Chapter 3: Understanding Wood and Grain

Wood Is Not Uniform

Unlike metal, plastic, or composite materials, wood is a natural, biological material. It grew as a living tree, and its internal structure reflects that origin. Understanding this structure is not optional — it is the key to working wood successfully. Every technique in this book, from sawing to planing to finishing, depends on understanding how wood behaves.

How a Tree Grows

A tree grows by adding a new layer of wood just beneath the bark each year. This is why you can count the rings in a cross-section to determine a tree’s age. Each ring consists of:

• Earlywood (springwood): Lighter, softer, less dense. Formed during the rapid growth of spring.
• Latewood (summerwood): Darker, harder, denser. Formed during the slower growth of summer and fall.

This alternation of hard and soft layers is what creates the grain pattern you see on a board’s surface. It also explains why wood behaves differently depending on which direction you cut, plane, or sand it.

Hardwoods vs. Softwoods

This classification is botanical, not necessarily about actual hardness:

• Softwoods come from conifers (evergreen trees with needles): pine, spruce, fir, cedar, redwood. They generally grow faster, are less expensive, and are easier to work. However, they dent and scratch more easily.
• Hardwoods come from deciduous trees (broadleaf trees): oak, maple, walnut, cherry, ash, beech, mahogany, poplar. They are generally denser, more durable, and have more interesting grain patterns.

Note: Some “hardwoods” are actually quite soft (balsa, poplar), and some “softwoods” are quite hard (yew). The terms refer to the type of tree, not the literal hardness.

Common Species and Their Properties

Grain Direction

Understanding grain direction is critical for clean work. Grain direction affects:

• How a plane or chisel cuts (with the grain vs. against it)
• Where tear-out occurs
• How the wood absorbs stain and finish
• How strong a joint is

Reading the Grain

Look at the edge of a board. You will see the growth rings appearing as lines. These lines indicate the grain angle. When you plane or cut, you want to work with the grain — in the direction that the fibers lay down, like stroking a cat’s fur in the right direction.

Planing with the grain: The blade slices cleanly under the fibers, producing a smooth surface and thin shavings.

Planing against the grain: The blade catches under the fibers and lifts them, causing tear-out — rough, chipped areas on the surface. This is the most common cause of poor surface quality.

Identifying Grain Direction

Several methods:

1. Look at the edge: The growth ring lines angle upward in the direction of the grain. Plane in the direction the lines point (uphill).
2. Run your hand along the surface: The grain feels smoother in one direction — that is with the grain.
3. Take a light shaving: If it planes cleanly, you are going with the grain. If it tears, reverse direction.
4. Look for the “cathedral” pattern: On flat-sawn boards, the pointed arches of the grain pattern point in the planing direction.

Interlocked and Reversing Grain

Some species (mahogany, sapele, some tropical woods) have grain that alternates direction in successive layers. This interlocked grain makes planing very challenging because the grain runs in both directions simultaneously. Techniques for dealing with this include:

• Using a very sharp blade set for a very thin cut
• Skewing the plane at an angle
• Using a high-angle (York pitch) plane
• Wetting the surface slightly before planing
• Using a card scraper instead of a plane

Flat-Sawn, Quarter-Sawn, and Rift-Sawn

How a log is cut into boards determines the grain pattern and stability of each board.

Flat-Sawn (Plain-Sawn)

The most common and economical cut. Boards are sliced straight through the log. Characteristics:

• Growth rings meet the face at less than 45°
• Produces the classic “cathedral” or “flame” grain pattern
• Most prone to cupping (the board curling across its width)
• Widest boards are obtained
• Least expensive

Quarter-Sawn

The log is quartered, then each quarter is sliced so the growth rings are roughly perpendicular (60-90°) to the face. Characteristics:

• Straight, parallel grain lines on the face
• Much more dimensionally stable — resists cupping
• In some species (white oak, sycamore), produces beautiful ray fleck (medullary rays visible as shimmering flecks)
• More expensive due to lower yield from each log
• Narrower boards

Rift-Sawn

Cut so the growth rings meet the face at 30-60°. Produces perfectly straight, consistent grain with no ray fleck. Rarely seen because of the very low yield and high cost. Used primarily for high-end furniture legs and table components where consistent grain appearance on all four faces is desired.

Moisture Content and Wood Movement

This is arguably the most important concept in woodworking. Wood moves. It expands and contracts with changes in moisture content, and it does so unevenly depending on the grain orientation.

How Wood Gains and Loses Moisture

Wood is hygroscopic — it absorbs and releases moisture from the air until it reaches equilibrium with its environment. In humid conditions, wood absorbs moisture and expands. In dry conditions, it releases moisture and shrinks.

Equilibrium Moisture Content (EMC) is the moisture level at which wood is stable in a given environment:

• Heated indoor environment (winter): ~6-8% EMC
• Average indoor environment: ~8-12% EMC
• Outdoor, sheltered: ~12-18% EMC
• Freshly cut green wood: 30-100%+ (water content as percentage of dry weight)

How Wood Moves

Wood moves primarily across the grain, not along it:

• Tangential movement (along the growth rings): Greatest movement, typically 6-8% from green to oven-dry
• Radial movement (across the growth rings): About half of tangential, typically 3-4%
• Longitudinal movement (along the grain): Negligible, typically 0.1%

This means a 300 mm wide flat-sawn board might expand or contract by 5-8 mm seasonally. A quarter-sawn board of the same width might move 3-4 mm. Lengthwise, the movement is essentially zero.

Why This Matters

Ignoring wood movement is the single most common cause of structural failure in woodworking. If you glue a solid wood panel into a rigid frame that does not allow for movement, the panel will crack as it shrinks, or the frame will break apart as the panel expands.

Rules for managing wood movement:

1. Use wood at the correct moisture content for its environment. For indoor furniture, this typically means 6-10%.
2. Allow cross-grain movement in your designs. Tabletops are attached with clips or slotted holes that allow the top to expand and contract. Frame-and-panel doors have the panel floating (not glued) in a groove.
3. Never glue cross-grain joints over large spans. A breadboard end must be attached with an elongated mortise or sliding dovetail that allows movement.
4. Orient grain consistently. In a glued-up panel, all boards should have the same grain direction.
5. Acclimate lumber. Bring lumber into your shop for at least a week before working it, so it adjusts to your shop’s humidity.

Measuring Moisture Content

A pin-type moisture meter is an inexpensive and valuable tool. It drives two small pins into the wood and measures electrical resistance, which correlates to moisture content. Use it to:

• Verify lumber is at the appropriate moisture content before building
• Check for consistency across multiple boards
• Monitor drying progress of green or air-dried lumber

Defects in Lumber

When selecting lumber, watch for:

• Knots: Where a branch grew. Can be sound (tight, solid) or loose. Knots are hard, can cause tear-out, and weaken the wood. Small, tight knots add character; large or loose knots should be cut out.
• Checks and splits: Cracks along the grain, usually at the ends. Cut off the checked portion.
• Warp: Boards that are not flat. Types include bow (lengthwise curve), cup (widthwise curve), twist (corners not in the same plane), and crook (edge curve). Mild warp can be flattened. Severe warp is waste.
• Sapwood: The lighter outer wood of the tree. Softer and less decay-resistant than heartwood but otherwise usable. Some woodworkers value the contrast.
• Reaction wood: Wood that grew under stress (compression wood in softwoods, tension wood in hardwoods). Unpredictable behavior — tends to warp severely when cut. Avoid it.

Selecting Lumber

At the Lumber Yard

• Sight down each board from one end to check for straightness, bow, and twist
• Check for moisture content if a meter is available
• Look for consistent color and grain if the pieces will be used together
• Buy extra — you will cut around defects, and having spare material reduces stress
• Buy longer than you need — ends often have checks or damage, and you need to square the ends

Rough-Sawn vs. Surfaced

• Rough-sawn (RS): Straight from the sawmill. Both faces and edges are rough. Less expensive, and you can thickness it to whatever dimension you want.
• S2S (surfaced two sides): Both faces planed smooth. You still need to joint edges and cut to width.
• S4S (surfaced four sides): Both faces and both edges planed. Ready to cut to length and use. Most expensive.

If you own a planer and jointer (or have a good jack plane), buying rough-sawn lumber saves significant money.

Plywood and Sheet Goods

Not all woodworking uses solid wood. Plywood and other sheet goods have important advantages:

• Dimensionally stable: Plywood does not expand and contract seasonally
• Available in large sheets: 1200 × 2400 mm (4 × 8 feet) standard
• Consistent thickness: No need to surface
• Strong in both directions: The cross-layered construction provides strength across and along the sheet

Types:

• Hardwood plywood: A veneer of hardwood (oak, birch, maple) over a plywood core. Excellent for cabinet carcasses, shelving, and drawer boxes.
• Baltic birch plywood: High-quality, void-free plywood with many thin layers. Excellent for jigs, drawer sides, and projects where edge quality matters.
• MDF (Medium-Density Fiberboard): Very flat and smooth, but heavy and weak. Excellent as a substrate for veneer or paint. Poor moisture resistance.
• OSB (Oriented Strand Board): Inexpensive structural panel. Not suitable for fine woodworking.

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