Timber, lumber, or wood is one of the oldest and remarkably versatile natural resource that is ideal for various applications. Timber for building construction is available from different species of plants. The quality of the timber is also determined based on the moisture content of it.
The properties of timber like dimensional stability, strength, and stiffness – all depend on moisture content of the wood. If sufficient moisture is not removed from the wood before it is used, the timber cannot be used as a building material. Therefore, the wood must be dried properly before using it for construction purposes. The seasoning of timber is the method of timber drying. Let us discuss the objectives, advantages, types, and methods of seasoning of timber.
What is Seasoning of Timber?
It is necessary to remove water from the wood before the wood can be used for various purposes. The process of drying wood is known as the seasoning of timber.
During this process, moisture is removed from the timber under a controlled environment. Also, it should be noted that the remaining moisture which cannot be extracted during seasoning is uniformly distributed throughout the mass of the wood.
Seasoning of timber does not only mean extracting of moisture but also plays an important role in providing strength and durability to the timber. If seasoning is not done the right way, shrinkage of wood will be irregular, and it will set up internal stresses between the fibers. When these stresses become excessive and are capable of overcoming the cohesion of fibers, the defects in timber like shakes and warp are formed.
Objectives of Seasoning of Timber
The main objectives of seasoning of wood are to:
- Maintain the size and shape of timber.
- Improve strength, hardness and stiffness of timber.
- Make it suitable for receiving various treatments like paints, preservatives, varnishes etc.
- Allow the wood to burn quickly if it is used as fuel.
- Reduce the tendency of the timber to crack, warp, shrink and bend.
- Improve the working properties of timber so that it can be smoothly worked on during conversion.
- Improve the resisting power of timber so that it becomes less prone to fungal and insect attacks.
- Decrease the weight of timber and as a result the cost of transportation and handling is reduced.
Advantages of Timber Seasoning
- Seasoned timber is much durable than unseasoned ones. Timbers that are high on moisture content are prone to fungal attack. Timber seasoning reduces the risk of fungal attack.
- The seasoning of timber can improve various properties of wood like its strength, hardness, durability, weight, resistance against insects and electrical resistance.
Classification of Timbers for Seasoning Purpose
Timber can be classified into the following three categories depending upon their behavior with respect to cracking, splitting and drying rate:01. Class A – Highly Refractory Woods
02. Class B – Moderately Refractory Woods
03. Class C – Non-Refractory Woods
01. Class A – Highly Refractory Woods
Highly refractory timbers are slow in drying. Also, these types of lumber are free from crack and split. The example of highly refractory woods are heavy structural timbers, i.e. Sal wood and laurel wood.
02. Class B – Moderately Refractory Woods
Moderately refractory timbers can be dried and can be made free from surface and end cracking within reasonable short periods. It also gives a little protection against rapid drying conditions. The examples of moderately refractory woods are teak wood and rosewood.
03. Class C – Non-Refractory Woods
Non-refractory timbers can be rapidly dried and can be made free from surface and end cracking even in the open air and sun. If these are not dried quickly, they develop blue stain and mould on the surface. The woods obtained from coniferous trees like deodar, fir, pine, spruce, chir etc. are the example of non-refractory woods.
Types of Seasoning of Timber
There are two methods of seasoning timber, i.e. Natural Seasoning and Artificial Seasoning.
01. Natural Seasoning of Timber
In natural or air seasoning of timber, the timbers of convenient sizes are stacked under a covered shed in a cross-wise direction in alternate layers to permit free circulation of air. Generally, air seasoning is a slow process.
The actual time of drying depends upon the size of timber, species of wood and seasonal variations of climate. According to Indian Standard code – IS – 1141, timber should not be considered as full air seasoned time is less than six months. The 25 mm thick timber may take three to four months to dry in a moderate climate.
Advantages of Natural Seasoning of Timber:
- Low investment.
- It does not require skilled labour.
- It is suitable when the market demand is low and non-uniform.
Disadvantages of Natural Seasoning of Timber:
- It needs a larger space for seasoning.
- This process is long and depends on natural air.
- In natural seasoning, the ends of timber dry fast and split. Interior portion of wood may remain moist while exterior part dries quickly.
- During the natural seasoning, there is a possibility of fungal and insect attack.
02. Artificial Seasoning of Timber
In the process of artificial seasoning, water is removed from the wood by artificial means. It can be either moderate or rapid. It depends on the temperature of the air injected into the chamber where the timber is piled and on the rate at which the air is circulated and extracted from the chamber.
Advantages of Artificial Seasoning of Timber:
- In artificial seasoning, the drying is fast and uniform. Therefore, defects in timber such as shrinkage, cracks and wrapping are minimum.
- Moisture content in wood can be reduced to the desired level due to the artificial seasoning. So the chances of fungi and insect attacks are less.
- Timber becomes more suitable for painting after artificial seasoning.
Disadvantages of Artificial Seasoning of Timber:
- It requires high investment, and if the demand for seasoned timber is low and intermittent, it is uneconomical.
- Skilled supervision is needed during artificial seasoning.
Types of Artificial Seasoning of Timber:
The various methods of artificial seasoning of timber are:
a) Kiln Seasoning of Timber: Kiln seasoning is a rapid method of timber seasoning that reduces the moisture content in the timber. In this method, wood is kept in a tight air chamber. The drying arrangements are available inside the chamber. The temperature inside the chamber can be raised with the help of heating elements. When the desired temperature is obtained inside the chamber, the moisture content in timber gets reduced.
b) Electrical Seasoning of Timber: In electrical seasoning, the electrodes are kept in contact with the two ends of the timber. Electric current is passed through it and the wood resists the flow of current passing through it. Due to this heat is generated during the process, which results in its drying.
c) Chemical Seasoning of Timber: Chemical seasoning is also called salt seasoning. In the chemical seasoning, green sawn timber is first soaked in a solution of an anti-shrink or anti-shrink cum hygroscopic chemical for an appropriate period. It is then taken out and treated as air seasoning or kiln seasoning.
Summing up, Timber Seasoning is the first step in the efficient utilization of timber, especially where several commercially important species of wood are hard, heavy and prone to develop defects during drying. It should be noted that all the types of timber either naturally durable or non-durable, need proper seasoning to give satisfactory service. Seasoning of timber should be regarded as an integral part of timber utilization. With the increased use of several secondary species that possesses higher shrinkage than teak, deodar, rosewood etc. it has become imperative that adequate attention should be paid to the process of seasoning of wood before use.
