Fishing Craft Technology (1+1)

Unit 4 - Boat building materials and construction of boat

4.3. Seasoning of wood
Freshly felled timber contains 100% moisture in many cases though in some light timbers, it may even go up to 200%. Excessive moisture in wood renders it weaker. It is susceptible to fungal attack and it is prone to dimensional instability. The water is in the form of sap or moisture. Seasoning of timber is the means adopted to reduce the moisture content to an amount suitable for the purpose for which the timber is to be used.
The objects of seasoning are

• To increase its strength, durability, workability and resistance.
• To reduce the tendency of split, shrink and warp
• To reduce its weight and thus minimize the cost of transport and handling.
• To make it immune from the attack of insects and fungi
• To make it receptive to finish (Preservatives, paints, varnish and polish)
• To make it burn readily, if used as fuel
• To maintain the shape and size of the components of the timber articles which are expected to remain unchanged in form
• To reduce the tendency of timber to crack, shrink and warp.

Seasoning helps in increasing its strength and service ability and also improves timbers dimensional stability and project from attack by insects and fungi.

i) Methods of seasoning:

• Natural seasoning
• Artificial seasoning

1) Natural seasoning:
In this method, seasoning of timber is carried out by natural air and hence, it is also sometimes referred to as air seasoning. Following procedure is adopted in air seasoning.

• Timber is log form is not usually fit for the process of seasoning. Hence, it is cut and sawn into suitable sections of planks or scantlings.
• Timber pieces can either be stacked horizontally or vertically, the former arrangement being very common.
• The ground, where stack is to be constructed is cleared and it is leveled for good drainage.
• The platform of stack is made slightly higher about 30cm, than the ground level. For this purpose, rows of brick or concrete pillars are constructed. The pillars may also be made of creosoted wood or wood coated with coal tar. The tops of pillars should be in the same horizontal plane. The pillars should be durable.
• The timber pieces are sorted out according to lengths and thickness. They are then arranged in layers, one above the other. Care should be taken to see that all members in a particular layer are of the same the thickness. If this precaution is not taken, there are chances of timber to become warped or cracked.
• Each layer is separated by spacer of sound dry wood. The usual dimension of spacers varies from 35 mm x 25 mm to 50 mm x 35 mm. The larger dimension of the timber is the width. The spacers are to be carefully placed is correct vertical alignment.
• The distance between spacers depends on the sizes of the timber members to be seasoned. It is less for thin sections and more for thick sections. It usually varies from 45 to 60 cm.
• Length of stack is equal to length of timber pieces. Width and height of stack are restricted to about 150 cm and 300 cm respectively. A distance of about 25 mm is kept between adjacent layers.
• The stack is to be protected from fast blowing wind, rain and extreme heat of sun. Hence the stack should preferably be covered by a roof of suitable material.

Advantages:

• Depending upon the climatic conditions, the moisture content of wood can be brought down to about 10-20%.
• It does not require skilled supervision.
• It is uneconomical to provide artificial seasoning to timber sections thicker than 100 mm. As such sections dry very slowly. Hence such thicker timber sections are usually seasoned by the process of air seasoning.
• This method of seasoning timber is cheap and simple.

Disadvantages:

• As the process depends on natural air, it sometimes becomes difficult to control it.
• Drying of different surface may not be even and uniform.
• If ends of thick sections of timber are not protected by suitable moisture proof coating there are chances for end splitting because the ends of such timbers dry rapidly in comparison to the central portion.
• If not properly attended, fungi and insects may attack timber during the process of seasoning and may there by damage it.
• Moisture content of wood may not be brought down to the desired level.
• Space required for this purpose will be more as timber will have to be stacked or stored for a sufficiently long time.
• The process of seasoning is very slow and it usually takes about 2 to 4 years to make timber fit for the work of carpenter.

2) Artificial seasoning:
Following are the reasons for adopting artificial seasoning to natural seasoning.

• Defects such as shrinkage, cracking and warping are minimized.
• Drying is controlled and there are practically no chances for the attack of fungi and insects.
• Drying of different surfaces is even and uniform.
• It considerably reduces the period of seasoning.
• There is better control of circulation of air, humidity and temperature
• Wood becomes more suitable for painting, gluing etc.,
• Wood with desired moisture content may be obtained by artificial seasoning

Various methods of artificial seasonings are as follows

• Boiling
• Chemical seasoning
• Electrical seasoning
• Kiln seasoning
• Water seasoning

Boiling:
In this method timber is immersed in water and water is then boiled. This is very quick method. Timber is thus boiled with water for about three to four hours. It is then dried very slowly. The period of seasoning and shrinkage are reduced by this method, but it affects the elasticity and strength of wood. In place of boiling water, timber may be exposed to the action of hot steam. This method of seasoning proves to be costly.

Chemical seasoning:
This is also known as salt seasoning. In this method, timber is immersed in a solution of suitable salt. It is then taken out and seasoned in the ordinary way. The interior surface of timber dries in advance of exterior one and chances of formation of external cracks are reduced.

Electrical seasoning:
In this method, use is made of high frequency alternating currents. Timber, when it is green, offers less resistance for the flow of electric current. The resistance increases as the wood dries alternately which also results in the production of heat. This is the most rapid method of seasoning. But initial and maintenance costs are so high that is becomes uneconomical to season timber on commercial base by this method.

Kiln seasoning:
In this method, drying of timber is carried out inside an air tight chamber or oven. The process of seasoning is as follows.

• Timber is arranged inside the chamber such that spaces are left for free circulation of air.
• Air which is fully saturated with moisture and which is heated to a temperature of about 35 ⁰C to 38 ⁰C is then forced inside the chamber by suitable arrangement.
• This forced air is allowed to circulate round the timber pieces. As air is fully saturated with moisture, evaporation from the surface of timber pieces is prevented. The heat gradually reaches inside the timber pieces.
• The relative humidity is now gradually reduced.
• The temperature is then raised and maintained till the desired degree of moisture content is attained.

Depending upon the mode of construction and operation kilns are of two types namely stationary kilns and progressive kilns

Stationary kiln:
A stationary kiln is also known as compartment kiln and in this kiln process of seasoning is carried out in a single compartment only. Drying operations are adjusted as drying proceeds. This kiln is adopted for seasoning timber which requires a close control of humidity and temperature. It gives better results.

Progressive kiln:
In a progressive kiln, carriage with timber sections travel slowly from one end of the kiln to the other and in doing it gets seasoned. The hot air is supplied from the discharging end so that the temperature is less at the charging end and it increases towards the discharging end.

Water seasoning:

• The timber is cut into pieces of suitable size
• These pieces are immersed wholly in water, preferably in running water of a stream. Care should be taken to see that timber is not partly immersed.
• The thicker or larger end of timber is kept pointing on the upstream side
• Timber is taken out after a period of about 2 to 4 weeks. During this period, sap contained in timber is washed away by water.
• Timber is than taken out of water and allowed to dry in free air. Water that has replaced sap from timber dries out and timber is seasoned.