Various defects which are likely to occur in timber may be grouped into the following three:
(i) Due to natural forces
(ii) Due to defective seasoning and conversions.
(iii) Due to attack by fungi and insects.
(i) Defects due to Natural Forces: The following defects are caused by natural forces:
(a) Knots (b) Shakes
(c) Wind cracks (d) Upsets
(a) Knots: When a tree grows, many of its branches fall and the stump of these branches in the trunk is covered. In the sawn pieces of timber the stump of fallen branches appear as knots. Knots are dark and hard pieces. Grains are distorted in this portion. Figure 1.9 shows some varieties of knots. If the knot is intact with surrounding wood, it is called live knot. If it is not held firmly it is dead knot.
(b) Shakes: The shakes are cracks in the timber which appear due to excessive heat, frost or twisting due to wind during the growth of a tree. Depending upon the shape and the positions shakes can be classified as star shake, cup shake, ring shakes and heart shakes [Ref. Fig. 1.10]
(d) Upsets: Figure 1.12 shows a typical upset in a timber. This type of defect is due to excessive compression in the tree when it was young. Upset is an injury by crushing. This is also known as rupture.
(ii) Defects due to Defective Seasoning and Conversion: If seasoning is not uniform, the converted timber may warp and twist in various directions. Sometimes honey combining and even cracks appear. This type of defects are more susceptible in case of kiln seasoning.
In the process of converting timber to commercial sizes and shapes the following types of defects are likely to airse: chip marks, torn grain etc.
(iii) Defects due to Fungi and Insects Attack: Fungi are minute microscopic plant organism.
They grow in wood if moisture content is more than 20°C and exposed to air. Due to fungi attack rotting of wood, takes place. Wood becomes weak and stains appear on it.
Beetles, marine borers and termites (white ants) are the insects which eat wood and weaken the timber. Some woods like teak have chemicals in their compositions and resist such attacks. Other woods are to be protected by chemical treatment.