Characteristics Of Good Timber
Good timber should possess the following qualities :
1. Hardness. It should be hard.
2. Strength. It should be able to resist heavy loads in structural members.
3. Toughness. It should be tough enough to resist shocks due to vibrations. It should not break in bending and should resist splitting. Timbers with narrow annual rings, are generally the strongest.
4. Elasticity. It should be elastic so as to regain its original shape after removal of loads. This property is very important for the timber used in sports goods.
5. Durability. It should be able to resist the attacks of fungi and worms and also atmospheric effects for a longer duration.
6. Defects. It should be from the heart of a sound tree and be free from sap, dead knots, shakes and other similar defects.
7. Fibres and Structure. It should have straight and closed fibres and compact medullary rays. It should give a clear ringing sound when struck. Dull heavy sound is an indication of internal decay. Its annual rings should be uniform in shape and colour.
8. Appearance and colour. Freshly cut surface should give sweet smell and present shining surface. It should have preferably dark colour, as light coloured timbers are generally weak in strength.
9. Shape and weight. It should retain its shape during the process of seasoning. Heavy timbers are always stronger than light weight timbers.
10. Workability. It should be well seasoned and easily workable. Teeth of saw should not get clogged during the process of sawing. It should provide smoothened surface easily
1.9 Structure of a timber
The cross-section of the trunk of a timber tree may be distinctly divided into four parts.
1. Pith, heart or medulla. Inner most part or core of the stem, which consists entirely of cellular tissues, is called pith.
2. Medullary sheath. The portion consisting of vascular tissues and which encloses the pith, is called medullary sheath. Medullary rays are vertical layers of cellular tissues and spider-like radial lines originating from the pith to the bark. Medullary rays bind the annual rings to one another. Large and distinct radial lines are called silver grains or flowers.
3. Annual rings. These consist of cellular tissues and woody fibres arranged in distinct concentric circles round the pith. Annual rings are generally formed in every year, due to the deposition of sap below dark. Number of annual rings indicates the age of a tree in a tropical climate. Sap wood consists of outer annual rings. Heart wood consists of inner annual rings round the pith.
4. Bark or Cortex. It consists of cells of wood fibre and is the outermost cover or skin of the stem.
3. Annual rings
4. Pith or heart
5. Medullary sheath
6. Heart wood or duramen
7. Sap wood
8. Medullary rays.
Characteristic Differences of Sap Wood and Heart Wood
The main characteristic differences of sap wood and heart wood are as under : 1. Sap wood, (i) Sap wood is younger in age and lighter in colour
(ii) It is easily attacked by insects.
(iii) Its annual rings are far apart.
(iv) It possesses less strength.
2. Heart wood, (i) Heart wood is older in age and darker in colour.
(ii) It is hard core of the stem and is not attacked by insects.
(iii) Its annual rings are nearer to each other.
(iv) It possesses more strength.
Characteristic differences of Hard Wood and Soft Wood
1. Annual rings - Less distinct and nearer to each other
2. Medullary rays - Distinct
3. Colour - Darker
4. Heart wood and sap wood- Can be distinguished
5. Fibres - Strength of fibres same in all directions.
6. Sawing - Difficult
7. Resinuneous material - Does not exist
8. Examples - Teak, mahagony, Sal, etc.
1. Annual rings - Clearly visible and far apart
2. Medullary rays - Indistinct
3. Colour - Lighter
4. Heart wood and sap wood- Can not be distinguished
5. Fibres - Straight and possess high tensile strength
6. Sawing - Easy
7. Resinuneous material - Exists in pores
8. Examples - Chir, and other coniferous trees
Preservation of Timber
A properly seasoned timber is most durable. If it is not seasoned properly, it is likely to be attacked by insects i.e. white ants, dry and wet rots. Timber should be used either fully dried in well ventilated positions or well immersed in water. In water the timber does not decay though it becomes soft and weak. In case timber is not seasoned before it is used, it should be preserved by the application of
preservatives. In masonry construction, the timber should not be used in direct contact with lime mortar.
Preservation of timber may be done by one of the following methods :
1. Charring. Lower ends of the timber posts before embedding in ground are generally charred to a depth of 15 cm and quenched in water, to prevent attack from dry rots and worms.
2.Tarring. Embedded portion of timber fence posts, ends of door and window frames, bettons and beams built in walls are usually tarred.
3. Painting. Painting the surface of timber members, protects it from moisture and thus prolongs its life. Paints possess excellent preservative properties and protect the timber against the attack of white ants. Paints are available in varieties of shade with different trade marks.
4. Creosoting. Creosote oil is a dark brown thick liquid. By applying creosotes to timber, chances of attacks of white ants and rots are reduced considerably. In this method well seasoned timber dried for 24 hours, is kept in air tight chamber and air is exhausted. Creosote is then pumped in at a pressure of 9 kg/cm² at a temperature of 50°C till it is fully saturated with oil. Creosoting is done for
railway sleepers, piles and transmission poles.
5. Wolmen Salt. A timber treated with wolmen salt which consists of creosote and sodium fluoride dissolved in water, is extremely fire resistant and free from fungi attacks. Zinc chloride, sodium fluoride, magnesium silco-fluoride and copper sulphate when applied to timber also help it from the attacks of fungi. On drying, such timbers are suitable for painting.
6. Ascu. A timber treated with Ascu powder developed by F.R.I. (Forest Research Institute) Dehra Dun is immune to the attacks of white ants and may be painted, varnished and polished.
7. Fire proofing of timber. A timber to some extent, may be made fire proof by soaking it in ammonium sulphate, ammonium chloride, ammonia phosphate, sodium arsenate or zinc chloride.
Defects in Timber
The following are the most common defects in timber.
1. Heart Shakes. [Fig. 1.2 (a)]. These are splits occurring in the centre of the tree and running from the pith towards the sapwood in the direction of medullary rays. In some timbers, these splits are hardly visible and in some timbers these are quite permanent. Heart shakes are caused due to shrinkage of the interior parts due to age. A heart shake straight across the trunk, is not a serious defect.
2. Star Shakes [Fig. 1.2. (6)1. These are splits which radiate either from the centre of timber or from the bark, running in the planes of medullary rays. Star shakes are mostly confined to sapwood and are caused due to severe frost and scorching heat of the sun.
3. Cup Shakes. [Fig. 1.2. (c)J. These are curved splits which separate the whole or part of one annual ring from another. These are caused due to unequal growth of timber.
4. Radial Shakes [Fig. 1.2 (d)]. These are similar to star shakes and occur in felled timbers when exposed to sun during seasoning. Radial shakes are generally irregular, fine and numerous. Many splits appear to start a few centimeters within the bark, run a short distance towards the centre, then following the course of an annual ring, approach the centre radially.
5. Rind-galls [Fig. 1.2. (e)]. These are typical enlarged swellings caused generally by the growth of layers over the wounds left after the branches have been cut off.
6. Rupture. These are caused due to fibres having been injured by crushing.
7. Twisted Fibres. The twisting of fibres is caused due to the action of prevalent wind twisting the young tree constantly in one direction.
8. Wind cracks. These are shakes or splits on the sides of a bark of timber due to the shrinkage of the exterior surface exposed to atmospheric influences.
9. Knots. These are the roots of small branches of the tree. These break the continuity of fibres. These are not much harmful if small, hard and rounds. Timber with large dead (loose) knots of many smaller ones, should be rejected as these do not provide specified strength.
10. Dead wood. It is deficient in strength and weight and is the result of trees being felled after maturity.
Seasoning of Timber And Its Necessity
Definition. The process of drying timber or removing moisture or sap from a freshly felled tree, is called seasoning of timber.
A well seasoned timber may contain about 10 to 12 per cent moisture which is necessary for proper retention of the shape and size of the articles manufactured from the timber. On the other hand if a timber is not properly seasoned before use, it is liable to shrink warp, crack, rot and decay. This is why properly seasoned timber need only be used for high class timber work.
Necessity of seasoning a timber. Seasoning of timber is done for the following purposes:
1. To reduce the weight of the timber for achieving economy in its transportation from the place of felling to the place of manufacturing the articles.
2. To minimise the tendency to shrink, split and warp in the manufactured wood work.
3. To increase the strength and durability of the timber and also to make the timber electrically resistant.
4. To improve the wood working qualities in timber for gluing, painting and polishing the surfaces of finished articles.
5. To enable to provide proper preservation treatment of the timber,
6. To make the timer free from the danger of being attacked by insects, fungus, etc.
7. To achieve good characteristics in timber,
Methods of Seasoning Timber
According to IS : 1141-1973, the following are the methods of seasoning the timber :
1. Natural or air seasoning ; 2. Artificial or kiln seasoning.
1. Natural or air seasoning.
Natural seasoning (dry natural seasoning)- Immediately after the tree is felled (cut), the bark of the tree is removed and the timber is cut into square sections called boulks'. The sections are stacked one over the other in such a way that there is sufficient gap left between the sections for air to pass through. The stack is covered with a roof and the timber is left to dry. This is known as dry natural seasoning. It takes more than 6 months for the timber to get seasoned.
To speed the process of seasoning, the logs of timber are left in the river to float down the river to the saw mill. The water passes.through the porous part of the logs and removing the sap. Afterwards the logs are cut into sections as described in dry natural seasoning and allowed to dry.
This method of seasoning is the best as it gives very strong and
durable timber but it takes longer time. It generally takes more than six months for timber to season to moderate climates. Timber seasoned by natural seasoning method, generally contains 18% of moisture.
2. Artificial or kiln seasoning.
1. This is done in Kilns. Time required is between 7 and 15 days
2. The timber is cut into sections and stacked leaving space between the sections for passage of air. The stacks are put on trucks (trollies) which run on rails and move through the kiln
3. Hot air generated by passing air over pipes containing steam is circulated by fans
4. The temperature and moisture is strictly controlled
Kiln seasoning is a quick method of seasoning timber to the desired moisture contents.
Advantages of kiln seasoning
1. Perfect control of drying
2. Economy of time
3. Moisture content may be reduced to desired level
4. Unlikely to be attacked by fungi and insects.
5. Wood receives paints well.
Disadvantages of kiln seasoning
1. It requires skilled supervision
2. Expensive in cost
3. Due to carelessness, the wood develops surface cracks,
warping and splitting
Constituent Parts of Paint and their Functions
The constituent parts of paints are the following :
1. Base. It is very finely grounded metallic oxide. It acts as a body of paint. Because of film of base, the paint becomes hard and resistive to weathering friction. The most commonly used bases in paints are : White lead, Lead sulphate, Sublimed lead, Red lead, Zinc oxide, and Titanium oxide.
2. Vehicle, The material used in paints to help it to spread the base over the surface is called vehicle. It acts as a binder between base and pigment and causes it to adhere to the surface to be painted. Vehicle is mixed with the bases to form a paste. Most commonly used vehicle, is Linseed Oil of the following four types i.e.
(i) Row Linseed Oil, (ii) Refined Linseed Oil. (iii) Pale boiled Linseed Oil. (iv) Wouble boiled Linseed Oil.
3. Colouring Pigments. The materials added to the paints to obtain desired final colour, is called colouring pigments. These are used to obtain the final colour of the paint different from that of the base. Depending upon the final colour of paints, the colouring pigments may be used. Such as lamp black, bone black, graphite, Indian red, chrome yellow, etc.
4. Thinner. The material used in paints to reduce its consistency, is called thinner. It enables the paint to be spread over the surface to be painted with the brush and to penetrate into the surface. Most commonly used thinner is turpentine oil which dries rapidly and helps to dry paint soon, Naptha and spirit are also sometimes used as thinner.
5. Drier. The material used in paints, to accelerate the action of drying is called drier. Paints need be dried soon to avoid the risk to catch dust and dirt. Most commonly used drier is Litharge whose use in finishing coat should be avoided, otherwise colour of paint may change due to change in atmospheric conditions.
6. Adulterants. The material which is used to reduce the cost of paint and also to reduce the weight and to increase its durability, is called Adulterant. Barium sulphate is widely used as adulterant because of its cheapness and its property not to react with paint. Calcium Carbonate, Magnesium Silicate and Silica are also used as adulterants.
Manufacture of Cement by Wet Process
For the manufacture of cement, following ingredients are required.
1. Lime stone 2. Clay 3. Coal 4. Gypsum.
Process. The cement is prepared by mixing 75% of limestone and 25% clay. Hard lime stone is powdered in a crusher. Clay is thoroughly mixed with water in a wash mill. Powdered lime stone and clay solution are then mixed and ground in a wet grinding mill to form a slurry having a moisture 32 to 40%, The slurry is stirred in a collecting basin, and is tested for its chemical composition as described below.
(i) Take the slurry in tube and mix HC1 to it.
(ii) The mixture is heated till precipitation occurs.
(iii) The mixture is cooled to obtain a jelly like material.
if the jelly so formed is hard, it indicates that the proportion of the mix is not correct. Either clay or limestone is then added as required in collecting basin itself. This test is very important and must be carried out by an expert, because if proportion is not correct, the properties of resulting cement will adversely change. From the correcting basin, slurry is dumped to a storage basin, where it is
constantly stirred by mechanical process. From storage basin, the slurry is pumped to upper chamber of the rotary kiln, regularly.
Rotary kiln consists of an inclined cylinder supported on masonry chamber 15 metres apart. Its length varies from 90 to 120 metres and diameter varies from 3 to 3.5 metres. The diameter of the cylinder in burning zone, is comparatively larger than that of other zones.
Slurry is admitted from the upper chamber of the kiln, to the higher portion of the cylinder which makes one revolution per minute and pulverised coal is entered from other end. When the slurry reaches the burning zone (temp. 1500° to 1600°C), CO₂ gas is evolved after heating and the moisture evaporates. Hot chamber is then cooled by blowing in cool air in the outlet pipe. To delay the setting time of the resulting cement, gypsum (3 to 4%) is added at this stage.
Klinker is then ground in ball mill and tube mill in which balls grind the klinkers to a very fine powder, called cement.