2.1: Understanding Wood

2.1 Understanding Wood

Wood is different from most materials you will use in a shop. It was once part of a living tree, and its structure continues to affect how it looks, cuts, bends, shrinks, sands, glues, and finishes.

Two boards from the same species—and even from the same tree—can behave differently. Learning to read the material is part of becoming a woodworker.

Learning Outcomes

After completing this section, you should be able to:

• Identify the major parts of a tree that are important to woodworkers.
• Define several basic wood-technology terms.
• Explain how the cellular structure of wood influences its properties.
• Describe why wood varies from board to board.

Wood Is Made of Cells

Wood is made primarily of long cells arranged generally along the length of the tree.

You can think of many of these cells as tiny tubes bundled together. Their size, shape, arrangement, and direction influence the properties of the lumber produced from the tree.

This fibrous structure helps explain why wood behaves differently depending on the direction in which it is cut, shaped, or stressed.

Parts of a Tree

Several parts of a tree are especially important to woodworkers.

Bark — The protective outer covering of the tree.

Cambium — The growing layer that produces new wood and bark.

Sapwood — The younger outer wood that conducts water upward through the tree.

Heartwood — The older central wood that no longer conducts sap. Heartwood is often darker than sapwood.

Pith — The central portion of the tree around which the earliest growth occurs.

Growth Ring — Wood produced during approximately one year of growth.

Earlywood — The portion of a growth ring produced earlier in the growing season. It generally has larger cells and lower density.

Latewood — The denser portion of the growth ring formed later in the growing season.

Wood Ray (Medullary Ray) — Cellular tissue extending outward from the center of the tree toward the bark. Rays can become a strong visual feature in some species and sawing orientations.

Cross-section of a tree labeled with bark, cambium, sapwood, heartwood, pith, wood rays, and annual growth rings.

Figure: Cross-Section of a Tree

Here's a more realistic image describing similar parts of the tree.

Another cross-section of a tree labeled with bark, cambium, sapwood, heartwood, pith, wood rays, and annual growth rings.

Figure: More realistic cross-Section of a Tree

Characteristics of Wood

Woodworkers use several terms to describe the material.

Color — The natural color of wood is affected by substances within the cell walls and by extractives found especially in heartwood. Some woods also darken when exposed to air and light.

Texture — The relative size and uniformity of the cellular structure. Woods with small, closely spaced cells are often described as fine-textured.

Grain — A general term describing the arrangement and direction of the wood's cellular structure and growth rings.

Figure — The visible pattern created by grain, growth structure, color variation, and other characteristics.

Density — Weight per unit volume. In general, denser woods tend to be harder and stronger than less-dense woods.

Moisture Content — The amount of water contained in wood relative to its oven-dry weight, expressed as a percentage.

For much indoor woodworking, a moisture content of approximately 6–10% is generally desirable.

Why This Matters

The structure of the tree follows the lumber into the shop.

It affects:

• how easily the wood cuts;
• how likely it is to tear out;
• how strong a part may be;
• how the surface looks;
• how the board responds to changes in moisture;
• how it should be oriented in a project; and
• how it accepts a finish.

A good woodworker does not simply force wood into a shape. You learn to understand the material and work with its characteristics.

Check Yourself

1. What is the difference between sapwood and heartwood?
2. What produces a growth ring?
3. What are wood rays?
4. What is the difference between grain and figure?
5. Why does the cellular structure of wood matter to a woodworker?