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:
(c) Wind cracks
(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]
(c) Wind Cracks: These are the cracks on the outside of a log due to the shrinkage of the exterior
surface. They appear as shown in Fig. 1.11.
(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
(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
Preservation of Timber
Preservation of timber means protecting timber from fungi and insects attack so that its life is increased.
Timber is to be seasoned well before application of preservatives. The following are the widely used
3. Chemical salt
1. Tar: Hot coal tar is applied to timber with brush. The coating of tar protects the timber from
the attack of fungi and insects. It is a cheapest way of protecting timber. Main disadvantage of this
method of preservation is that appearance is not good after tar is applied it is not possible to apply other
attractive paints. Hence tarring is made only for the unimportant structures like fence poles.
2. Paints: Two to three coats of oil paints are applied on clean surface of wood. The paint
protects the timber from moisture. The paint is to be applied from time to time. Paint improves the
appearance of the timber. Solignum paint is a special paint which protects the timber from the attack of
3. Chemical salt: These are the preservatives made by dissolving salts in water. The salts used
are copper sulphate, masonry chloride, zinc chloride and sodium fluoride. After treating the timber with
these chemical salt paints and varnishes can be applied to get good appearance.
4. Creosote: Creosote oil is obtained by distillation of coal tar. The seasoned timber is kept in
an air tight chamber and air is exhausted. Then creosote oil is pumped into the chamber at a pressure of
0.8 to 1.0 N/mm2 at a temperature of 50°C. After 1 to 2 hours timber is taken out of the chamber.
5. ASCO: This preservative is developed by the Forest Research Institute, Dehradun. It consists
of 1 part by weight of hydrated arsenic pentoxide (As2O5, 2 H2O), 3 parts by weight of copper sulphate
(CuSO4⋅5 H2O) and 4 parts by weight of potassium dichromate (K2Cr2O7) or sodium dichromate
(Na2Cr2O7⋅2 H2O). This preservative is available in powder form. By mixing six parts of this powder
with 100 parts of water, the solution is prepared. The solution is then sprayed over the surface of timber.
This treatment prevents attack from termites. The surface may be painted to get desired appearance.