Q.1 Give a brief description of wood.
Ans. Wood: -
Wood is a natural organic material, which can be cut or pressed into suitable shapes and sizes by mechanical or chemical means. The primary structural (or microconstituents) of wood are cellulose and an amorphous carbohydrate material lignin. The cellulose is a long chain polymer, which has the structure as shown in Fig. (a). Here n is the number of molecules per chain. Its value may be as high as three to four thousand molecules per chain.
The presence of hydroxyl group (OH groups), in the cellulose structure. makes it highly polarised. This gives rise to the formation of secondary bonds between the chains. As a result of this, the cellulose possesses a crystalline structure. The crystals of the cellulose take the form of tabular cells as shown in Fig. 9.1(b). These crystals are bonded together by amorphous lignin. The amorphous lignin also provides a passage for moisture and sap to flow during the growth of wood (i.e., tree or plant).
Types Of Wood: -
It can be classified into three categories: softwood, hardwood and manufactured panels such as plywood.
Softwood: -
Softwood is from an evergreen or coniferous (cone-bearing) tree. Common varieties are pine, fir, spruce, hemlock, cedar and redwood. These woods are mostly used in the home construction industry. Cedar and redwood are excellent choices for outdoor projects, while pine is often used for “Early American Country Style” furniture.
Pine and most other softwoods will absorb and lose moisture more than hardwoods so are not as stable.
Hardwood: -
Hardwood lumber comes from deciduous trees, the ones that shed their leaves annually. Popular domestic species are oak, maple, cherry, birch, walnut, ash and poplar. Of these common native hardwoods, only red oak and poplar are usually stocked in home centers and lumberyards, the others have to be obtained from specialty stores.
Oak and ash, are known as open-grain woods. These species have alternating areas of relatively porous and dense wood, when stained the open-grain areas absorb the color readily while the harder areas are more resistant. This accentuates the grain patterns, creating a dramatic effect.
Cherry, maple and birch are closed-grain woods, demonstrating a more uniform texture throughout a board. Poplar is also a closed-grain wood, but its color ranges from a beige to olive green, and often has purple highlights thrown into the mix. Because of this unusual coloration, it is rarely used if a furniture piece is going to have a clear finish. This wood is best when stained or even painted. Poplar, being less expensive, is also a good choice for framing hardwood projects.
Characteristics of Wood: -
The wood used for carpentry should have the following characteristics
1. It should be from the heart of tree and free from sap.
2. It should be free from defects like knots, shakes, resin pockets, worm holes, twisted fibres etc.
3. It should have uniform dark colour with close fibres.
4. It should not clog the teeth of the saw when it is cut.
5. It should smell sweet on cutting.
6. It should give clean sound when struck with hammer.
7. It should be properly seasoned.
8. It should be easily workable.
9. It should be strong and heavy.
10. It should have high resistance to fire.
Advantages of Wood: -
1. It is easily available and can be transported by simple means.
2. It is light in weight yet strong.
3. It is relatively easy to handle and can be sawn and joined with carpenter’s tool.
4. Boards can be cut rapidly by a saw and fastened firmly together with nails.
5. It is a good insulator of heat and sound.
6. It can withstand shocks and bumps.
7. Because of its light weight it is preferred for building works in earthquake regions.
8. When properly protected timber structures can be maintained for hundreds of years.
9. It can resist corrosion.
10. Repairs, additions and alterations to wood construction are easy.
Disadvantages of Wood: -
1. It is highly combustible material.
2. Frame buildings built closely together present a serious hazard.
3. Wood is destroyed by decay induced by fungi and by insects that feed upon wood under favourable conditions.
4. Atmospheric humidity makes wood swell and shrink.
Uses of Wood: -
1. Used in the form of piles, posts, beams, lintels, door-window frames, roof members etc.
2. Used for flooring, ceiling, paneling and construction of partition walls.
3. Used for frame-work for concrete, centering for arch work, transmission poles and fencing.
4. Used in wagon and coach building, marine installations and
5. Used in making furniture, agricultural implements, sports goods, musical instruments, well curbs, motor bodies, carts, carriages, railway sleepers, toys etc.
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