Home » COMMON TIMBER DEFECTS

A defect is anything present in wood, which results in the lowering of its quality or value. A defect can be caused by the action of man, insects, and fungi or may naturally occur in the growing tree. Some defects caused by man, insect or fungi action, collectively known as artificial defects are the following:

 

A) Artificial Defects

Artificial defects are those, which are caused by organisms or situations, which are external to the growing tree or felled timber.

(i) Cup:- This is a bend across the face of a plank.

 

(ii) Bow:- This is a bend along the length of a plank.

 

(iii) Twist or warp:- Multiple bend about the axis of a plank.

 

(iv) Check:- A split along the grain of wood.

 

(v) Waney edge:- This is the bark of tree showing on the edge of a plank.

 

(vi) Dry rot:- This results when tiny organisms known as fungi destroy the wood.

 

(vii) Fungus stain:- Stain occurring on wood as a result of fungus attack.

 

(viii) Insect attack:- This results when termites and other insects live in the wood and steadily destroy it.

Defects listed in numbers (i) to (iv) above are usually caused by bad seasoning practices.

 

B) Natural Defects

Natural defects are those, which occur in the growing tree and are usually manifested only after it has been felled. Some natural defects are explained below.

 

(i) Heart shake:- This is a split across the centre of a log, usually along a medullary ray.

 

(ii) Star shake:- This is different from the heart shake only by the fact that there are several splits, each along a medullary ray.

 

(iii) Cup-shake:- This is a discontinuous (i.e. short) split between two growth rings.

 

(iv) Ring shake: – A circular crack, which completely separates two annual rings.

 

(v) Knot:- This is a portion of a branch enclosed inside the stem of a tree.

 

C) Shrinkage in Timber 

Seasoning causes timber to shrink. Some shrinkage occurs along the circumference and this causes the growth rings to shorten. This is called circumferential shrinkage.

Shrinkage also occurs longitudinally along the grain of wood thus shortening the length of the log. This is known as longitudinal shrinkage.

Radial shrinkage also occurs from the bark towards the centre of the trunk. In terms of extent, usually the longitudinal shrinkage is the least while the circumferential shrinkage is the largest.

 

 

Veneer and Manufactured boards

When a tree trunk is converted by using plain, tangential or quarter sawing methods, what is obtained is referred to as timber. However, a tree trunk can also be cut by peeling or slicing method to obtain what is referred to as veneer. A veneer is a thin sheet of wood usually having a thickness not exceeding 3 mm.

 

(i) Peeling method

This is also referred to as rotary method of cutting veneers. A trunk is softened by steaming it and then rotated about its longitudinal axis. A stationary sharp cutting edge placed against the edge of the trunk peels off a continuous sheet of veneer.

 

ii) Slicing method 

In this case, a trunk is softened by steaming and then held in a stationary position. A sharp cutting edge placed against the trunk is then moved forward and backward to cut off discontinuous sheets of veneers. Veneers may be glued together to solid wood to produce manufactured boards.

 

Manufactured boards have the following advantages when compared to timber.

  1. Despite their thinness, they are considerably strong.
  2. They are available in large sheets.
  3. Compared to timber, expansion or contraction is quite minimal.
  4. A wide variety of manufactured boards is available.
  5. They can be easily worked, cut or nailed without splitting.
  6. Some of them can be easily bent to form curved shapes.

 

Examples of manufactured boards are as follows:

(i) Plywood

Plywood is obtained by gluing veneers positioned in such a way that the grains of neighbouring sheets are at right angles to each other. The term grain means the direction of the majority of wood fibres in relation to the vertical axis of the wood. Plywood having more than three veneers is referred to as multi-ply while that having a thick centre layer is called stout heart. It is usual to make the veneers of both surfaces of the plywood to lie in the same direction. Consequently, plywood contains an odd number of veneers (e.g. 3, 5, and others).

 

(ii) Chipboard

Chipboard is produced by using synthetic glue to bond together wood chips of comparable sizes. This process is carried out under high heat and pressure. Chipboard does not hold screws well and it does not produce satisfactory joints.

 

(iii) Blockboard, Laminated board (also called Laminboard), and Battenboard. 

These three are similar in construction. In each case, a core comprised of thin strips of wood glued together is covered with one or more sheets of veneers. The strips of the blackboard are usually between 13mm to about 25mm wide and the grain of the outer veneers is at right angles to the grain of the strips. The strips of the laminboard are narrow, usually 12mm wide at the most. Due to the narrowness of these strips, the possibility of appreciable distortion of the surface of the laminboard as a result of shrinking of the strips is reduced. The strips of a battenboard are usually about 26mm or more in width.

 

(iv) Laminated plastic

Laminated plastic is made by bonding paper with synthetic resin under high heat and pressure. It is very durable and attractive. It is much used for making table tops.

 

(V) Hardboard

This is produced from thick pulp usually obtained by reducing softwood chip to fibres and mixing with hardening and bonding agents. This pulp is then converted to hardboard under high heat and pressure.

 

(vi) Particle board and composite board 

Essentially, each is a combination of wood chips, paper and water. The main difference is that during manufacture, particle board is subjected to a much higher pressure than composite board and this is partly responsible for their eventual textural differences.

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