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SEASONING OF TIMBER

Seasoning is the process of reducing moisture content in a freshly

cut tree to the desired level.

Objectives of seasoning

The objects of seasoning are to:

1. Increase the durability by protecting it from fungi, insects

and other causes related to moisture content;

2. Impart hardness, stiffness, strength and resistance to electric

shocks;

3. Maintain shape and size;
4. Make it workable;
5. Make it fit to receive painting;
6. Make it suitable for gluing;
7. Reduce the tendency of cracking and warping;
8. Decrease weight and save transportation cost and
9. Allow to burn easily, if used as fuel.

Methods of seasoning

Methods of seasoning may be broadly grouped into:

 (a) Natural seasoning

 (b) Artificial seasoning

 (a) Natural Seasoning

This type of seasoning may be in the form of:

 (i) Air seasoning

 (ii) Water seasoning

Air Seasoning:

This is where timber is stacked in a shed with a platform of height 300 mm Care is taken to see that air can circulate around each timber balk. Moisture content decreases over a period of time. Well-seasoned timber contains only 15 percent moisture. This process is slow but it is the best method of seasoning.

Water seasoning:

In this method logs are placed in a river with thicker ends pointing upstream. A number of logs are tied together and the group is anchored to a standing tree or to rock to see that it is not carried away by the river. The sap contained in the timber is washed out over a period of 2-4 weeks. The timber is then stacked in a dry place. Compared to air seasoning this method takes less times.

(b) Artificial Seasoning

The various methods of artificial seasoning are:

 (i) Boiling

 (ii) Kiln seasoning

 (iii) Chemical seasoning

 (iv) Electrical seasoning

• Boiling:

 In this method timber is immersed in water and then water is boiled for 3-hours. Instead of boiling hot steam may be passed over the timber. Them it is dried slowly. This process of seasoning is fast but costly. It reduces the strength of timber to some extent.

• Kiln seasoning:

A kiln is an airtight chamber. In this timber to be seasoned is placed and hot air is pumped in. After moisture content is brought down, the temperature is reduced and the chamber is allowed to cool. The kiln used may be progressive type also. In such a kiln, carriages carrying timber moves from one to the other end slowly.

Hot air is pumped from discharging end so that temperature is higher at that end compared to at the charging end. As timber comes out at discharging end sufficient moisture is removed.

This method is suitable if seasoning is required on large scale, Since in this case investment is high.

• Chemical seasoning:

     It is also known as salt seasoning. In this method, the timber is dipped in a solution of sodium chloride or sodium nitrate. The surface salt draws out inner moisture. This preliminary treatment by chemicals ensures uniform seasoning across the section. It is then taken out and seasoned in the ordinary way.

• Electrical seasoning:

     In this method, high frequency alternating current is passed through the timber. Heat is introduced internally and the timber starts drying. As the moisture content reduces resistance to electricity increases. The measure of resistance may be used to stop seasoning at appropriate level. This method is fast and uniform. Many plywood companies adopt this method. It is a costly technique.

Advantages of natural seasoning

1. It requires low investment.
2. It does not require skilled labor.
3. It is ideally suited, to low and non-uniform market demand.

Disadvantages of Natural Seasoning

1. It needs larger space for seasoning.
2. The process is very slow, usually takes 2-4 years.
3. As the process depends on natural air, there is no control on it.
4. Ends may dry fast and split. Interiors may remain moist while exteriors dry fast.
5. The moisture content may not be brought down to the desired level.
6. Chances of fungi and insect attack cannot be ruled out during the seasoning period.

Advantages of artificial seasoning

1. Drying is uniform, hence defect such as shrinkage, cracks and wrapping are minimum.
2. Drying process is fast, hence economical.
3. Moisture content can be reduced to the desired level.
4. Chances of fungi and insects attack are low.
5. Wood becomes more suitable for painting.

Disadvantages of artificial seasoning

1. It needs high investment.
2. Skilled supervision is required.
3. If demand is low and intermittent, it is uneconomical.