For most cabinetmakers and furniture makers, wood is the raw material of choice. Wood has thousands of uses. Although wood may appear to be a relatively simple substance, closer examination shows that wood is one of the most complicated and unusual natural materials. The more you know about wood and its properties, the more valuable wood can become to you.
Wood is made up of many tiny tubular cells which are held together by the wood’s own cement, lignin. These tubular cells are similar to a bundle of drinking straws (see Figure 1). These cells carry the supply of necessary nutrients and water which nourish the life processes of the tree. The walls of these cells also provide support and strength to the tree. These cells run up and down the tree, and produce grain that is visible on the cut surfaces and edges of lumber.
Each of the cells is very narrow and rather long. These cells consist of a cell wall and a cell cavity (lumen) inside the cell wall (see Figure 2). Most of the cells in a tree are dead. The only living cells in a tree are the recent growth produced by the cambium (how the tree grows in width) and some cells in the sapwood.
When the wood is sawn, the openings in the cells are exposed, forming openings on all the wood surfaces. These openings are small pores, and the quantity of pores is called porosity. This porosity is quite extensive. This structure of open space and cell walls gives wood its strength and its properties.
The terms hardwood and softwood are botanical terms and do not indicate the actual hardness or softness of the wood. Some hardwoods are softer than some softwoods, and vice versa. Hardwoods are actually broadleaf trees. Some examples of hardwoods include walnut, oak, ash, maple, cherry and mahogany. Softwoods come from conifers, which are actually trees that bear cones or have needle-like leaves. Some examples of softwoods are the southern yellow pines, white pines, fir, cedar and redwood. Figure 3 compares the softwoods and hardwoods.
The growth ring is often used in reference to the annual growth of a tree. The rings are not always as easy to see as the ones shown in Figure 4. Some woods do not show any visible indications of annual growth. Some species have quite distinct growth rings; others are not easily visible.
The growth rings of wood are made of springwood and summerwood. The portion of the growth ring formed early in the growing season is called the springwood or early wood. That which forms later in the season is called the summerwood or late wood. Generally, the springwood has larger cell cavities and thinner walls, and is less dense than the late wood (see Figure 2).
Hardwoods are classified into three groups, based on the pattern of growth of the annual rings:
Ring-porous species have springwood cells that are wide and distinct, usually several cells wide. The summerwood cells are small, indistinct and thick walled, making the rings very distinct. Some examples are oak and ash. Figure 5 illustrates a ring porous wood, red oak.
Semi-ring porous (semi-diffuse porous) species have fairly distinct springwood cells but are not as wide and obvious as the ring-porous wood. The summerwood, which comprises most of the annual growth ring, has distinct, thick cell walls. Some examples are black walnut and pecan. Figure 6 shows a semi-ring porous wood, black walnut.
Diffuse porous species have no distinct difference between the springwood and summerwood and no distinct ring or annual grain patterns. Some examples of diffuse porous woods are birch, poplar, basswood, maple and cherry. Figure 7 shows a diffuse porous wood, maple. Structural arrangement of wood.
Meet the Author
Dr. Todd Shupe is the President of Wood Science Consulting, LLC. He is a well-recognized expert on wood forensics, wood preservation, wood decay and degradation, and wood species identification. He has a broad background in new product development, quality management, and marketing and sales in both the public and private sectors. For more information please visit DrToddShupe.com.
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