ENCYCLOPEDIA OF WOOD

UNDERSTANDING WOOD

Подпись: A pile of logs sit at a sawmill in Oregon, ready to be milled into lumber.

As you strive to improve your mastery of the demanding craft of woodworking, much of your attention will be devoted to learning about tools and the tech­niques for using them. But in your quest for perfection, do not neglect the most fundamental component of every project—the wood itself.

Rarely perfect and always vary­ing, each piece of wood exhibits its own character, just as certainly as a human being: Some woods are plain, some colorful; some are sta­ble, some unpredictable; some work easily, some with difficulty. A knowl­edge of these properties will allow you to make the most of your abilities, achieving a wedding of form, substance and technique that can transform even an ordinary project into a work of art.

You can obtain much factual information about the prop­erties of wood in readily available books and articles. Learning to apply that knowledge is more challenging. For example, the knowledge that maple boards may contain wide variations in color, texture and figure will assume greater meaning as you learn to use these characteristics to best advantage. Likewise, although Douglas-fir is an attractive, easily worked wood, vari­ations in its surface porosity can make it difficult to finish well. But when you learn how to seal the wood, you will find many uses for Douglas-fir. Experience will also tell you that a resilient
wood such as pine is more forgiv­ing of less precise joinery, while dense, brittle species such as ma­hogany demand joints that are cut to close tolerances. And every beginner quickly learns that sanding wood across the grain, rather than parallel to it, results in scratches that are accentuated when a finish is applied to the piece.

Remember, too, that how a par­ticular piece of wood behaves in your shop depends in large measure on what happened to it before it reached the lumberyard. How the wood grew in the tree, the weather the tree endured and how the wood was cut and dried all affect the final product. The wood of a leaning tree, for example, will react differently during machining than that sawn from the trunk of an erect tree. And whether a board is quartersawn or plain-sawn has an impact on its dimensional stability.

One way to obtain intimate knowledge of your material is to saw it yourself from a tree using a portable lumber mill (page 36). Selecting and felling a tree, bucking—or crosscut­ting—it into logs, and milling the planks impart a hands-on understanding that is impossible to acquire any other way. The work is arduous, and it also takes considerable time to cut and dry the boards. But the rewards—both in the unique lumber produced and the personal satisfaction in producing it—are well worth the effort.

A stand of Douglas-fir trees basks in the sunlight in a West Coast forest. Many softwoods, like Douglas - fir, are ideal for interior trim or cabinet work.

Harvested from the trunks and branches of trees, wood is a resilient, dynamic building material. Understanding how trees grow can shed considerable light on why wood behaves as it does when it is worked or finished.

Подпись:Подпись:UNDERSTANDING WOODПодпись: Roots Anchor tree and absorb water and minerals from the soil

Подпись: ANATOMY OF A TREE

All trees consist of three major sys­tems: a root network that draws water and minerals from the soil; a crown of leaves, where water and minerals are combined with carbon dioxide in the presence of sunlight to produce food for the tree (photosynthesis); and—of most interest to woodworkers—a supporting trunk that transports the water and food.

Viewed in cross section, a tree trunk at first appears to be a fairly homoge­nous column of wood, marked by a series of concentric bands called growth rings. However, a close view reveals a series of distinct layers wrapped around each other, some living, some not. At the center is the heartwood, the dens­est—and dead—part of the trunk. Encircling the heartwood is the paler sapwood, which in turn is surrounded by the cambium, the trunk’s only active­ly growing segment. The cambium’s growth accounts for the layers of sap - wood that are added each year. On either side of the cambium are layers that trans­port sap throughout the tree and store surplus food. As the inner sapwood recedes from the cambium, its pores gradually clog with resins and gums, and become heartwood. As the outer sec­tions become dormant, they form a trunk’s outermost layer, the bark.

The differences between sapwood and heartwood are important to every wood­worker. Because it is more porous than heartwood, sapwood absorbs finishes better. But the denser heartwood is usu­ally more durable and decay-resistant. The carbohydrates present in sapwood cells make the wood vulnerable to fun­gi and insects. The colors of heartwood are also generally richer and more vibrant than those of sapwood.

Growth ring

Подпись:Подпись:Подпись:UNDERSTANDING WOODПодпись:

Подпись: Growth rings In regions where a tree’s growth is interrupted by seasonal change, its wood is characterized by growth rings: concentric bands, usually fractions of an inch wide, perpendicular to the axis of the trunk. Trees that grow in temperate areas with a winter season display distinct rings. In the tropics, where growth is more or less continuous, a sharply defined ring may only be visible as the result of a dry season. The rings are intersected by a series of rays: flattened bands of tissue that radiate outward from the pith to the phloem of the tree. Growth rings consist of two separate layers. The first, called earlywood, is laid down at the beginning of the growing season; the second layer, or late-
Подпись: wood, is formed toward the end. Earlywood is more porous than latewood, which accounts for the contrast between the two. Taken together, the earlywood and latewood of a growth ring in temperate climates represent one year in a tree’s life. The width of a ring depends on growing conditions and varies from species to species, but changes from year to year reveal a tree’s history. A wide ring suggests a growing season with ample sun and moisture, while a narrow ring is evidence of disease, unfavorable weather or insect attacks. For the woodworker, growth rings are also clues to the strength of the wood: uncharacteristi-cally narrow or wide rings can signal weak timber.

A concentric ring divided into earlywood and latewood indi­cating the amount of wood added to a tree’s diameter in one growing season

Sapwood

Подпись: Phloem A thin, spongy layer of tubes that carry dissolved sugars and growth hormones from the leaves to other parts of a tree Подпись: Cambium  A thin reproductive layer that forms new tissue, adding to the phloem and sapwood to increase a tree’s girth

Active part of the tree’s wood through which water and minerals are con­ducted from the roots to the leaves; also stores nutri­ents and helps to support the tree

Подпись: UNDERSTANDING WOODA thick slice from the trunk of a mature oak forms an oval-shaped tabletop. The growth rings that characterized this tree are clearly visible: Light-colored early­wood alternates with darker bands of latewood, etching a distinct line between each year’s growing periods.

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ENCYCLOPEDIA OF WOOD

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