|Carpentry for Vocational Schools - A Teachers Handbook|
Since timber is a natural product, developed through many years of growth in the open air, exposed to continual and varying climate conditions, it is prone to many defects.
Defects cannot be corrected and therefore each individual piece must be inspected before use and judged on its own merits. Defects can be caused during growth, during drying, through insects, through fungi or during subsequent handling or machining, and each should be known, so that imperfect pieces can be detected and rejected.
a) During growth
- Shake is called a partial or complete separation between adjoining layers of wood, due initially to causes other than drying. The three types are Heart shake, Cup shake and Star shake.
- Knots can be caused by a branch or limb being cut through the process of sawing up the log. Knots are classified in two groups: live knot and dead knot. Live knot is left by a branch when the tree is felled.
- Dead knot is left by branches that have been cut off or broken before felling, and which cannot be relied on to remain in position in the piece.
- Gum pocket is a cavity which has contained or contains gum.
b) During or after drying process
- Bow is a deviation from the flat, the piece being arched.
- Twist is a spiral distortion along the length of a piece of timber.
- Cup is a curvature across the width of a piece of timber.
c) Defects through insects
- Termites or white ants attack timber structures and are a serious problem in Papua New Guinea.
The species that causes the damage live in the ground. Precautions involve treating timber with a preservative or avoiding direct timber contact with the ground.
Left unchecked termites eat the wood, leaving the outer shell intact. Often their presence is not evident until serious damage has occurred