Wood

Wood is the hard substance of trees and shrubs that makes up the trunk and the branches. It is one of the most important plant products that is used both in its natural state and in a variety of processed forms.

Two-thirds of the world’s wood requirements are supplied from the boreal conifer forests, which form a zone at high northern latitudes spanning Russia, Canada, and Scandinavia. Softwoods, such as fir (Abies sp.), pine (.Pinus sp.), and spruce (Picea sp.), grow mainly in this region, whereas hardwoods come from many parts of the world. Teak (Tectona gran-dis) and mahogany (Swietenia mahagoni), for example, grow in the tropical rain forests while the temperate forests are populated with many species, including oak (Quercus sp.), ash (Fraxinus), and maple (Acer).

Logs are transported by water in countries with good river systems. During the cold months of the year, the logs are piled onto the frozen rivers. When the ice melts they are floated downstream, often held together to form large rafts that are pulled to the sawmills by powerful tugboats. This is the most economical means of transportation, although large quantities of logs are also moved by rail and road.

Wood preparation

Before wood can be used industrially it must go through a number of processes. Felled trees have their branches removed and enter the sawmill as “green” round logs. After they have had their bark removed the logs are sawn into planks, sorted, and trimmed. The lumber must then be seasoned to match the moisture levels of its destined environment.

Wood acts like a sponge; it absorbs moisture and swells in damp air, but shrinks in dry atmospheres when the water evaporates. The wood of the balsa tree (Ochroma lagopus), for example, contains enough air space in its cells to absorb hundreds of times its weight in water. Seasoning not only minimizes shrinkage, but also increases resistance to fungal decay, insect attacks, and metal corrosion. In addition, seasoning makes the wood more receptive to paints, varnishes, and preservatives.

The traditional method of treating wood was air seasoning. Lumber was stacked outside under cover, each length separated by “stickers” that allowed the free flow of air. The moisture must, however, evaporate from the surface of the wood at about the same speed as it moves out from the center, or the drier surface will split and shrink. Metal cleats may be driven into dense hardwoods to prevent splitting, and sheets may be draped over very green lumber to slow surface evaporation.

Today, the faster kiln drying method of seasoning is more popular. The lumber is fed into a chamber where the temperature and humidity levels are initially low. As the moisture content of the wood falls, the humidity is reduced further, and the temperature increased until the wood reaches the desired moisture level.

A fan circulates the air and dehumidifiers remove the moisture.

Fully automated kilns are costly because each type of lumber requires different treatment. Kiln drying allows for faster turnover of stock, although large planks of wood, such as oak, may still take several weeks to dry. This method of seasoning also achieves the lower moisture levels in woods that are required for precision work such as high-class joinery and flooring in modern centrally-heated houses and offices where the humidity is low.

Logs can be cut in several ways. Method A produces wide planks but these tend to warp. Methods B and C produce planks that are more stable but less economical. Waste wood can be burned to power the sawmill or processed into pulp.

Veneer and composite woods

Many good-quality logs are sliced or peeled into thin sheets known as veneers. The logs are usually softened first to reduce the risk of splitting, and then cut to the requisite length and clamped into a lathe which turns the log against a peeling knife. The sheets are dried to a moisture content of 6 to 8 per cent and graded as face, back, or core, depending on the quality. Highly patterned veneers such as walnut [Juglans sp.) are used for decorative furniture. Woods that are too valuable to use in a solid form—for example, teak—are also used as veneers over a cheaper core material, such as blockboard.

Plywood was the first composite wood material to be manufactured on a large scale. It is still the most widely used today. Traditional plywood timbers include birch [Betula sp.) in Europe, Douglas fir (Pseudotsuga menziesii) in North America, and spruce in Scandinavia. Plywood is made by gluing several sheets of veneer together. The quality of each sheet in the plywood “sandwich” usually varies and the back and inner plies are generally of poorer-quality grades.

In the plywood sandwich, the grain of each veneer lies at right angles to the grain of the neighboring layer. The number of layers is usually odd, so that the grain on the two outer layers runs in the same direction. This graincrossing structure gives the wood greater tensile strength and resistance to humidity and temperature changes because it minimizes “movement.”

Because of its strength and resistance to variations in heat, plywood is used extensively for furniture and partitions. It also is used instead of solid wood when a thin but rigid material is needed. Curved plywood products, such as boats and chairs, can be made from molded plywood. The glued veneers are put between two halves of a shaped mold before being pressed. Alternatively, the plywood is molded into shape by fluid pressure inside a flexible bag.

Sandwich” construction timbers are increasingly used in the building industry as an alternative to solid wood. The development of effective adhesives has made composite wood stronger and more adaptable.

Laminated lumber and blockboard

Laminated lumber is made from boards of seasoned lumber that are glued together with the grain running parallel in each layer but with the outer veneers at right angles to the core. For curved products, thin, malleable boards are simultaneously glued and bent to shape. Large columns, arches, boat keels, and decks are often made from this material because large planks can be made of uniformly seasoned wood and the thickness of planks can be increased at points of maximum stress. Laminated maple [Acer sp.), for example, is used for high-stress sports equipment, such as golf clubs, tennis rackets, and hockey sticks.

Blockboard is another form of sandwich construction consisting of a core of wooden blocks held together by surface veneers. The solid core prevents the timber from bending, and its grain runs at right angles to the veneers for strength and stability. Light woods, such as pine, may be used for the core blocks because the strength of the board lies in its construction rather than the strength of the wood.

Wood-frame construction is a traditional form of housebuilding. This method is widely employed in the United States, Canada, Scandinavia, and Australia.