Wood Species Selection Guide
Selecting the appropriate wood species for a project requires balancing aesthetic goals with physical requirements such as strength, stability, and durability. Wood is an anisotropic material, meaning its properties differ depending on the grain direction.
1. Physical and Mechanical Properties
1.1 Janka Hardness
The Janka hardness test measures the resistance of a wood sample to denting and wear. It is defined as the force required to embed an 11.28mm (0.444 in) steel ball halfway into the wood.
- Low Hardness (e.g., White Pine, Basswood): Easy to work with hand tools but prone to denting.
- Medium Hardness (e.g., Black Walnut, Cherry): Good balance of workability and durability for furniture.
- High Hardness (e.g., White Oak, Hard Maple, Hickory): Difficult to work but extremely wear-resistant.
1.2 Dimensional Stability (Movement in Service)
Wood expands and contracts with changes in relative humidity. The degree of movement varies by species and grain orientation (tangential vs. radial).
- Stable Species (e.g., Genuine Mahogany, Teak): Minimal movement, making them ideal for precise joinery and exterior use.
- Unstable Species (e.g., Beech, Hickory): Significant movement requires careful joinery design (e.g., breadboard ends or floating panels) to prevent cracking or warping.
1.3 Specific Gravity and Density
Density often correlates with strength and screw-holding power but also affects the ease of gluing and finishing. High-density woods (like Ipe or Cocobolo) often contain natural oils that can interfere with traditional wood glues.
2. Botanical Classification: Hardwood vs. Softwood
The distinction is botanical, not necessarily a measure of physical hardness.
- Hardwoods (Angiosperms): Deciduous trees with broad leaves. They possess specialized water-conducting vessels (pores).
- Ring-Porous (e.g., Oak, Ash): Large pores in earlywood and small pores in latewood. Strong grain patterns and often difficult to finish to a glass-smooth surface without grain filler.
- Diffuse-Porous (e.g., Maple, Cherry): Small pores distributed evenly. They provide a more uniform texture and take finishes smoothly.
- Softwoods (Gymnosperms): Conifers, usually evergreen with needles. They lack vessels and consist primarily of tracheids. They are generally easier to machine but can be resinous (e.g., Pine, Fir).
3. Durability and Rot Resistance
For exterior projects or items in contact with the ground, the wood's natural resistance to decay is paramount. This resistance is found in the heartwood (the older, non-living inner core), which contains extractives like tannins and oils. The sapwood (the living outer layer) of almost all species has little to no rot resistance.
- Exceptional Resistance: Ipe, Teak, Black Locust, Western Red Cedar.
- Moderate Resistance: White Oak, Black Walnut, Douglas Fir.
- Poor Resistance: Hard Maple, Beech, Birch, Red Oak.
4. Workability Characteristics
- Interlocked Grain (e.g., Sapele, Sycamore): The grain direction reverses in alternating layers, making the wood difficult to plane by hand without "tear-out."
- Silica Content (e.g., Teak, Iroko): Some species contain microscopic silica particles that dull steel tools rapidly. Carbide-tipped tools are recommended.
- Steam Bending: Species like White Oak and Ash have long, straight fibers that allow them to be steamed and bent into tight curves without fracturing.
4.2 Finishing and Gluing
- Tannin Content: High-tannin woods like Oak and Walnut can react with water-based finishes or iron fasteners, causing dark blue or black stains.
- Oil Content: Oily exotics like Teak or Rosewood may require wiping the joint with acetone or denatured alcohol immediately before gluing to ensure a good bond.
5. Summary Table of Common Species
| Species | Hardness (lbf) | Stability | Durability | Typical Use |
|---|
| Black Walnut | 1,010 | Excellent | High | Fine furniture, gunstocks |
| White Oak | 1,360 | Average | High | Boatbuilding, flooring, barrels |
| Hard Maple | 1,450 | Average | Low | Workbenches, cutting boards |
| Black Cherry | 950 | Excellent | Low | Cabinetry, turnings |
| Western Red Cedar | 350 | Excellent | High | Siding, decking, outdoor structures |
| Hickory | 1,820 | Poor | Low | Tool handles, ladder rungs |