Timber is widely used in construction from ancient times. It can be used at the time of construction like shuttering material, scaffolding, formwork, etc. Timber is also used in furniture, doors and many decorative works. Basically timber is an inevitable material for construction. So study of timber is very necessary.
Mainly there are two types of trees which are trees which grow outwards and others which grow inwards. Trees that grow outwards are called exogenous and trees that grow inwards are called endogenous.
Exogenous
These trees grow outwards. There are also two types of exogenous trees.
- Conifers
These trees are evergreen trees. They produce softwoods and show distinct annual rings, from which the age of the tree can be predicted. They do not show distinct medullary rays.
Ex. chir, kail, pine, deodar.
- Deciduous
These trees are not evergreen. Their leaves fall in autumn and new leaves appear in spring. They do not show distinct annual rings but show distinct medullary rays.
Ex. teak, sal, shisham.
Structure of Tree
- Bark
It is the outside layer of the trunk, branches, and twigs of trees. It serves as a protective layer for the inside wood of the tree.
Trees actually have inner bark and outer bark. The inner layer of bark is made up of living cells and the outer layer is made of dead cells. The inner layer of bark is also called phloem. The main function of the inner layer is to carry sap full of sugar from the leaves to the rest of the tree.
- Cambium
The thin layer of living cells just inside the bark is called cambium. The function of cambium is to produce new cells and help the tree to grow wider. It can be also said that it is material yet to be converted to sapwood.
- Sapwood
It is made up of a network of living cells that bring water and nutrients up from the roots to the branches, twigs, and leaves. It is the youngest wood of a tree. After a period of years, sapwood dies to become heartwood. It is also called alburnum.
- Heartwood
It is dead sapwood in the center of the trunk. It is the hardest part of wood giving it support and strength. It is usually darker in color than the sapwood. It does not play any active role in the growth of the tree. It consists of the inner annual rings around the pith.
- Pith
It is the tiny dark spot of spongy living cells in the center of the tree trunk.
Defects in a timber
- Heart shake
It usually occurs in matured trees. It is due to the shrinkage of heartwood.
- Star shake
It is caused by severe heat of the sun and it affects sapwood.
- Knot
This is caused by the bases of small branches of a tree that form circular rings. The knot may be a live knot or a dead knot.
- Dry rot
This defect is caused by fungi where there is no free circulation of air; dampness is the main cause.
- Wet rot
It occurs due to alternate drying and wetting of timber. It converts timber into powder form.
- Honeycombing
This is caused due to improper seasoning of timber.
- Foxiness
This defect is caused due to over maturity and unventilated storage of wood during its transit.
- Cup shake
This defect is caused by strong winds that sway the tree, or by excessive frost acting on the moisture present in the tree when it is still young.
Seasoning of timber
There are two processes of seasoning of timber: natural seasoning and artificial seasoning.
- Natural seasoning
This method of seasoning is simple and cheap, but it is very slow; it requires 3 to 5 months.
In this process, timber is kept in a shade at about 30 cm high above the ground level. Care is taken to see that there is proper air circulation. Naturally, the moisture content in timber reduces. A well-seasoned timber contains only 10 to 20% moisture.
Artificial seasoning
It is the quickest method of wood seasoning and keeps the moisture content under control. It takes 4 to 5 days to complete under normal conditions. The section of timber obtained is uniform and even. There are also different types of artificial seasoning like boiling, kiln seasoning, chemical seasoning, and electrical seasoning.
- Boiling
In this method, timber is immersed in water and then it is boiled for 3 to 4 hours. After boiling, it is dried slowly. The process of seasoning is fast but costly, and if not controlled closely the timber will break.
- Kiln seasoning
In this method, timber is kept in the oven for about 12 to 20 days. The temperature is initially kept at about 35 to 38 degrees Celsius and then gradually increased.
- Chemical seasoning
In this method, timber is immersed in a solution of salt and then dried in a kiln.
- Electrical seasoning
This is the most rapid method of seasoning and costlier than the other methods. In this method, an electric current is passed through the timber. As the moisture content reduces, the resistance increases.