Timber: growth, conversion, seasoning and defects

Hardwoods and softwoods

• Hardwoods come from broad-leaved trees: mukwa (kiaat), Zimbabwean teak, mahogany, msasa. Usually durable and attractive, used for furniture.
• Softwoods come from conifers: pine, grown in plantations in the Eastern Highlands. Light, cheaper and easy to work, used for building and shelving.
• Eucalyptus (gum) is a fast-growing hardwood used for poles and fencing.

Tree structure

Bark, cambium (growth layer), sapwood (carries sap), heartwood (strong, durable), annual rings, pith and medullary rays.

Conversion

Sawing logs into planks: through-and-through (plain sawing) is quick and economical; quarter sawing gives more stable boards with attractive figure.

Seasoning

Reducing moisture content (to about 12–15% for indoor furniture).

• Air seasoning: stacked with stickers under a roof – cheap but slow.
• Kiln seasoning: in a heated chamber – fast and controlled but costly.

Seasoning makes timber stronger, lighter, more stable and resistant to decay.

Defects

Knots, shakes (splits along the grain), checks, warping (cupping, bowing, twisting), and attack by fungi and insects (borers, termites).

Manufactured boards

Plywood, blockboard, chipboard (particle board), hardboard and MDF: available in large sheets, stable and cheaper, but edges need covering.

Check yourself

1. Name one Zimbabwean hardwood and one softwood.
2. Give one advantage of kiln seasoning over air seasoning.
3. Name two types of manufactured board.