Seasoning of Timber

 

 

SEASONING OF TIMBER

Seasoning is the process of reducing moisture content in a freshly cut tree to the desired level.

In other words, Reduction of moisture content along with improving some qualities before the use of woods is called seasoning of timber. By seasoning, generally, the moisture is reduced to about 15% where new cut woods bear about 50%.

Timber is wood that is used for building houses and making furniture.

Basically, timbers are used for structural purposes. Those woods, which are adapted for building purpose, are timber.

When the trees are cut down, it will be having approximately 50% of moisture content. Timber cannot be used immediately after the cutting of trees because of the tree contains an uneven sap. If the excess moisture is present within the timber will shrink excessively.It will even have a tendency to decay so, there is the only seasoning of timber is one of the solutions. The 100% amount of the moisture cannot be removed from the Tree.

12-15℅ of moisture remains within the tree after seasoning. Carpentry tools can easily be used on the well-seasoned timber. Well-seasoning of timber may contain up to 15%.

 

Objects 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.

10. To change and improve the properties of wood.

11. To make a correct percentage of shrinking of woods.

 

Methods of Seasoning of Timber -

There are Two Methods of Seasoning of Timber:

1. Natural Seasoning

2. Artificial Seasoning

 

1. Natural Seasoning -

Natural seasoning is the process in which timber is seasoned by subjecting it to the natural elements such as air or water.

This type of seasoning may be in the form of:

 (i) Air seasoning

 (ii) Water seasoning

 

(i) Air Seasoning -

It is also known as air drying. In the process of air seasoning timber logs are arranged in layers in a shed. The arrangement is done by maintaining some gap with the ground. So, platform is built on ground at 300mm height from ground. In air seasoning, timber in properly cut forms is stacked in a proper manner in the open air for losing moisture by process of evaporation.

The Stacks are so constructed to allow free circulation of air around each part as far as possible. The stacks are properly sheltered from direct sun and winds and rain. It may take 6 months to 4 years for bringing down the original moisture content to allowable limits of 14-16 percent by this method.

(ii) Water seasoning -

Water seasoning is the process in which timber is immersed in water flow which helps to remove the sap present in the timber. It will take 2 to 4 weeks of time and after that the timber is allowed to dry. Well-seasoned timber is ready to use. Sometimes logs are placed in running water before subjecting to air seasoning. This helps in replacement of “sap” from wood cells by water. The water-saturated wood dried quicker as compared to sap-rich wood. The process of placing timber logs in water is sometimes referred to 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.

Advantages of natural seasoning -

• It requires low investment.

• It does not require skilled labour.

• It is ideally suited to low and non-uniform market demand.

Disadvantages of Natural Seasoning -

• It needs larger space for seasoning.

• The process is very slow, usually takes 2-4 years.

• As the process depends on natural air, there is no control on it.

• Ends may dry fast and split. Interiors may remain moist while exteriors dry fast.

• The moisture content may not be brought down to the desired level.

• Chances of fungi and insect attack cannot be ruled out during the seasoning period.

2. Artificial Seasoning -

 The various methods of artificial seasoning are:

 (i) Boiling

 (ii) Kiln seasoning

 (iii) Chemical seasoning

 (iv) Electrical seasoning

(i) Seasoning by Boiling -

In this method timber is immersed in water and then water is boiled for 3-4 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.

 

(ii) 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.

(iii) 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.

(iv) 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 artificial seasoning -

• Drying is uniform, hence defect such as shrinkage, cracks and wrapping are minimum.

• Drying process is fast, hence economical.

• Moisture content can be reduced to the desired level.

• Chances of fungi and insects attack are low.

• Wood becomes more suitable for painting.

Disadvantages of artificial seasoning -

• It needs high investment.

• Skilled supervision is required.

• If demand is low and intermittent, it is uneconomical.

 

PRESERVATION OF TIMBER -

Preservation of timber is carried out to increase the life of timber. Preservation is done using different types of preservatives. Increasing life makes timber more durable and it can be used for longer periods. Preservation also helps the timber to get rid of insects and fungi etc. If preservation is not done, then wood will be diseased and damaged badly.

Properties of Good Preservative for Timber -

·       The preservative used to protect the timber should contain following requirements or properties.

·       It should be effortlessly and cheaply available.

·       It should not contain any harmful substances, gases etc.

·       It should cover larger area with small quantity. Hence, it should be economical.

·       Decorative treatment or any surface treatment should be allowed on timber after the application of preservative.

·       Strength of timber should not be affected by the preservative.

·       It should not contain any unpleasant smell.

·       It should not get affected by light, heat, water etc.

·       It should not get affected by fungi, insects etc. and should also efficient to kill them.

·       It should not generate flame when contacts with fire.

·       It should not corrode metals when it makes a contact with them.

·       The depth of penetration of preservative in wood fibers should be minimum 6mm to 25mm.

 

Different Types of Preservatives for Timber -

·       Coal tar

·       ASCU

·       Chemical slats

·       Oil paints

·       Solignum paints

·       Creosote oil

Coal Tar for Preservation of Timber -

Coal tar is heated and obtained liquid hot tar is applied on timber surface using brush. Coal tar contains unpleasant smell and does not allow paint on it. So, it is used for door frames, window frames etc. It is very cheap and has good fire resistance.

ASCU Preservative for Timber -

ASCU is a special preservative which is available in powder form. It is dissolved in water to get preservative solution. It should be added 6 parts by weight of ASCU in 100 parts by weight of water. The final solution is applied on timber by spraying. This solution does not contain any odor. It is useful mainly to get rid of from white ants. ASCU contains hydrated arsenic pent oxide, copper sulphate or blue vitriol and sodium dichromate or potassium dichromate in it. After applying ASCU, the timber can be coated with paint, varnished etc.

Chemical Slats for Preservation of Timber -

Chemical salts like copper sulphate, mercury chloride and zinc chloride are used as preservative which can be dissolved in water to get liquid solution. They are odorless and do not generate flames when contact with fire.

Oil Paints Preservatives for Timber -

Oil paints are suitable for well-seasoned wood. They are generally applied in 2 or 3 coats. Oil paints prevents timber from moisture. If timber is not seasoned, then oil paints may lead to decay of timber by confining sap.

Solignum Paints for Preservation of Timber -

Solignum paints are applied in hot condition using brush. They are well suitable for preserving timber from white ants. Solignum paints can be used by adding color pigments so, the timber has good appearance.

Creosote Oil for Preservation of Timber -

Creosote oil is prepared by the distillation of tar. It is black or brown in color. It contains unpleasant smell. It is applied in a special manner. Firstly, the timber is well seasoned and dried. Then, it is placed in airtight chamber and inside air is pumped out. Finally creosote oil is pumped into the chamber with high pressure about 0.7 to 1 N/mm2 at a temperature of 50oC.

After allowing it for 2 hours, the timber absorbs creosote oil sufficiently and taken out from the chamber. Creosote oil is flammable so, it is not used for timber works in fireplaces. It is generally used for wood piles, poles, railway sleepers etc.

Methods of Timber Preservation -

·       Brushing

·       Spraying

·       Injecting under pressure

·       Dipping and stepping

·       Charring

·       Hot and cold open tank treatment

Brushing of Timber Preservatives -

Brushings the simplest method of applying preservatives. For well-seasoned timber, oil type preservatives are applied with good quality brushes. For better results, the applied preservative should in hot condition. Multiple coats should be applied and certain time interval should be maintained between successive coats.

Spraying of Timber Preservatives -

Spraying is an effective technique than brushing. In this case, preservative solution is sprayed on to the surface using spray gun. It is time saving and quite effective.

Preservative Injecting Under Pressure -

The preservative is injected into the timber under high pressure conditions. Generally, creosote oil is applied in this manner which is already discussed above. It is costly treatment process and required special treatment plant.

Dipping and Stepping Method of Timber Preservation -

Dipping is another type of preserving in which, timber is dipped directly in the preservative solution. Hence, the solution penetrates the timber better than the case of brushing or spraying. In Some case, the stepping or wetting of timber with preservative solution is allowed for few days or weeks which is also quite effective process.