In this article learn seasoning of timber,air seasoning or natural seasoning,kiln seasoning or artificial seasoning,object of seasoning,methods of stacking, seasoning methods, Preventing drying of logs,comparison of air seasoning and kiln seasoning.
SEASONING OF TIMBER
Newly felled tree contains a considerable quantity of sap. If this sap is not removed the timber is likely to warp, crack and shrink. It may even decay. The art of seasoning of timber is to extract the moisture under controlled conditions as nearly as possible at a uniform rate from all parts of timber and to leave the remaining moisture, that cannot be extracted, uniformly distributed throughout the mass. Irregular drying will cause irregular shrinkage resulting in the setting up of internal stresses between the fibres. When these stresses become strong enough to overcome the cohesion of the fibres then the timber warps and shakes are formed.
Objects of seasoning.
(i) Wet timber is an easy prey to decay by fungi, borers, termites etc. Seasoning thus makes timber resistant to decay.
(ii) Seasoning makes timber lighter.
(iii) It becomes easier to paint and polish seasoned timber.
(iv) It is easier to treat seasoned timber with preservatives.
(v) Seasoned timber becomes stronger and more stable.
(vi) Seasoning stops shrinkage of timber on drying.
(vii) Seasoned timber has better electrical resistance.
Preventing drying of logs.
After the tree has been felled its bark is removed and it is roughly squarred and sawn as quickly as possible to avoid cracking of timber and to expedite seasoning. There is possibility of faster evaporation of moisture from ends of timber piece and if not checked then it is likely to result in cracking and splitting of ends. To avoid this, painting ends of logs or planks with sealing liquid, tar or asphalt, paraffin wax, mixture of molasses and lime or of rosin and lamp black and shading them with canvass or plywood etc., is adopted.
If however the logs cannot immediately on felling, be converted into planks or scantlings then rapid drying of logs should be minimised by storing them completely submerged under water. If the water is stagnant then the same should be changed at least once fortnight so as to remove the fermenting material.
Methods of stacking.
The one and nine methods of stacking sleepers
Timber, before seasoning, should be stacked in yards free from weeds and debris. The yard should have big shady trees to protect the timber from direct sun.
Ends of logs should be protected against splitting by applying anti-splitting compositions and stacked on foundation in closed stacks in one or more layers. Stacks should be protected against direct sun by providing a covering – if needed.
The one and nine methods of stacking sleepers (Fig. 9.16.) is best suited for moderately heavy conferous sleepers in hot climate and for heavy timbers in moist climates.
Close Crib method
In the close Crib method (Fig. 9.17) reduced air circulation slows down the pace of seasoning.
This method is recommended for stacking heavy structural timbers like sal in hot and dry localities.
Open Crib method (Fig. 9.18.)
is a modification of the close crib method and because of more air circulation taking place it is more akin to the one and nine method in its effects. Stacks of not more than 100 sleepers are recommended. Poles are stacked either in closed heaps or with crossers. If stacked in closed heaps (Fig. 9.19.) then there should be alternate layers of butt ends and of top ends so that the two ends of the stack are level. Poles themselves could be used as crossers, which should not be spaced more than three metres.
Fence posts should be stacked in open crib fashion (Fig. 9.20) in which successive layers of posts are at right angles to each other and there is a gap of about 8 cm between adjacent posts in the same layer. Centre to centre distance between crossers should not exceed 1.5 m and the height of stack should not exceed 3 metres.
Horizontal stacking of sawn timber is done on vertical pillars of treated timber, brick masonry or of cement concrete 30 cm square in section and 30 to 45 cm high. The pillars are spaced 1.2 m centre to centre along the length and the breadth of the stack. The length of material to be stacked decides the length of stacking unit. Long beams of cross section 10 cm × 10 cm and above are placed on the foundation pillars to form a frame work for stacking timber. These beams should obviously be from strong timbers.
Seantling and squares should be stacked with crossers 5 cm × 4 cm in section and spaced 2.5 to 3 m apart. The ends should be protected with moisture proof coatings.
Planks should be stacked on level platform with crossers of uniform thickness and section, which (the crossers) should be in vertical alignment in a stack (Fig. 9.21.). Longer planks should form the bottom of the stack and the shorter one’s the top. Heavy wooden beams should be placed on the top to prevent top layers from warping. A gap of about 2.5 cm should be left between adjoining planks for free circulation of air in the centre of stack. The stack should be protected against rain and sun by providing a shed over it.
For details refer to IS : 1141-1973.
Based on the recommendations of IS : 1141-1973 the seasoning methods should be classified as : (a) Air seasoning or natural seasoning, and (b) Kiln seasoning or artificial seasoning.
(i) Air seasoning or natural seasoning.
As soon as possible after felling, the log is converted by sawing it into battens and planks etc. These are then stacked on a well drained place in the shade. While stacking care should be taken to ensure free circulation of fresh air all around each piece (Fig. 9.22). The stacking should be done on masonry or concrete supports a few centimeters above the ground.
Care should be taken not to expose the freshly converted timber stacked for seasoning to severe winds or to sun.
This process of seasoning timber is the best as it gives very strong and durable timber, but it is extremely slow. It takes more than six months for timber to season in moderate climates.
(ii) Kiln seasoning or artificial seasoning.
Artificial method of seasoning or kiln seasoning speeds up the seasoning process. For large scale production of seasoned timber kiln seasoning is a must.
Kiln seasoning is done in a chamber equipped with rrangements for heating and humidifying the air to required conditions of relative humidity and temperature and for its circulation across the timber stacked in the chamber for seasoning. Usually it is steam that it used for heating and humidifying the air in the kiln. The seasoning of the timber is started at a comparatively lower temperature and high humidity. As the timber dries these conditions are gradually altered until at the end of the seasoning the temperature of the air inside the chamber is fairly high and the humidity is low. The kiln charge is allowed to cool inside the kiln to within 15 to 20ºC of the outside temperature before removal. Seasoning of timber by this method takes about four to five days under normal conditions.
COMPARISON OF AIR SEASONING AND KILN SEASONING
Relative merits and demerits of the two methods of seasoning are as follows :
|Air seasoning||Kiln seasoning|
|1. It is a slow process.||1. It is a quick process.|
|2. It is simple and economical.||2. It is quite technical and expensive.|
|3. Air seasonied timber is more amenable to attacks of insects and funti.||3. Kiln seasoned timber is less amenable to attack of insects and fungi.|
|4. It requires more stacking space.||4. It requires less stacking space.|
|5. It gives stronger timber.||5. A little weaker timber is obtained.|