Different kinds of defects are found in timber. Generally, they are caused by the nature of the soil on which the tree is grown, the fertility it gained during growth and changes occurred in the environment during its growth. The defects to be generally looked into while selecting timber for woodwork are discussed below.
Knots
These are the sections of the branches of the tree present on the surface of the wood. This appears in the form of hard and dark pieces. Knots are the spots of weaknesses in timber, particularly when the timber has to bear a compressive force (Fig. 10.2). When the diameter of the knot is less than 6 mm, it is called a nail knot; when it is 6–20 mm, it is called a small knot; when it is 20–40 mm, it is called a medium knot; and when it is more than 40 mm, it is called a large knot. Tight knots are those which are joined to the wood securely. A knot is said to be a live knot or a sound knot when the formation of the knot is free from decay and other defects, and is also intact with the surrounding wood. A knot that is not held firmly in the wood is called a dead knot or a loose knot. Timber containing a large number of knots should be rejected as they are weak in strength and poor in appearance.
Figure 10.2 A typical knot
Shakes
These are the cracks found in felled trees, which are formed due to some reason or the other. Different kinds of shakes are identified and are presented in Fig. 10.3.
Heart shakes are those which split or crack with more width at the centre and diminishes towards the circumference. This type of crack occurs in over-matured trees. Star shakes are radical splits or cracks wider at the circumference, which reduce towards the centre of the tree. These are caused mostly due to severe frost or more heat. These cracks are confined to sapwood only. Cup shakes are presumably formed by the rupture of the tissues in a circular direction across the cross-section of log, usually along the annular rings. Radial shakes are similar to star shakes, but they are thin, irregular and numerous. They are caused when there is a delay in sawing after felling. They usually radiate to a small distance from the bark towards the centre.
Figure 10.3 Types of shakes in timber
Twisted Fibres
These are also known as twisted grains or wandering hearts. They are caused by the twisting of young trees by fast-blowing winds. The fibres of the wood are twisted in one direction only which causes problems while sawing. However, they may be used as poles or posts without any planing (Fig. 10.4).
Figure 10.4 Typical twisted fibres and upset timbers
Upset
It is also called as rupture. This is caused due to some injury during the growth of the tree. It may be caused due to heavy winds and due to faulty felling of the tree.
Rind Galls
Rind means bark, and gall represents abnormal growth. Thus, rind galls are peculiar curved swellings formed on the body of a tree. These are caused due to the growth of layers over the injuries left over after improper felling of trees. The timber taken from this part is very weak and is not durable (Fig. 10.5).
Figure 10.5 Rind galls
Wind Cracks
The exterior surface of a tree shrinks when it is exposed to atmospheric agencies. Such a shrinkage results in cracks known as wind cracks (Fig. 10.6).
Figure 10.6 Typical wind cracks of timber
Sloping Grains
The cells in living trees do not always grow perfectly vertical or straight and parallel to the length of the truck but in a sloping manner. Such an action causes the grains to slope when the timber is sawn parallel to the pitch. Such sloping grains are considered to be a defect.
Presence of Sapwood
As discussed earlier sapwood is less durable than heartwood and hence its presence should be avoided. The presence of sapwood can be identified by the colour, which is much lighter than that of heartwood. Further, it does not take good polish as compared to heartwood.
Apart from the natural causes discussed above there may be some defects that occur in the process of seasoning, which is also to be considered while selecting timber for building construction works.
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