What is Seasoning of Timber? Various Methods of Seasoning of Timber

Seasoning of timber is defined as the process of drying out water from "wet" or "green" timber. Drying the moisture out of timber or wood enhances its properties in terms of strength and durability. 

The moisture content is timber can be calculated as the ratio of the weight of water contained in a piece of timber compared with the weight of the actual woody substance in the same piece. It is expressed in percentage as:

Moisture Content of Timber = [Weight of Water/ Weight of Wood Substance ] x 100

How Timber Dries

Seasoning of timber is the process of reducing the moisture content of trees obtained after felling. Seasoning reduces the moisture content by drying the moisture at controlled conditions as nearly as possible at uniform rate from all parts of timber.

This controlled state prevents the timber from possible fermentation and making it suitable for use. 

When a tree is felled, it contains 50 percent or more of its own dry weight as water. 

A well-seasoned timber contains an optimum moisture content between 10 to 12 %.  The process helps to increase the durability of timber. 

The moisture content of timber used for building frames 12 to 18 % as per IS 287-1993.

Objectives of Seasoning 

• Reduce shrinkage and warping after placement in structure
• Increase strength, durability, resistance and dimensional stability
• Reduce tendency to split and decay
• Make it suitable for painting
• Reduce the weight

Methods of Seasoning Timber

Seasoning can be performed by two methods:

1. Natural Seasoning
2. Artificial Seasoning

1. Natural Seasoning

Natural seasoning is performed by natural means using air or water. Hence, there is air seasoning and water seasoning. 

a. Air Seasoning

Natural Air is allowed to flow through timber balks. For better exposure to air, timber balks are stacked on a platform at an arrangement of 150 mm x 300 mm. It takes a period of 180 to 200 days. The method is cheap and economical.

Fig. Air Seasoning

b. Water Seasoning

Water seasoning is carried out on the banks of the rivers. Logs are placed in the river with thicker ends pointing upstream. Within a period of 2 to 4 weeks all the sap gets washed away. Compared to air seasoning, water seasoning required less seasoning time. 

Fig. Water Seasoning Timber

2. Artificial Seasoning

Artificial seasoning is carried out on a chamber of timber logs supplied with regular heat, controlled humidity and air circulation. The timber is seasoned with 4 to 5 days and dried slowly. Various methods of artificial seasoning are:

• Boiling
• Kiln Seasoning
• Chemical Seasoning or Salt Seasoning
• Electrical Seasoning
• MC Neil Seasoning Method

a. Boiling

The timber is immersed in water and boiled for 3 to 4 hours. Hot steam may be circulated instead of boiling water and then allowed to dry slowly. The process is fast but costly. The method reduce strength and elasticity of timber. 

b. Kiln Seasoning

Timber is seasoned in an airtight chamber with saturated air at a temperature of 35 to 38 degree celsius. This can be stationary or progressive kiln seasoning. In stationary or compartment kin, the relative humidity is gradually reduced and the temperature is increased and maintained. Once moisture reduces to 5 to 6 percent the temperature is reduced and chamber is cooled. 

In progressive kiln seasoning, carriages carry timber and travel from one end to the other end of the kiln gradually. This method is suitable if the seasoning is to be performed on large scale.

c. Chemical Seasoning

Chemical seasoning also called as salt seasoning , involves dipping timber in a solution of suitable salt. Later it is dried in a kiln. As the vapour pressure of chemical salt is less, they draws out the inner moisture to the surface at a uniform rate. 

Chemical seasoning at preliminary stage helps to achieve uniform seasoning. The commonly used salt for chemical seasoning are sodium chloride and sodium nitrate. 

d. Electrical Seasoning

High frequency AC current is allowed to pass through the timber. Here the resistance of the electric current is measured to know the amount of seasoning. With the decrease in moisture content, the resistance of the current increases. Hence, the value of resistance give the rate of seasoning. 

It is a fast method of seasoning giving uniform quality timber. It is commonly used in plywood companies, but is a costly process. 

e. Mc Neil's Seasoning

Timbers are stacked in a brick-lined chamber with ample air space and a large water surface. Hot gases from a fireplace within the chamber create a moist, warm atmosphere. The process takes 15 to 2 months. McNeil's seasoning is a relatively expensive method.

Important Points

1. Uniform seasoning of timber is achieved by humidity drying.

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