makes it the wood of choice for the manufacture of baseball bats; rake, shovel, and other tool handles; and certain parts of chairs and other heavy-use furniture. Various species of maple (Acer spp.) are used for butcher blocks, bowling alley surfaces, bowling pins, the backs of violins, and various parts in the construction of pianos—all relying on maple's hard and durable characteristics.
Hardwoods are harvested in nearly every country capable of supporting a lumber industry. Thus, a considerable variety of wood species is available on the world lumber market. Those woods originating within the country of intended use are called domestic lumber, versus wood from those species obtained from foreign countries, which are called exotic woods. Hundreds of hardwood species are traded globally and have a diversity of color, grain, hardness, machineability, and finishing characteristics. One of the softest and lightest woods, balsa (Ochroma lagopus ), a tropical species from South and Central America, is technically a hardwood, although it is much softer than most coniferous softwoods. Despite this fact, it is remarkably strong for its light weight, and modern uses for this wood include model building, insulation, and flotation devices. In contrast, one of the world's heaviest hardwoods is lignum vitae (Guaiacum officinale ), another tropical tree species from the same general region as balsa. This wood has a specific gravity of 1.3 and will not float in water. The wood of lignum vitae is very heavy, and up to 30 percent of its weight is in the form of resins and oils. This wood resists decay very well and can withstand pressures of greater than 2,000 pounds per square inch. These characteristics make lignum vitae a very useful wood for various industrial, manufacturing, and marine applications. Some hardwoods are in plentiful supply, while others are uncommon or rare due to the relative scarcity of the species in natural forests. While many of these woods possess beautiful figure and color in their grain patterns and are highly sought by furniture builders, their continued harvest may pose problems in maintaining the tree species in its habitat, since many grow slowly and reproduce infrequently.
One way of using rare or uncommon hardwoods in furniture-making is through the process of veneering. Thin slices of highly desirable hardwoods are prepared and carefully dried. The dried wood slices (veneers) are glued to the surfaces of more common, structurally stable materials (such as plywood or fiberboard), and the veneer is protected with a suitable finish such as lacquer or varnish. The use of veneer saves valuable timber resources while enabling the enjoyment of beautiful grain and color characteristics of these hardwoods.
Lumber Production and Preparation
All production of wood products begins with the logging of timber by a number of methods (e.g., clear-cutting versus selective cutting). In intensively managed forest stands, softwood lumber (particularly dimension lumber or pulp lumber) can be produced quite efficiently. In contrast, the inherently slow growth of hardwood species restricts the methods of harvest primarily to selective methods from naturally occurring timber stands. Once logs have been felled and are ready for transport, they are moved from the timber stands by vehicle or by using waterways to float them to a milling operation. At the sawmill, the logs are sawn to optimize the quantity and quality of the lumber. The sawyer needs to know the characteristics of the wood being cut and how the lumber will respond to drying and further milling operations. When cut, live timber is very wet; the cell spaces in the xylem are filled with water (a moisture content of approximately 100 percent). This water (free water) must be removed; any remaining water within the cell wall complexes (bound water) must be removed so that the final moisture content of the wood is brought down to approximately 6 to 12 percent of the total weight of the dried wood.
Two main forms of drying wood are used. If an air-drying method is used, the rough-cut lumber is stacked with spacer boards (stickers) between the pieces of wood to promote air circulation around the boards. The lumber is then set out in open areas to allow the water to evaporate slowly from the cut surfaces of the wood, which eventually reduces the moisture content to an acceptable level. The other method uses a drying kiln—essentially a very large, forced-air oven in which temperature and air movement can be carefully controlled. The wet (or partially air-dried) stickered lumber is placed in the kiln, and the temperature and air circulation is increased to evaporate the water from within the wood; this process takes much less time than air-drying. When wood dries, it tends to shrink (particularly across the grain of the wood), so the kiln operator must be aware of the drying properties of each species of wood. If dried too quickly, the lumber can split, twist, or become damaged. When the wood has been sufficiently dried, it will remain relatively stable and is less likely to be attacked by wood rotting fungi when compared to wet woods. The dry, rough-cut lumber is then planed to final dimensions and is transported to distribution centers and lumber yards for use. Further processing of some woods is done with special rotary cutters to produce millwork, architectural trim pieces commonly used around doors and windows and along floors and ceilings. Other millwork items include railings, banisters, balusters, doors, and window frames—all essential components found in most traditionally built homes and other buildings.
Manufactured Wood Products
Some wood products are actually manufactured; that is, constructed from raw wood materials, but utilizing adhesives or other filler components to create new products useful to the construction industry. Perhaps the most important of these is plywood, a wood product made of several layers or plys of thinly cut wood. The grain patterns are normally oriented at right angles to one another prior to their lamination with various forms of adhesives. The result is a sheet stock product that is very dimensionally stable, maintains its flatness when installed properly, and can be used for a wide variety of applications: flooring, sheathing for the outer shell of framed buildings, and roofing. Development of plywood has revolutionized the home construction industry, which previously relied upon sawn and processed lumber planks for these purposes. Other manufactured wood products make use of chips, coarse wood particles, and, in some cases, wood fibers or sawdust, all of which were previously discarded as waste by-products of the lumber processing industry. Advances in wood technology have permitted the use of these materials with the addition of modern adhesives to produce products such as oriented strand board (OSB), particle board, and medium-density fiberboard (MDF). These are sheet-stock materials used in home construction, furniture manufacturing, and other industrial applications.
Other Wood Products
In addition to wood's uses for construction and furniture/cabinetry manufacture, other significant uses for wood products include the harvesting of wood for use by the pulp and paper industry, production of fibers for use in industry, conversion of raw wood materials into charcoal, extraction of turpentine and similar compounds for use as solvents and paint additives, and the use of cork in the beverage and manufacturing industries. Even the material previously stripped from the logs and discarded prior to transport to the mill is now used. Tree bark is a valued commodity for use in the landscaping/horticultural industry, and in some cases forms an organic component to artificial soil mixes. It is encouraging to note that today none of the parts of harvested trees are wasted—the technology of wood processing is sufficiently advanced to ensure that one of the world's most valuable and sustainable resources is used as efficiently as possible. Management of forests and natural habitats as sources of wood products is essential to provide for increasing needs of wood by humans.
see also Conifers; Cork; Fiber and Fiber Products; Forestry; Paper; Trees; Wood Anatomy.
Robert S. Wallace
Constantine, Albert, Jr. Know Your Woods. New York: Albert Constantine and Son, Inc., 1969.
Forest Products Laboratory. Wood Handbook: Wood as an Engineering Material. Madison, WI: U.S. Forest Products Laboratory, 1974.
Hoadley, R. Bruce. Identifying Wood—Accurate Results with Simple Tools. Newtown, CT: Taunton Press, Inc., 1995.
——. Understanding Wood—A Craftsman's Guide to Wood Technology. Newtown, CT: Taunton Press, Inc., 1997.
Jackson, Albert, and David Day. Good Wood Handbook. Cincinnati, OH: Betterway Books, 1996.
Lincoln, William A. World Woods in Color. Fresno, CA: Linden Publishing Co., Inc., 1986.
Wood and Wood Products
Wood and Wood Products
Wood and wood products have played a critical role in the evolution of humankind. From the most primitive of beginnings, humans have used wood for survival and to improve the quality of life. In the twenty-first century, people continue to use wood for many of the same purposes that their most ancient ancestors did. Fuel for heating and cooking is still the largest consumer of wood fiber. Construction of shelter and furniture is secondary as is pulp and paper production.
As a raw material, nothing else in history possesses the versatility of timber. On a volumetric basis, in the year 2000 worldwide annual consumption was between 3 and 4 billion cubic meters. To put this in perspective, if all of this wood was lashed together it could make a floating bridge 2 meters (6.54 feet) thick and 40 meters (131 feet) wide that stretches around the world at the equator. As evidence of wood's exceptional utility value, there are approximately 5,000 distinctly different types and applications of wood products in society.
As a raw material, wood has no equal peers. On a strength-to-weight ratio, wood is stronger than steel. Heavy timber has unique thermal insulating properties, which often allow it to retain structural integrity during building and warehouse fires that completely soften and deform structural steel members. While concrete has excellent structural properties, its density is so high that it is often ruled out as a building material. High shipping and handling costs associated with concrete limit its use to areas close in proximity to the raw mineral mines.
Petrochemical-based plastics offer new alternatives to wood use in construction but such plastics are not renewable. The energy requirements and pollution volumes associated with the production of each of these products is significantly higher than those levels associated with wood products conversion, often four to ten times greater. Additionally, of the materials concrete, plastic, steel, and wood, only wood is naturally renewable.
Forest to Lumber Yard
In the United States, most round wood (in the form of stems or logs) comes from managed forest land (both public and private). The growing population's demand for land, renewable raw materials, and environmental protection has pushed forest management and wood products production to become highly efficient and ecologically healthy. Most management strategies are geared toward multiple use; that is, recreation, wildlife, sporting, ecology, and other uses are included with timber production.
Both natural regeneration and manual replanting are used to restock forest land after timber harvesting. North America is rich with productive land on which to grow trees and with intellectual capital to promote the wise, efficient, and sustainable use of wood fiber. Third-party certification or "green" labeling is emerging as a means to assure wise and efficient forest management and timber conversion.
Also, approval by the International Standards Organization (ISO) is a key tool for companies involved in worldwide wood products trading. Companies that are able to gain wood certification and/or ISO approval have a distinct market advantage because consumers have assurance that the products or materials they consume have conformed to stringent manufacturing and sound environmental principles.
The techniques of timber conversion (turning trees into wood products) have been evolving for thousands of years. During the latter half of the twentieth century, conversion technology advanced exponentially. Computer power has been a key ingredient for optimizing production of wood products. Sawmilling (turning trees into lumber) has been practiced for hundreds of years. Formerly, only the biggest and best trees were cut and brought to mills, and people made decisions regarding how to cut these into lumber.
During the early 1900s, sawmill timber conversion efficiency was approximately 35 to 40 percent, meaning more than half of each trunk was wasted. In the twenty-first century, computers and automated equipment often make most of the decisions regarding how tree stems will be converted to products. The scanning and automation technology used in the forest products industry is the same as that used by the military and the automobile and aerospace industries. Development and application of such technology has allowed smaller and less valuable trees to be used for products.
Historically, the wood fiber that did not become lumber was burned or landfilled. Today, conversion efficiency can reach 70 percent. (There is always some inherent loss associated with turning round stems into rectangular boards.) The residual chips and sawdust from sawmilling are turned into pulp for paper and particles for pressed wood composites. Bark is mainly used as fuel or mulch. The close alliance of these different industries can raise conversion efficiency to 100 percent.
In addition to solid lumber production, there are a variety of composite wood products that have been developed. Plywood and laminated veneer lumber use thin sheets of wood veneer as lamina (layers) for panel-type and lumber-type products, respectively. The conversion efficiency associated with wood veneer production is higher than that of lumber production. Oriented strand products use thin wafers of wood as a raw material. In this case, low-grade trees are reduced to thousands of small strands, and the strands are subsequently pressed together with adhesive into panel products. When trees are reduced to strands, conversion efficiency is 90 to 95 percent. The only fraction not used for strands is the bark, which is converted to fuel or mulch.
The United Nation's Food and Agriculture Organization has reported that the world's overall forest area declined by 1.6 percent (140 million acres) between 1990 and 1995. Because of this sharp decline, there is a growing trend toward harvesting from plantation forests versus natural forests.
Structural panel products like plywood and oriented strand board have revolutionized the building construction industry in the Americas and worldwide. Panel products allow rapid housing construction and provide many superior properties compared to the materials used previously. Non-structural wood composites offer further utilization potential for wood fiber. Often using waste sawdust or shavings as raw materials, particleboard and medium-density fiber board are used extensively in the furniture and cabinet industry. These stable products are used as core materials for both lowand high-cost furniture. When used properly, each of these composite products offers advantages over traditional solid wood products. Dimensional stability, uniformity, long spans, and engineered strength enhancement are just a few such advantages.
The quest to develop stronger, straighter, and more efficient renewable products is ongoing. In an effort to better utilize agricultural byproducts, materials such as wheat straw, kenaf, cotton gin trash, and bagasse are being researched as supplemental fiber sources in wood-based composites. Advanced hybrid composites between wood and materials such as carbon fiber, plastics, and fiberglass have been investigated and are becoming more common in highly specialized structural materials. The level of information-sharing currently available through technology continues to foster the research and development of amazing products at a record pace.
see also Conifers; Forester
Haygreen, John G., and Jim L. Bowyer. Forest Products and Wood Science: An Introduction, 3rd ed. Ames, IA: Iowa State University Press, 1996.
Hoadley, R. Bruce. Understanding Wood: A Craftsman's Guide to Wood Technology. Newtown, CT: The Taunton Press, Inc, 1990.
Walker, Aiden, ed. The Encyclopedia of Wood. London, England: Quarto Publishing, 1989.
Wood Handbook: Wood as an Engineering Material. Publication No. FPL-GTR-113. Madison, WI: U.S. Forest Service, 1999.