Following are the four steps of timber processing: 1. Felling of Trees 2. Seasoning of Timber 3. Conversion of Timber 4. Preservation of Timber.
Step # 1. Felling of Trees:
To get timber, the trees are knocked down or cut down or caused to fall to the ground. This is known as the felling of trees. The important facts to be remembered in connection with felling of trees are as follows:
(i) Age of Trees for Felling:
The trees should be felled when they have just matured or when they are very near to maturity. If they are felled before they have attained maturity, the sap wood would be in excess and timber obtained from such trees would not be durable and it will be in less quantity. On the other hand, it is also not desirable to fell trees after they have fully matured as heart wood starts decaying after maturity. The age of good trees for felling varies from 50 to 100 years.
(ii) Method of Felling:
The trees should be felled by experienced persons. The tree should be cut from a place a little above its roots and very near to the ground level. Such a practice would help in getting more timber from the trunk of tree.
The process of felling of timber is as follows:
(a) A cut is made at the lowest possible point of the trunk and it is extended beyond the centre of gravity of cross- section of tree as shown on cut on left hand side of fig. 9-2. This cut should be made on the side opposite to that on which it is desired to fell the tree.
(ii) Another parallel cut is made above the first cut in the opposite direction as shown in fig. 9-2.
(iii) The top of tree is then tied with ropes on all the four diametrically opposite sides.
(iv) The rope on the side the tree is to be felled is pulled and at the same time, the rope on the opposite side is slowly loosened.
(v) By suitable swinging the tree, the trunk of tree will break at the level of cuts and then it is slowly allowed to fall gently on the ground. The labourers cutting the tree should shift away when the tree falls to avoid any accident.
(vi) The branches are chopped off and the log is cut to the required sizes. Its bark is removed and the log is sawn as early as possible. The timber should be protected from rapid drying and possible attack from insects.
The various appliances required in the process of felling of trees include axes, ropes, saws, wedges, wire cables, etc. A wedge is used to relieve the blade of the saw and at the same time, it is made use of to ascertain the reaction of the tree when it is struck.
The number and size of wedges will be decided by the lean or inclination of tree, distribution of weight and thickness of tree. The wedges should be driven gradually to cause the smooth felling of tree.
(iii) Season for Felling:
The trees should be felled when sap is at rest. The season for felling of trees should be carefully determined by keeping in mind the climatic conditions of the locality and types of trees. In autumn and spring, the sap is in vigorous motion and hence the felling of trees in these seasons should be avoided.
For hilly areas, the mid-summer would be the proper season for felling as there is heavy rainfall in winter. For plain areas, the mid-winter would be the proper season for felling as in summer, the water contained in sap would be easily evaporated and it will lead to the formation of cracks.
Step # 2. Seasoning of Timber:
Meaning of Seasoning:
When a tree is newly felled, it contains about 50 per cent or more of its own dry weight as water. This water is in the form of sap and moisture. The water is to be removed before the timber can be used for any engineering purpose. In other words, the timber is to be dried.
This process of drying of timber is known as the seasoning of timber and the moisture should be extracted during seasoning under controlled conditions as nearly as possible at a uniform rate from all parts of the timber.
It should also be seen during seasoning that the remaining moisture, which cannot be extracted, is uniformly distributed throughout the mass. If the drying is irregular, the shrinkage of timber will also be irregular and it will set up internal stresses between the fibres. When these stresses become excessive and are capable of overcoming the cohesion of fibres, the timber warps and the shakes are formed.
The wood is a hygroscopic material. The capacity of wood to absorb water vapours from air is called the hygroscopicity of wood. The dry wood absorbs the moisture from the surrounding air. Now, the air humidity is not constant and hence the wood moisture content also varies accordingly.
The fluctuations in wood moisture content from zero to the fibre saturation point cause corresponding volume changes in wood leading to cracking, warping, swelling and shrinkage of wood.
The wood attains a level of equilibrium moisture content under the given climatic conditions of temperature and relative humidity. By the process of seasoning, the excess water of timber is extracted in such a way that the moisture content of seasoned timber corresponds to the required moisture content in timber for the environments in which it is to be used.
The relationship between the climatic conditions and moisture content in timber has been established from tests on various types of timber. It is to be noted that the seasoned timber should be protected from exposure to the rain and excessively high humidity.
Free Moisture and Bound Moisture:
The moisture in timber can be present either in the cell cavities or in the cell walls. The former is known as the free moisture or free water and major part of moisture in timber is present as free water. The latter is known as the bound moisture and it is closely associated with the body of timber.
When timber containing moisture is exposed to the atmospheric conditions, it starts losing its moisture content. The free water is evaporated first and the point at which the cell cavities no longer contain free water is known as the fibre saturation point. After the fibre saturation point has been reached, the tendency of timber to shrink appears and it is more or less proportional to the loss in bound moisture.
Determination of Moisture Content:
The moisture content of timber is determined as follows –
Where, P = Percentage of moisture
W1 = Original weight of timber
W2 = Oven-dry weight of timber.
The samples of timber are taken in the form of pieces having dimensions of 50 mm X 50 mm X 25 mm. The test piece is taken and weighed fresh. It is then dried in an oven at a temperature of 103°C ± 2°C. The weight of test piece in the oven is regularly observed till the variation in last two consecutive observations does not exceed 0.002 gm. The test piece is then considered to be oven-dry and its weight is noted.
The oven-dry method of determining moisture content is a standard method. But for field observations, the electronic instruments are available for readily working out the moisture content of timber.
Objects of Seasoning:
The seasoning of timber is carried out to achieve the following objects:
(i) To allow timber to burn readily, if used as fuel.
(ii) To decrease the weight of timber and thereby to lower the cost of transport and handling.
(iii) To impart hardness, stiffness, strength and better electrical resistance to timber.
(iv) To increase the resisting power of timber, as most of the causes of decay of timber are more or less related to the moisture.
(v) To maintain the shape and size of the components of the timber articles which are expected to remain unchanged in form.
(vi) To make timber easily workable and to facilitate operations during conversion.
(vii) To make timber fit for receiving treatment of paints, preservatives, varnishes, etc.
(viii) To make timber safe from the attack of fungi and insects.
(ix) To make timber suitable for gluing i.e. effectively joining two members of timber with the aid of glue.
(x) To reduce the tendency of timber to crack, shrink and warp.
Methods of Seasoning:
The methods of seasoning can broadly be divided into the following two categories:
i. Natural seasoning
ii. Artificial seasoning.
i. Natural Seasoning:
In this method, the 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 the air seasoning:
(a) The timber in log form is not usually fit for the process of seasoning. Hence it is cut and sawn into suitable sections of planks or scantlings.
(b) The timber pieces can either be stacked horizontally or vertically, the former arrangement being very common. Fig. 9-3 shows a typical horizontal stack for air seasoning.
(c) The ground, where stack is to be constructed, is cleared and it is levelled for good drainage.
(d) The platform of stack is made slightly higher, about 300 mm, than the ground level. For this purpose, the 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.
(e) The timber pieces are sorted out according to lengths and thicknesses. They are then arranged in layers, one above the other. The care should be taken to see that all members in a particular layer are of the same thickness. If this precaution is not taken, there are chances for timber to become warped or cracked.
(f) Each layer is separated by spacers of sound dry wood. The usual dimensions of spacers vary from 35 mm x 25 mm to 50 mm x 35 mm, the larger dimension being the width. The spacers are to be carefully placed in correct vertical alignment.
(g) The distance between spacers depends on the sizes of timber members to be seasoned. It is less for thin sections and more for thick sections. It usually varies from 450 mm to 600 mm.
(h) The length of stack is equal to length of timber pieces. The width and height of stack are restricted to about 1500 mm and 3000 mm respectively. A distance of about 25 mm is kept between adjacent layers.
(i) 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.
(j) Similar stacks may be constructed. The minimum distance between adjacent stacks should be at least 600 mm.
Following are the advantages of the natural seasoning:
(i) Depending upon the climatic conditions, the moisture content of wood can be brought down to about 10 to 20 per cent.
(ii) It does not require skilled supervision.
(iii) 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.
(iv) This method of seasoning timber is cheap and simple.
Following are the disadvantages of the natural seasoning:
(i) As the process depends on the natural air, it sometimes becomes difficult to control it.
(ii) The drying of different surfaces may not be even and uniform.
(iii) 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 portions,
(iv) If not properly attended, the fungi and insects may attack timber during the process of seasoning and may thereby damage it.
(v) The moisture content of wood may not be brought down to the desired level.
(vi) The space required for this process will be more as timber will have to be stacked or stored for a sufficiently long time.
(vii) 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.
ii. Artificial Seasoning:
Following are the reasons for adopting the artificial seasoning to the natural seasoning:
(a) The defects such as shrinkage, cracking and warping are minimized.
(b) The drying is controlled and there are practically no chances for the attack of fungi and insects.
(c) The drying of different surfaces is even and uniform.
(d) It considerably reduces the period of seasoning.
(e) There is better control of circulation of air, humidity and temperature.
(f) The wood becomes more suitable for painting, gluing, etc.
(g) The wood with desired moisture content may be obtained by the artificial seasoning.
The various methods of artificial seasoning are as follows:
(b) Chemical seasoning
(c) Electrical seasoning
(d) Kiln seasoning
(e) Water seasoning.
Each of these methods of artificial seasoning will be now briefly discussed.
In this method of artificial seasoning, the timber is immersed in water and water is then boiled. This is a very quick method. The timber is thus boiled with water for about three to four hours. It is then dried very slowly under a shed. The periods of seasoning and shrinkage are reduced by this method, but it affects the elasticity and strength of wood. In place of boiling water, the timber may be exposed to the action of hot steam. This method of seasoning proves to be costly.
(b) Chemical Seasoning:
This is also known as the salt seasoning. In this method, the 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.
(c) Electrical Seasoning:
In this method, the use is made of high frequency alternating currents. The timber, when it is green, offers less resistance to the flow of electric current. The resistance increases as the wood dries internally which also results in the production of heat. This is the most rapid method of seasoning. But the initial and maintenance costs are so high that it becomes uneconomical to season timber on commercial base by this method.
(d) Kiln Seasoning:
In this method, the drying of timber is carried out inside an airtight chamber or oven.
The process of seasoning is as follows:
(i) The timber is arranged inside the chamber such that spaces are left for free circulation of air.
(ii) The air which is fully saturated with moisture and which is heated to a temperature of about 35°C to 38°C is then forced inside the chamber by suitable arrangement.
(iii) This forced air is allowed to circulate round the timber pieces. As air is fully saturated with moisture, the evaporation from the surfaces of timber pieces is prevented. The heat gradually reaches inside the timber pieces.
(iv) The relative humidity is now gradually reduced.
(v) The temperature is then raised and maintained till the desired degree of moisture content is attained.
Depending upon the mode of construction and operation, the kilns are of two types, namely, stationary kilns and progressive kilns. A stationary kiln is also known as a compartment kiln and in this kiln, the process of seasoning is carried out in a single compartment only.
The 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. In a progressive kiln, the carriage with timber sections travels slowly from one end of kiln to the other and in doing so, 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.
It is used for seasoning timber on a large scale. If not properly attended, the drying in this kiln may prove to be unsatisfactory. The kiln seasoning, though costly, gives well-seasoned timber as it controls three important conditions, namely, circulating air, relative humidity and temperature.
(e) Water Seasoning:
In this method, the following procedure is adopted:
(i) The timber is cut into pieces of suitable sizes.
(ii) These pieces are immersed wholly in water, preferably in running water of a stream. The care should be taken to see that the timber is not partly immersed.
(iii) The thicker or larger end of timber is kept pointing on the upstream side.
(iv) The timber is taken out after a period of about 2 to 4 weeks. During this period, the sap contained in timber is washed away by water.
(v) The timber is then taken out of water and allowed to dry under a shed having free circulation of air. The water that has replaced sap from the timber dries out and the timber is seasoned.
The water seasoning is a quick method and it renders timber which is less liable to shrink or warp. It also removes organic materials contained in sap of timber. It however weakens the timber and makes it brittle.
Comparison between Natural Seasoning and Artificial Seasoning:
1. Moisture Content:
It is difficult to reduce the moisture content below 15 to 18%.
It is simple and economical
3. Quality of Timber:
The air seasoned timber is more liable to the attacks of insects and fungi.
It requires more space for stacking.
It is a slow process.
It gives stronger timber.
1. Moisture Content:
The moisture content can be reduce to any desired level.
It is expensive and quite technical
3. Quality of Timber:
The kiln seasoned timber is less liable to the attacks of insects and fungi.
It requires less space for stacking
It is a quick process.
It gives a little weaker timber.
Classification of Timbers with Respect to Seasoning:
Depending upon the ease with which Indian timbers can be seasoned, they are divided into three groups, namely, non-refractory timbers, moderately refractory timbers and highly refractory timbers.
The non-refractory timbers can be rapidly seasoned without any trouble. They can be seasoned even in the open air and sun. The examples are deodar, simul, etc.
The moderately refractory timbers have tendency to split and to crack during seasoning. They are therefore to be protected against rapid drying conditions. The examples are mango, rosewood, sissoo, teak, etc.
The highly refractory timbers are likely to be damaged severely during seasoning. They are difficult to season. The examples are axle wood, hopea, laurel, sal, etc.
The cost of seasoning of timber will naturally depend on the thickness of timber and type of timber with respect to seasoning. It will be more for highly refractory timbers and less for non-refractory timbers.
The period or time for seasoning of timber will also vary with the thickness of timber and type of timber with respect to seasoning. Table 9-3 shows the time required for seasoning of different timbers with different thicknesses.
Step # 3. Conversion of Timber:
The process by which timber is cut and sawn into suitable sections is known as the conversion. For this purpose, the power machines may be employed at different stages of process.
Following important facts in connection with the conversion of timber are to be remembered:
(i) The conversion is a skilled art and it should be carried out in such a way that there is minimum wastage of useful timber.
(ii) The allowance should be made for shrinkage, squaring and planning. It is about 3 mm to 6 mm.
(iii) The wooden beams should be sawn in such a way that they do not contain pith in their cross-section. To achieve this, the timber is first sawn through pith into two halves.
(iv) To obtain strong timber pieces, the saw cuts should be made tangential to the annual rings and practically parallel to the direction of medullary rays.
(v) The conversion may be achieved either by ordinary sawing, quarter sawing, tangential sawing or radial sawing as shown in fig. 9-4 to fig 9-7.
Fig. 9-4 shows ordinary sawing or bastard sawing or flat sawing or slab sawing. The saw cuts are tangential to the annual rings and right through the cross-section of timber piece. The log is moved forward and backward on the platform of a sawing mill. Thus the parallel cuts are made throughout the length of the log and parallel slices of planks are obtained.
This is a very easy and quick method of sawing and it is widely adopted in our country. It is also most economical method and wastage of useful timber is minimum. However the planks obtained by this method are liable to warp and twist as a result of unequal shrinkage. The outer portion is of sap wood and it shrinks more.
The central portion is of heart wood and it shrinks less. Thus, the thickness at the centre remains almost unaltered while the circumferential shrinkage causes warping and twisting of planks. Fig. 9-5 shows quarter sawing.
The saw cuts are at right angles to each other. It may produce fine figure wood when adopted in case of timber having no distinct medullary rays. In this method, there is a tendency for the timber to bend in a transverse direction.
Fig. 9-6 shows tangential sawing or plain sawing or flat grained sawing. The saw cuts are tangential to the annual rings and they meet each other at right angles. This method is adopted when the annual rings are very distinct and the medullary rays are not clearly defined.
The planks obtained by this sawing warp too much because the sections are weak as the medullary rays which impart strength to the longitudinal fibres are cut. The timber obtained by this method is unsuitable for flooring and it cannot be polished evenly.
Fig. 9-7 shows radial sawing or rift sawing. The saw cuts are made radially in a parallel direction to the medullary rays. This method is used for conversion of hard timber. It gives wood with decorative effect.
The timber obtained by this method shrinks and warps to a less degree and it is distorted to the minimum. However the wastage is maximum and the cost of sawing proves to be high as more time and labour are required in turning wood several times.
(vi) During conversion, there will be loss of timber due to sawing and it may extend upto 40 per cent in reduction of weight of log.
The probable loss in conversion of timber from log to the required size of timber may be taken as follows:
Round log to plan – 40%
Round log to scantling – 50%
Square log to plan – 30%
Square log to scantling – 40%
Step # 4. Preservation of Timber:
Objects of Preservation of Timber:
The preservation of timber is carried out to achieve the following three objects:
(i) To increase the life of timber structures,
(ii) To make the timber structures durable, and
(iii) To protect the timber structures from the attack of destroying agencies such as fungi, insects, etc.
Requirements of a Good Preservative:
Following are the requirements of a good preservative:
(i) It should allow decorative treatment on timber after being applied over timber surface.
(ii) It should be capable of covering a large area with small quantity.
(iii) It should be cheap and easily available.
(iv) It should be durable and should not be affected by light, heat, etc.
(v) It should be free from unpleasant smell.
(vi) It should be non-inflammable.
(vii) It should be quite efficient in killing fungi, insects, etc.
(viii) It should be safe and harmless for persons and animals.
(ix) It should give pleasant appearance to the timber after being applied over it.
(x) It should not affect the strength characteristics of timber.
(xi) It should not be easily washed away by water.
(xii) It should not corrode the metals with which it comes into contact.
(xiii) It should offer high resistance to the moisture and dampness.
(xiv) Its penetrating power into wood fibres should be high. Is. is necessary for the preservative to be effective to penetrate at least for a depth of 6 mm to 25 mm.
Types of Preservatives:
Following preservatives are commonly used for the preservation of timber:
(i) Ascu treatment
(ii) Chemical salts
(iii) Coal tar
(iv) Creosote oil
(v) Oil paints
(vi) Solignum paints.
(i) Ascu Treatment:
The ascu is special preservative which is developed at the Forest Research Institute, Dehradun.
Its composition is as follows:
(a) Part by weight of hydrated arsenic pentoxide, (As2O5, 2H2O)
(b) Parts by weight of blue vitriol or copper sulphate, (CuSO4, 5H2O)
(c) Parts by weight of potassium dichromate, (K2Cr2O7) or sodium dichromate (Na2Cr2O7, 2H2O).
This material is available in powder form. To prepare a solution of this material, six parts by weight of ascu are mixed in 100 parts by weight of water. The solution is then sprayed or applied on timber surface. This preservative gives timber protection against the attack of white ants. The surface treated with this preservative can be painted, polished, varnished or waxed. The solution is odourless.
The other compositions of water soluble preservatives on this line are mentioned in table 9-4.
(ii) Chemical Salts:
These are water-borne preservatives and they are mostly salts dissolved in water. The usual salts used are copper sulphate, mercury chloride, sodium fluoride and zinc chloride
The solutions are prepared from these salts and they are applied on the timber surface. These preservatives are odourless and non-inflammable. The treated surface can be painted or varnished after drying. These preservatives have good penetration and the timbers treated with these preservatives will show an immediate increase in weight of 2400 to 4800 N per m3. After drying, the net increase in weight will come down to about 50 to 300 N per m3.
(iii) Coal Tar:
The timber surface is coated with hot coal tar with the help of brush. The coal tar becomes workable when heated. The process is known as the tarring. The coal tar has unpleasant smell and appearance. It makes timber unsuitable for painting. Hence the tarring is adopted for frames of doors and windows, rough timber work, etc. and it is found to be most useful for parts embedded in ground because of its cheapness and effective resistance. The coal tar is fire-resistant.
(iv) Creosote Oil:
In this case, the timber surface is coated with creosote oil. The process is known as the creosoting or Bethel’s method of preservation of timber. The creosote oil is obtained by the distillation of tar.
The creosoting is carried out as follows:
(a) The timber is thoroughly seasoned and dried.
(b) It is then placed in an air tight chamber.
(c) The air is pumped out from the chamber.
(d) The creosote oil is then pumped under a high pressure of about 0.70 to 1 N/mm2 and a temperature of about 50°C.
(e) After a period of about 1 to 2 hours, when timber has sufficiently absorbed creosote oil, it is taken out of chamber.
The creosote oil is one of the best antiseptic i.e., substance poisonous for wood-attacking fungi. It is a black or brown liquid, weakly affected by water, neither volatile nor hygroscopic, harmless to wood or metal, inflammable, with an unpleasant odour and having low wood-penetrating ability to the extent of 1 mm to 2 mm only.
The creosoting practically doubles the life of timber and it is generally adopted for piles, poles, railway sleepers, etc. Depending upon the net retention and type of timber, the creosote treated timber will normally increase in weight by 800 to 3200 N per m3. The creosote oil is highly toxic in nature and gives out highly unpleasant smell.
The process of creosoting proves to be costly. The creosote oil should not be used for interior surfaces of dwelling houses, foodstuff-storage premises, in underground installations and near inflammable surfaces.
(v) Oil Paints:
The timber surface is coated with 2 or 3 coats of oil paint. The wood should be seasoned. Otherwise sap will be confined and it will lead to the decay of timber. The oil paints preserve timber from moisture and make it durable.
(vi) Solignum Paints:
These paints preserve timber from white ants as they are highly toxic in nature. They can be mixed with colour pigments and applied in hot state with the help of brush. The timber surface may therefore be given the desired colour or appearance.
Methods for Preservation of Timber:
Following are the six methods adopted for preservation of timber:
(iii) Dipping and steeping
(iv) Hot and cold open tank treatment
(v) Injecting under pressure
The solution prepared from preservative is applied on timber surface by good quality of brushes. This is the simplest method and it is generally adopted for seasoned timber. The cracks should be filled up before the application of preservative.
For better penetration, the oil type preservatives may be applied hot and the preservative should be liberally used on the surface. Several coats of preservatives may be applied and enough interval of time should be kept between successive coats for absorption of preservative.
This method of charring is rather very old and as such, no preservative is used in this method. The surface to be charred is kept wet for about half an hour and it is then burnt upto a depth of about 15 mm over a wood fire. The charred portion is then cooled with water. Due to burning, a layer of coal is formed on the surface. This layer is not affected by moisture and it is not attacked by white ants, fungi, etc.
The disadvantages of this method are:
(a) The charred surface becomes black in appearance and hence it cannot be used for exterior work.
(b) There is some loss of strength of timber as the cross-section is reduced due to charring.
The process of charring is generally adopted for lower ends of posts for fencing, telephone, etc. which are to be embedded in the ground or to be inserted in moist soil.
(iii) Dipping and Steeping:
In this method, the timber to be given preservative treatment is dipped or soaked for a short period in the solution of preservative. This method gives slightly better penetration of preservative than in case of brushing or spraying. Instead of dipping, the steeping or wetting of timber with preservative may be carried out for periods varying from a few hours to days or weeks. The depth of penetration of preservative depends on the type of timber.
(iv) Hot and Cold Open Tank Treatment:
In this method, the timber is submerged in a tank containing solution of preservative which is heated for a few hours at temperature of 85°C to 95°C. The tank is then allowed to cool down gradually while the timber is still submerged in the tank. This method is effective in giving protection to the sap wood.
(v) Injecting under Pressure:
In this method, the preservative is injected under pressure into the timber. This method is usually adopted in creosoting. This is the most effective method of treating timber with the preservative. But it requires special treatment plant. This method proves to be essential for treating non-durable timbers which are to be used at places where there is danger of attack by fungi and insects.
In this method, the solution of preservative is filled in a spraying pistol and it is then applied on timber surface under pressure. The pistol works under compressed air. This method is also quite effective and it is superior to brushing.