Wood as a Material: Structure, Identification and Properties
Scope
This domain tests the anatomy of the tree, the structure of the wood cell wall, how species are identified in the workshop, and the physical and mechanical properties that decide which timber suits which job.
Structure
- A cross-section shows the pith at the centre, heartwood (dead, darker, extractive-rich, durable), sapwood (living, pale, sap-conducting, low durability), the cambium growth layer, and inner and outer bark.
- An annual ring is earlywood (spring, thin-walled, light) plus latewood (summer, thick-walled, dense). Ring width shows growth rate, not strength directly.
- The cell wall is cellulose microfibrils in a lignin matrix with hemicellulose; the S2 layer controls stiffness and shrinkage.
- Hardwoods (angiosperms) have vessels or pores; softwoods (gymnosperms) have tracheids only - a softwood is not automatically soft.
Properties
Wood is hygroscopic and anisotropic. Shrinkage from green to oven-dry is roughly 6–10% tangentially, 3–6% radially and only 0.1–0.3% longitudinally, so flat-sawn boards cup and distort more than quarter-sawn ones. Density and moisture content (MC) govern strength; MC is found by the oven-dry method or an electrical moisture meter.
Identification
Colour, odour, grain figure, pore arrangement (ring-porous such as sal, diffuse-porous such as mango), weight, hardness and the fingernail or knife test all contribute.
Worked example
A 100 mm wide flat-sawn board at 20% MC dries to 10% MC. With a tangential shrinkage coefficient of about 0.28% per 1% MC, the change in width is 100 × 0.28 × (20 − 10) ÷ 100 = 2.8 mm - which is why a drawer or frame is sized with clearance, not dead tight.
Traps
Claiming sapwood is stronger than heartwood (durability differs, not strength); assuming all softwoods are weak; confusing grain (fibre direction) with texture (cell size); applying shrinkage figures above the fibre saturation point of about 28–30% MC, where no shrinkage occurs.