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Monday, June 20, 2011

WHAT IS WOOD?

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Accessible, light, easily worked and replaceable, wood is an essential element of basic survival�the weapon, shelter and fuel. Wood is a hard, fibrous substance also known as xylem, which is the principal strengthening and water-conducting tissue found in the stems and roots of many plants, including trees and shrubs. The versatility of wood is basically attributable to its structure, chemical composition, and properties. Produced by many botanical species, it is available in various colours and grain patterns.


In contemporary times, in spite of technological advancement and competition from metals, plastics, cement, and other materials, wood maintains a place in most of its traditional roles, and its serviceability is expanding through new uses with the result that its consumption is steadily increasing. The long list of present wood uses includes products in which its natural texture is retained and others in which the wood is mechanically and chemically modified to the extent that its presence cannot be recognized. In addition to well-known products, such as lumber, furniture, and plywood, wood is the raw material for wood-based panels, for pulp and paper, and many other products, especially chemical derivatives of cellulose and lignin. Finally, wood is still an important fuel in much of the world.


The principal compound in cell walls is cellulose. Its molecules are linear chains of glucose, which may reach four microns in length. Orderly arrangement of cellulose molecules in fibrils (micelles) accounts for its crystalline properties. Noncellulosic constituents (hemicelluloses, lignin, and pectin) encrust the matrix among fibrils. Some hemicelluloses appear to serve as an important cross-link between the noncellulosic polymer8 and cellulose. Lignin is a complex substance that imparts rigidity to ceil walls. Pectins are important constituents of the layer between cell walls (middle lamella).


Cellulose and the other chemical constituents are contained in wood in the following proportions (in percent of the oven-dry weight of wood) cellulose 40-45 percent (about the same in gymnosperms and dicotyledonous angiosperms); hemicelluloses 0 percent i gymnosperms and 15-5 percent in angiosperms; lignin 5-5 percent in gymnosperms and 17-5 percent in angiosperms; and pectic substances in very small proportion. In addition, wood contains extractives (gums, fats, resins, waxes, sugars, oils, starches alkaloids, tannins) in various amounts (usually 1-10 percent, sometimes 0 percent or more). Extractives are not structural components but are deposited in cell cavities and intercellular spaces and may be removed (extracted) without change of wood structure. Most mechanical properties of wood are closely correlated to density and specific gravity. It is possible to learn more about the nature of a wood sample by determining its specific gravity than by any other simple measurement.


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Density is the weight or mass of a unit volume of wood, and specific gravity is the ratio of the density of wood to that of water. Determination of the density of wood in relation to that of other materials is difficult because wood is hygroscopic, and both its weight and volume are greatly influenced by moisture content. In order to obtain comparable figures, weight and volume are determined at specified moisture contents. The standards are oven-dry weight (practically zero moisture content) and either oven-dry or green volume (moisture content above fiber saturation point, which averages about 0 percent).


PROPERTIES OF WOOD


MECHANICAL PROPERTIES


The mechanical or strength properties of wood measure its ability to resist applied forces that might tend to change its shape and size. Resistance to such forces depends on the magnitude and manner of application of the force. It also depends on various characteristics of the wood, such as moisture content and density. The term strength is often used in a general sense to refer to ail mechanical properties. The mechanical properties of wood include strength in tension and compression (axial and transverse), shear, cleavage, hardness, static bending, and shock (impact bending, toughness). Respective tests determine stresses per unit of loaded area (at elastic limit and maximum load) and other criteria of strength, such as modulus of elasticity (a criterion of stiffness), modulus of rupture (bending strength), elastic resilience, and toughness.


THERMAL PROPERTIES


Although wood expands and contracts with varying Temperature, these dimensional changes are small in comparison to the shrinkage and swelling caused by variation of moisture content. In most cases expansion and contraction are negligible and without practical importance; only temperatures below 0° C, (° F) may cause surface checks, and in living trees, unequal contraction of outer and inner layers may result in frost cracks.


PHYSICAL PROPERTIES


Familiarity with most kinds of wood makes it possible to identify them by their appearance. Some woods such as black walnut can he identified by their color others can be distinguished by their, odor. Many woods have a pronounced difference in color between sapwood and heartwood, whereas in others there is difference in color exposed to fire. In addition, the low heat conductivity of wood (high insulating value) makes it desirable for building construction. Heat conductivity is about two to two and a half times greater axially than transversely and increases with density and moisture content.


Colour � This varies from the white of a holly tree to the red in redwood or black of an ebony.


Odour � This is due to certain Chemical deposition the heartwood when it is more pronounced than sapwood.


Grain � This is the direction of the wood fibers i.e. the alignment of the cells w.r.t. the axis of a tree. It can be described as cross, straight, interlocked, spiral curly, or wavy.


Texture � This refers to the size of the cells in the wood & their proportion in unit volume; or simply, it is the uniformity of a woods surface. Texture can be fine (e.g. boxwood & sandalwood) or coarse (e.g. teak & sal).


Weight � this depends on the density & hygroscopicity of the wood.


Density & specific gravity � this is the weight or mass of a unit volume of wood, & specific. Gravity is the ratio of the density of wood to that of water. Differences in these quantities occur due to different proportions of wood substance (excluding the volume of cell humans & wall spaces) & the content of extractives.


Hygroscopicity � wood is hygroscopic i.e. it exhibits on affinity for water and can absorb moisture. This Property varies among different species. The moisture content of cell walls varies from 0 to 5%. Hygroscopicity is of primary importance because all wood properties such as shrinkage, swelling, hardness, strength, heat producing capacity & resistance to decay depend on it.


Shrinkage and Swelling � wood is subject to dimensional changes depending on its moisture content fluctuations. Shrinkage of the wood, proceeding that of the cell wall, occurs as moisture escapes from the wood structure & results in the molecules moving closer together. Swelling is just the reverse. These dimensional changes are anisotropic i.e. different in axial, radial & tangential directions. They may result in change of shape. Checking (cracks formation) warping, case hardening (release of stresses in resawing), honey combing & collapse.


ELECTRICAL PROPERTIES


Very dry wood is an excellent insulator � But electrical conductivity increases with an increase in moisture content. Wood also acts as a dielectric i.e. although a non-conductor, it sustain the force of an electric field passing through it. Wood also exhibits the piezo electric effect i.e. electric polarization under mechanical shares & strain, in an electric field.


ACOUSTIC PROPERTIES


Wood can also produce sound by direct striking & can amplify or absorb sound waves originated from other bodies. The pitch of the sounds increases with larger dimension, lower moisture contents & higher density & elasticity.


TYPES OF WOOD


HARDWOOD


Timber obtained from angiospermous, or flower-bearing, trees that have broad leaves. Hardwood trees are deciduous, except in the warmest regions, and shed their leaves at the end of each growing season. Hardwood is the source of about 0 percent of the worlds production of lumber. The term hardwood is a classification of material, known originally from such hard European woods as beech and oak but actually including both the hardest and the softest of woods available. Many beautiful hardwoods include cabinet timbers such as American black walnut, oak, elm, yew, rosewood, teak, ebony, primavera, maple, satinwood, greenheart, and various mahoganies.


COTTON WOOD


Cottonwood includes several species of the genus populous. Most important are eastern cottonwoods, also known as Carolina Poplar and white wood, swamp cottonwood and black cottonwood. The heartwood of the three cotton woods, eastern, black, and swamp, is grayish white to light brown. The wood is comparatively uniform in texture, and generally straight grained. It is odorless when well seasoned. Cottonwood is used principally for lumber, veneer, pulpwood, and fuel.


MAPLE


Commercial species of maple include sugar maple, black maple, silver maple, red maple, boxelder, and big leaf maple. The sapwood of the maples is commonly white with a slight reddish-brown, tinge. Maple wood has a fine, uniform texture. It is heavy, strong, hard, stiff and resistant to shock and has large shrinkage. Maple is used principally for (lumber, veneer, crossties, distillation wood and pulpwood. A large portion is manufactured into flooring, furniture, boxes, and crates, Shoe lasts, handles, woodenware, novelties etc.





ASPEN


The heartwood of Aspen is grayish white to light grayish Drown. The sapwood is lighter colored and generally merges gradually into heartwood without being clearly marked. Aspen wood is usually straight grained with a fine, uniform texture. It is easily worked. This type of wood is lightweight and soft. It is low in strength, moderately stiff, moderately low in resistance to shock, and has a moderately high shrinkage.


ASH


Ash, black ash and Oregon Ash.


Commercial white ash is a group of species that consists mostly of white Ash and green ash. Blue Ash is also included in this group. White ash is particularly sought because of the Inherent qualities of this wood; it is heavy, strong, hard. Stiff, and is high resistance to shock. Because of these qualities, such tough Ash is used particularly for handles oars vehicle parts, and sporting goods.


Oregon ash has on the other hand somewhat lower strength properties, but is generally used for the same purposes.


The wood of black Ash runs considerably lighter in weight than that of white ash, and therefore, is sold as cabinet Ash, and is suitable for storage, furniture and shipping containers.


Hardwoods can be classified as


Deciduous Hardwoods


These trees lose their leaves in winter. They grow in warmer temperate climate (including the British Isles, Europe, Japan, New Zealand, Chile and Central America), and are slow growing and expensive.


Evergreen hardwoods


These trees keep their leaves all year round, and therefore grow more quickly and to a greater size. They are usually softer and easier to work with than deciduous hardwoods. The grow mainly in tropical and sub-tropical climates (including most of South America, Central America, India, China, Africa, Burma, India and East and West Indies).


SOFT WOODS


Soft Woods include firs, pines, spruce, and all conifirous timbers. They are characterized by distinct annual rings, but indistinct medullary rays, (groups of horizontally arranged cells radiation from the center towards the barks). Their traits also include light colour, straight fibbers, long steins of uniform section, long & narrow pointed leaves, exudation of issuing and mostly turpentine as well. soft woods are very strong for direct pull, but are weak in resisting thrust or shear.


Examples of softwoods (found mainly in India & Pakistan) are


CHIR


Himalayan softwood; easy to work; liable to surface and cracking unless protected against too rapid drying.


DEODAR


Has a natural preventative; light, moderately strong, easy to season & work; retains its shape well.


MANGO


Moderately staring; general utility wood; easy to season but liable to stain if not dived quickly; easy to work; & keeps its shape well.


WORKING WITH WOOD


Woodworking is defined as the process of constructing, or working with articles of wood, as in carpentry, joinery, cabinet making, and engraving. The different techniques include


WHITTLING


Woodworking techniques begin with the simple one of whittling. This is the using of a knife to cut away extra wood to achieve a form required by the craftsman.


TURNING


This is the fashioning of a piece of wood with a chisel while the wood is rotated on a lathe, much like the potters wheel. Most round forms such as the legs and arms of tables and chairs are formed in this way.


WOOD CARVING


The designing and decoration on a piece of furniture done by craftsmen using tools is the art of carving.


WOOD CUTS


In this process the artist cuts away the wood along the grain, leaving lines which when inked, will make the print in relief.


WOOD ENGRAVING


This process is similar to wood cuts, except instead of being done on a wood block cut out with the grain it is done cutting across the grain.


WOOD SCULPTURE


As wood is more perishable very few examples of early wood sculptures remain. The sculptor working on the piece of wood must carry a mental image or drawings of the finished product in three dimensions. The block to be worked on must first be marked out with chalk before the artist starts cutting away wood using a lot of different techniques.


WAYS OF JOINING WOOD.


The object of the joint is to fix two members together so that the joint has the greatest possible mechanical strength and is as unobtrusive as possible. Though there are many joints in use, they fall into a few basic groups, many being variations and elaborations of fundamentally simple ideas. Practically all are based on handwork, and with few exceptions most machine-made joints follow the traditional patterns; most joints rely to a considerable extent on a combination of mechanical fit and glue for their strength


GLUING OF WOOD AND MECHANICALLY FASTENING OF WOOD


Generally there are two ways of joining wood, gluing and fastening. Technically fastenings are considered to be more reliable than that of the glue. Of mechanical means of fastening, screws are the best, though a good joiner would pride himself in being able to make most things without a single screw in It. They are used for concealed fixings, for work which may have to be taken apart. Smaller nails, such as panel pins, are used for fixing small members, such as moldings, and sheet materials, like plywood. Their narrow heads are to be punched and puttied over. The joiner except for fixing lower- quality work does not use longer nails. Thus fastening can be done in a number of ways, such as with nails, spikes, screws, bolts, lag screws, drift pins, etc.


NAILS


Nails are the most common mechanical fastenings used in temporary and permanent constructions. There are many types, sizes, and forms of standard nail, and in addition many special-purpose nails. In general, nails have stronger joints when driven into the side grain than into the end grain of wood. Also nails should be preferably be used so that their literal resistance, rather than direct withdrawal resistance, is utilized.


SPIKES


Common wire spikes are manufactured in the same manner as common wire nails. They have either a chisel point or a diamond point and are made in length of 7 to inches; they have larger diameters than the common wire nails. The allowable withdrawal and lateral resistance formulas and limitations given for common wire• nails are also applicable to spikes, except that in calculating the withdrawal load of spikes, the depth of penetration should be reduced by two- thirds the length of the point.


DRIFT BOLTS


Drift bolts are driven into prebored holes of diameters 1/8th inches less than that of the bolts. The allowable load for a drift bolt in lateral resistance should not exceed that for an ordinary bolt of the same dimension. The drift bolts should be of greater length than the common bolt to make up for the lack of washes & nut.


WOOD SCREWS


The common types of wood screws have flat, oval, or round heads, their principal parts also include the shank, thread & core. Flat head screws are used if a flush surface is desired inheres oval & round head screws are used for appearance.


LAG SCREWS


These are used because they are highly convenient especially whose bolts are difficult to fasten & nuts on the surface are objectionable lag screws range from about 0, to 1 inch in diameter & to 16 inches in length.


CONNECTOR � JOINT


Timbers can be joined by metal or other types of connectors. These types include split-ring, soothed, shear � plate, bulldog, claw � plate, circular spike, & kubler wood-dowel connectors. The latter was originally made of cast iron but now of oak & is very satisfactory


CROSS BOLTS


Cross bolts placed at or near the end of timbers joined with connectors or at intermediate panel point will provide additional safety. They may also be used to reinforce members that have, through change in moisture content in service, developed checks to an undesirable degree.


SPECIAL FASTENERS


Wood workers use a variety of metal fasteners besides nails and screws. All kinds of special bolts, screws, hooks, plates, and braces are used to fasten objects together. Many of the fasteners are designed for use with metals. Many of special fasteners used by wood workers are briefly described here.


CARRIAGE BOLTS


They have oval heads and are square just below the heads. The square part sinks into the wood thereby preventing turning once the bolt is set in the hole. These bolts are used in rough construction and for joints that do not show.


SHEET METAL SCREWS


Sheet metal screws, or tapping screws are threaded the full length of the screw. They are used to attach materials such as plastic and sheet metal to wood. They are especially handy when attaching thin pieces.


CORRUGATED FASTENERS


They are used when appearance is not a factor. They provide a quick way of joining stock. They will not make strong joints. They are staggered and driven at an angle to the grain.


MENDING PLATES


These are flat pieces of steel. They come in several shapes And Sizes. Mending plates are used to reinforce and repair broken and weakened joints. They are usually used in places where they will not show.


ADVANTAGES OF JOINTS


1n the wood working process, the importance of joints cannot be denied. In fact it is almost impossible to work with wood without a proper command on joint- making. Moreover, the joints used are of various kinds; the application of a joint in the production of something is decided keeping in view the requirements and purpose of the product.


The Dowelled joint for instance, is a very strong and easily by machine. To ensure the true alignment, a tongue is an advantage, and this necessitates a double row of dowels, which gives the greatest strength. No end grain is exposed as with an ordinary tenon. The dowels, and less timber is used than for a tenon joint.


The combed joint is essentially a glued joint. The glue must take the whole of the stress; so the larger the area of glued sauces the better. Three glue surfaces are most common, but more can be made it the section is thick enough for each tenon to be of workable thickness.


The combed joint does not hold itself in position while glue is setting. Clamping has to be arranged so that the parts are held in their true positioning.


WOOD DESTROYING AGENTS


MOLDS AND STAINS


Molds and stains are confined largely to sapwood and are of various colors. Little direct staining of the wood is caused by molds, since the discoloring caused by them is largely superficial and is due far the most part to cottony or powdery surface growths. Which vary from white or light colors to black. Such blemishes often are easily brushed or surfaced off.


Stains penetrate into the sapwood and cannot be removed by surfacing. The discoloring of the wood occurs as specks, spots, streaks, or patches of varying intensities of color. The so-called °blue stains, which vary from bluish to bluish black and brown, are the most common although various shades of yellow, orange, purple and red are sometimes encountered. The exact color ot the stain depends on the infecting organisms and the species and moisture condition of the wood.


INSECTS


The worst of these are termites, popularly called ‘white ants’. The insects live in a colony, each colony having a queen, much larger in size, which alone lays millions of eggs during her lifetime of about ten years.


WOOD-BORING BEETLE


This beetle is capable of biting holes into the wood the larva is directly responsible for the damage brought about by its endless tunneling while it feeds on the wood substance, leading to the eventual collapse of the wood structure.


Then there are wood wasps much less harmful, the larvae of which bore tunnels in the dead wood of Deodar, Spruce and Fir, extending for several inches.


MARINE BORERS


There are two types of these; one popularly called ‘Teredo navalis’, these do not actually feed on the wood but bore tunnels into it for shelter from their enemies. The other species called ‘Limnoria’ feed on wood and in a short time, wood becomes honey-combed with tunnels, and ultimately collapse.


USES OF TIMBER


Wood and its products have a huge variety of uses all over the world. Some products are used directly but most wood products serve as intermediate materials that undergo processing and are manufactured into final products or structures.


ROUND WOOD PRODUCTS


Poles , posts and certain mine timbers are products in round forms. Poles are used in telecommunication lines e.g. telegraphs, telephones, or as pilings i.e. foundations for buildings, and posts are used in fences highway guards and various supports.


SAWN WOOD


The main sawn wood product is lumber. It is used for heavy construction. Railroad ties are also made by sawing. This lumber is the product of the sawmill and is produced in varying sizes from logs.


VENEER


This is a thin sheet of wood that is uniform in thickness. Veneers are used primarily for plywood and furniture but are also used in toys, containers, matches, battery separations etc.


PLYWOOD AND LAMINATED WOOD CONSTRUCTIONS


These are glued wood products. Plywood is panel product manufactured by glueing together veneers to both sides of a single veneer or solid wood. In addition to flat panels, plywood is manufactured in curved form used for boats, furniture etc.


Laminated wood is used in beams, columns and arches for buildings, boat keels, aircraft carrier deckings, helicopter propellers and mine sweepers.


PARTICLE BORAD


This is manufactured from particles of wood glued together. It contributes to greater wood utilization by permitting the use of residues of other wood using industries and of harvesting operations in forests.


FIBER BOARD


This is made of fibers of wood. It serves in building constructions, exterior siding, interior finishing and shelves, furniture, ship-building, automobile manufacture, refrigeration cars, toys and concrete framework.


PULP AND PAPER


Wood is the main source of pulp and paper which serve million of purposes.


MECHANICALLY DERIVED PRODUCTS


Some of the principal applications of wood include agricultural tools, aircrafts, baseball bats, baskets blinds, blackboards, cloths pins, crates, fishing roads, golf clubs, handles, ice cream spoons, ironing boards, ladders, oars, pallets, pencils, picture frames, rules, skies and sleighs, scientific instruments, tanks, tennis rackets, toothpicks and many others.


CHEMICALLY DERIVED PRODUCTS


These include acetic acid, acetone, cellophane, cellulose acetate, charcoal, dyestuffs, explosives, lacquers, methanol, molasses, oils, paper products, photo graphic films, plastics, rayons, sugars, synthetic spongestar, turpentine and yeast etc. Wood has also been used as a fuel for long periods.





WHAT IS METAL ?


Metals are usually crystalline solids, which constitute almost 75% of all chemical elements found in nature. Mostly, they have simple crystal structures distinguished by a close packing of atoms and a high degree of symmetry. The art and science of extracting metals from their ores and modifying the metals for use, is known as metallurgy.


The most abundant varieties in the Earths crust are aluminum, iron, calcium, sodium, potassium, and magnesium. The vast majority of metals are found in ores (mineral-bearing substances~, but a few such as copper, gold, platinum, and silver frequently occur in the free state because they do not readily react with other elements.


PROPERTIES OF METAL


• High density and melting point.


• Malleable and ductile (i.e. can be drawn into wire).


• Thermal conductivity.


• Electric conductivity.


• Lustrous (when polished).


• Sonorous (i.e. can be beaten into sheets).


• High tensile strength.


MANUFACTURE OF METALS


Most metals are found as minerals (compounds of the metal mixed with earthy material (gangue). The mixture is called an ore. Metals can bee manufactured from their sources e.g. sulphides, oxides, nitrates, chlorides, suppurates etc. By electrolysis or reduction. The ore is concentrated after mining to eliminate worthless materials as possible. One of the most important methods for concentrating sulphide ores is known as the ore flotation process.


In this process the finally pulverized ore is mixed with water, to which one or more chemical frothing agents are added. When air is blown into the mixture froth is produced and the earthy material is wetted and sinks. The sulphide ore particles, however, rise to the surface in the froth, where there can be skimmed off the surface. After the addition of acids to break up the froth, the concentrated ore is filtered and dried.


The metal is then manufactured in the following way


ROASTING OF THE ORE


In this process the concentrated ore is heated in a controlled amount of air. The purpose of this operation might be


1. To convert the sulphide ore into its oxide prior to reduction of the oxide to the metal itself. At this stage impurities such as arsenic are driven off.


. To covert the sulphide ore partially into its oxide, which is then reduced to the metal by further reaction with the sulphide ore.


SINTERING


This involves heating the material until partial fusion occurs and larger, more easily handled material is obtained.


SMELTING


This involves the reduction f the ore to the molten metal at a high temperature. Substances called fluxes are added, their function being to combine with the gangue to forma liquid slag that floats on the surface of the molten metal.


REFINING


The purpose of refining metals is to make them as puree as is necessary. Numerous techniques include the following


• Electrolytic refining


Electrolytes can be used to purify metals e.g. copper is purified electrolytically by making the impure copper the anode of an electrolytic cell, which contains an electrolyte of copper sulphate solution and a thin strip of pure copper as the cathode. By the appropriate choice of voltage, pure copper is transferred from the anode to the cathode.


• Zone-refining


This method is applied on a very small scale to produce metals and some non--metals in an extremely high degree of purity. The method depends upon the principle that an impure molten metal will deposit puree crystals on solidifying. The metal, in the form of a rod, is melted over a very narrow region at one end; this molten region is transferred from one end of the rod to the other by slowly moving a furnace. Impurities collect in the molten region and are swept to one end of the metal.





TYPES OF METALS


SODIUM


• Physical properties


The metal is soft and silvery coloured it is an extremely good conductor of heat and electricity and less dense than water. because it rapidly transits and loses its slivery appearance in air, it is generally stored under oil.


• Chemical Properties


The metal is very reactive; sodium reacts with water with increasing vigor.


Na(s) + H O(1) ------------ Na + OH � (aq) +H(g)


It reacts with a variety of non-metals when heated to give oxides sulphides, hydrides, etc. e.g.


Na(s) + S(s) ---------------(Na+) Sa �(s)


(Lithium alone reacts with nitrogen to give the nitride (Li+)N-, since both the lithium and nitrogen atoms are very small and the resulting nitride has a very compact structure with a high lattice energy).


The metal burns in a steam of hydrogen chlorideand reacts with ammonia when heated e.g.


Na(s) + HCI(g) ------------ Na+CI-(s) + H(g)


Na(s) + NH(g) ----------- Na+NH-(s) + H(g)


CALCIUM


• Physical properties


The metal is very hard. Ti is a good conductor of heat and electricity. When pure it is silvery coloured, but quickly transits on exposure to air because an oxide film covers its surface. It has a high melting & boiling Pt).





• Chemical Properties


The metal is very reactive. It decomposes water with increasing vigor to give the hydroxide and hydrogen e.g.


Ca(s) + HO(1) ------------- Ca+(OH)-)(aq)/s) +H(g)


At a suitable temperature it combines with a variety of non-metals to give oxides, sulphides,halides nitrides; and hydrides.


Ca(s) + H(g) ----------- Ca+(H-)(s)


With dilute hydrochloric and dilute sulphuric acids it gives a saltand hydrogen e.g.


Ca(s) + H+C1-(aq) ------------ Ca+(C1-)(aq)+H(g)


ALUMINUM


Aluminum is a light metal possessing considerable strength, yet is malleable and ductile. It isnot very reactive because normally there is a very thin oxidelayer on its surface. Whenthis oxide layer is removedby rubbing with mercury,the metal reacts rapidly with moisture in the air, forming a moss-like growth of aluminum hydroxide, and becomes very hot in the process.


It combines directly with oxygen, sulphur, nitrogen and the halogens when heated to a sufficiently high temperature.


Aluminum reacts with moderately concentrated hydrochloric acid to give the chloride and hydrogen. The pure metal is not readily attacked by dilute sulphuric acid, but with concentrated HSo4 it gives the sulphate and sulphur dioxide. It is made passive by nitric acid and this has been attributed to the formation of an impenetrable oxide layer on its surface. It is attacked by sodium hydroxide solution with the liberation of hydrogen.


A1(s)+OH-(aq) + 6HO(1) -------- A1 (OH)4 � (aq) + H (g)


IRON


Pure iron is a silvery coloured metal with a melting point of 155 C. It is easily magnetized when placed inside a coil carrying an electric current, but loses its magnetism when the current is switched off. A number of non-metals combine with it on heating, e.g. oxygen, the halogens, nitrogen, sulphur and carbon; iron filings burn in oxygen with a shower of bright sparks forming iron (III) oxide, FeO, but in the massive form iron is coated with a layer of magnetic oxide of iron, FeO4. Pure iron does not action of dry air or air-free water; however the combined action of air and water results in the formation of rust, essentially hydrated iron (III) oxide. At red heat iron is attacked by steam with the formation of magnetic oxide of iron and hydrogen. Dilute non-oxiding acids such as sulphuric and hydrochloric acids attack iron, with the formation of iron (II) ions, Fe+ (aq), and hydrogen; in the presence of air the iron (II) ions are slowly oxidized to iron (III) ions, Fe+(aq), concentrated nitric acid renders the metal passive. Impure irons such as wrought iron & cast iron are also very useful.


COPPER


Copper has a melting point of 108 C and a density of 8.4 g cm-. It is an attractive golden coloured metal, being very malleable and ductile. The metal is slowly attacked by moist air and its surface gradually becomes covered with an attractive green layer of basic copper carbonate. At about 00 C it is attacked by air and a black coating of copper (II) oxide forms on its surface; at a temperature of about 1000 C copper (I) oxide is formed instead. Copper is also attacked by sulphur vapour, with the formation of copper (I) sulphide, and by the halogens which form the copper (II) halide.


The metal is not attacked by water or steam and dilute non-oxidising acids such as dilute hydrochloric and dilute sulphuric acid are without effect in the absence of an oxidising agent. Boiling concentrated hydrochloric acid attached the metal, with the evolution of hydrogen.


Hot concentrated sulphuric acid attacks the metal and so too does dilute and concentrated nitric acid.


Cu(s) + HSO4(1) ------------- CuSO4(s) +HO(1) + SO(g)


Cu(s) + 8HNO(aq) ----------- Cu(NO)(aq) +4HO(1)+NO(g)


USES OF COPPER


Copper is used for the windings of dynamos and for conveying electrical power, it’s a useful metal for the construction of condensers for chemical plants and car radiators. Finely divided copper is used as an industrial catalyst.


SILVER


Silver has a melting point of 61 C and a density of 10.5 g cm �. It is a white lustrous metal and is very malleable and ductile; it is the best conductor known. The metal is resistant to attack by air and mositure, although the presence of hydrogen sulphide results in the black stain of silver sulphide. Steam and dilute non-axidising acids are without effect on the metal; however, it is attacked by hot concentrated sulphuric acid and cold dilute nitric acid, with the formation of silver (I), Ag+(aq), ions.


Ag(s) + HSO4(1) ------- AgSO4(s) + SO(g) + HO(1)


Ag(s) + 4HNO(aq) ------- AgNO(aq) + HO(1) + NO(g)


USES OF SILVER


Concentrated nitric acid produces mainly nitrogen dioxide. Silver is still used in coinage as an alloy with copper. Large quantities are also used for the manufacture of tableware and jewelry. The deposition of silver on cheaper articles is carried out to produce silverplated cutlery.


GOLD


Gold has a melting point of 106 C and density of 1. g cm-. It is extremely malleable and ductile, e.g. it can be beaten into sheets no thicker than 0.000 01 mm and pulled into wire of extremely small diameter; its thermal and electrical conductivities are very high. The metal is one of the most unreactive elements; it is not attacked by air, water or steam, and the common mineral acids leave the metal untouched.


USES OF GOLD


In view of its lack of chemical reactivity and its attractive bright yellow colour, gold is used in the manufacture of jewelry. Gold is sold by the carat. Cheaper articles are made by plating copper alloys with gold, using an electrolytic process.


ALLOYS OF METALS


An alloy is a substance prepared by adding other metals or non-metalsto a basic metal, so as to obtain certain desirable qualities. Thus it can be considered as a uniform mixture of two or more metallic elements.


Some Common alloys of metals include brass, bronze, duralumin, steel and stainless steel.


BRASS


This is an alloy copper (60 � 8%) and zinc 0-40%. It is stronger to more malleable than copper. It has greater workability due to a lower melting point and an attractive appearance.


BRONZE


This contains copper (0-5%), and tin (5-10%). It is very strong, and resistant to chemical attack. It is also very shiny in appearance.


Bronze is used to make coins, medals, and sculptures.


DURALUMIN


This contains aluminum (0-5%), copper (-5) and magnesium (-5%) and carbon (0.15 � 1.5%). Steel is more malleable and ductile than iron. It is much harder and stronger & can withstand great stress & strain.


Steel is used in the construction of ships, cars, bridges to machinery.


STAINLESS STEEL


This is a mixture of iron (0-5%), and chromium & nickel (5-10%). It is very hard & resistant to corrosion. Has a very lustrous appearance. Stainless steel is used to make cutlery, Tools & surgical instruments.


TECHNIQUES OF METAL WORK


The techniques of working metal developed very slowly and for long only in connection with the progress of metallurgy itself the mining of a mass of metal from the earth. Scholarly opinion now holds that the first steps were taken after the adoption of settled ways of life-represented by agriculture and stock breeding-in northeastern Iran, the first area in which this occurred. In this area were native copper, metal-bearing rocks, malachite, and abundant timber, which allowed a steady progress of discoveries to be made. The Iranians learned the essentials of metalworking by using native copper variations of the techniques were applied to other metals as they were recognized. A diffusionist theory is now generally accepted The techniques were developed in northeastern Iran, but the products, and possibly also the producers, gradually were carried by trade and emigration to other areas. They went to the valley civilizations of Mesopotamia, across western Persia and through the east Mediterranean littoral to Egypt, across North Africa, and on into Spain. A second route lay from western Iran into Anatolia and then across the Hellespont to Europe. This diffusion began in about the 5th millennium BC and was continued for over 000 years.


EARLY TECHNIQUES


The earliest metalworking was of copper, perhaps as early as the th millennium. Using small nuggets of native copper picked up in streams or from the ground. These nuggets were presumably at first considered a special kind of attractively colored stone, and by grinding and beating-methods already used for working stones, flints, and obsidian-they could be shaped into ornaments.


ANNEALING


The newt step was the discovery, about 5000 BC that these special stones could be worked on with repeated hammering if the mass was heated to a full red color and cooled from time to time, and that this kept the metal soft and workable. Ordinary wood fires produced sufficient heat for this process, called annealing. Repeated hammering without annealing will cause the metal to become too hard and brittle, with resultant jagged tracks.


SMELTING


The next discovery came after the development of the closed two-chamber pottery kiln, which produced a far greater heat than the open fires adequate for the earlier low-fired pottery. This took place probably before 4000 BC and led, after some 500 years or so, to the smelting first of small pieces of native copper, malachite (which under certain conditions will render into copper), and finally large amounts of copper ores, in furnaces that initially resembled the two-chamber pottery kilns. It was not until copper ores were smelted that any significant increase in the supply of copper and copper products could take place.


CASTINGS


Most metals are produced by melting and casting in molds. The mould may be shaped and dimensioned to the final size or it may simply be a prism which is intended for further processing. When intended for turther processing the metal will solidify in a rather coarse grain structure and will contain a number of casting defects such as porosity, shrinkage and cavities.


HOT WORKING


The working of metals and alloys depends upon plasticity. They can be heavily deformed, especially in compression, without breaking. For steel structural members the most usual method is by hot rolling between shaped rolls at temperatures around 1000 C. after rolling, the members are left to cool naturally and, during this process, a heavy film or iron oxide develops. Thus steel sections deliverd as rolled end to be shot blasted or sand blasted before receiving any protective coating.


Many familiar articles, e.g. crankshafts, are forged into shape.this involves placing a hot blank into one half of a shaped mould and then impressing the other half of the mould on to the blank. This is generally done under impact using such methods as drop forging, die stamping etc.


COLD WORKING.


Because of their ductility many metals and alloys can be bold worked, that is to say, shaped at temperatures below the recrystallization temperature. This create an immense number of dislocations and the metal becomes harder and its yield point is raised. Metal sheets area shaped into cups, bowls or motor car parts by deep drawing or stretch forming.


JOINING


Although adhesive bonding may be used for joining metals, the ecumenist methods are welding, brazing, soldering or inserting metal pins such as rivers and bolts.


BRAZING AND SOLDERING


Both processes involve jointing by means of a thin film of a material which has a lower melting point than that of the parent material and which, whenmelted, flows into the joint, oftenby capillary action, to form a thin film. A good brazed or soldered joint should have a strength which is not too different from that of the parent material.


PINNING


For some materials (such as cast iron and wrought iron) bolting or rivetingare the only possible ways of making joints. Both rely on friction-the tightened bolt forces the two members together and the friction between nut and bolt at the threads holds it in place. In riveting, the hot rivet is hammered into prepared holes. As it cools it contracts and develops a tensile stress which effectively lochs the members together. High strength friction grip (HSFG) bolts used in structural steelworks combine both aspects � the bolt is prestressed to a given level and this tensile prestress acts in the same way as does the tensile stress in a river.


WELDING


The welding technique᠐, which involved inter-layering relatively soft and tough iron with high-carbon material, followed by hammer forging᠐produced a strong, tough blade. In modern times the improvement in iron-making techniques, especially the introduction of cast iron, restricted welding to the blacksmith and the jeweller. Other joining techniques, such as fastening by bolts or rivets, were widely applied to new products, from bridges and railway engines to kitchen utensils. Modern fusion welding processes are an outgrowth of the need to obtain a continuous joint on large steel plates. Riveting had been shown to have disadvantages, especially for an enclosed container such as a boiler. Gas welding, arc welding, and resistance welding all appeared at the end of the 1th century.


The first real attempt to adopt welding processes on a wide scale was made during World War I. By 116 the oxyacetylene process was well developed, and the welding techniques employed then are still used. The main improvements since then have been in equipment and safety. Arc welding, using a consumable electrode, was also introduced in this period, but the bare wires initially used produced brittle welds. A solution was found by wrapping the bare wire with asbestos and an entwined aluminium wire.


The modern electrode, introduced in 107, consists of a bare wire with a complex coating of minerals and metals. Arc welding was not universally used until World War II, when the urgent need for rapid means of construction for shipping, power plants, transportation, and structures spurred the necessary development work. Resistance welding, invented in 1877 by Elijah Thomson, was accepted long before arc welding for spot and seam joining of sheet. Butt welding for chain making and joining bars and rods was developed during the 10s. In the 140s the tungsten-inert gas process, using an inconsumable tungsten electrode to perform fusion welds, was introduced.


In 148 a new gas-shielded process utilized a wire electrode that was consumed in the weld. More recently, electron-beam welding, laser welding, and several solid-phase processes such as diffusion bonding, friction welding, and ultrasonic joining have been developed.


GAS WELDING


Gas welding is a non-pressure process using heat from a gas flame. The flame is applied directly to the metal edges to be joined and simultaneously to filler metal in wire or rod form, called the welding rod, which is melted to the joint. Gas welding has the advantage of involving equipment that is portable and does not require an electric power source. The surfaces to be welded and the welding rod are coated with flux, a fusible material that shields the material from air, which would result in a defective weld.


ARC WELDING


Arc-welding processes, which have become the most important welding processes, particularly for joining steels, require a continuous supply of either direct or alternating electrical current. This current is used to create an electric arc, which generates enough heat to melt metal and create a weld.


Are welding has several advantages over other welding methods. Arc welding is faster because of its high heat concentration, which also tends to reduce distortion in the weld. Also, in certain methods of arc welding, fluxes are not necessary. The most widely used arc-welding processes are shielded metal arc, gas-tungsten arc. gas-metal arc, and submerged arc.


SHILEDED METAL ARC


In shielded metal-arc welding, a metallic electroude, which conducts electricity, is coated with flux and connected to a source of electric current. The metal to be welded is connected to the other and of the same source of current. By touching the tip of the electrode to the metal and then drawing it away, an electric arc is formed. The intense heat of the arc melts both parts to be welded and the point of the metal electrode, which supplies filler metal for the weld. This process, developed in the early 0th century, is used primarily for welding steels.


GAS-TUNGSTEN ARC


In gas-tungsten arc welding, a tungsten electrode is used in place of the metal electrode used in shielded metal-arc welding. A chemically inert gas, such as argon, helium, or hydrogen, is used to shield the metal from oxidation. The heat from the arc formed between the electrode and the metal melts the edges of the metal. Metal for the weld may be added by placing a bare wire in the arc or the point of the weld. This process can be used with nearly all metals and produces a high-quality weld. Moreover, the rate of welding is considerably slower than in other processes.


GAS METAL ARC


In gas-metal welding, a bare electrode is shielded from the air by surrounding it with argon or carbon dioxide gas or by coating the electrode with flux. The electrode is fed into the electric arc, and melts off in droplets to enter the liquid metal that forms the weld Moat common metals can be joined by this process.


SUBMERGED ARC


Submerged-arc welding is similar to gas-metal arc welding, but in this process no gas is used to shield the weld. Instead, the arc and tip of the wire are submerged beneath a layer of granular, fusible material formulated to produce a proper weld. This process is very efficient but is generally only used with steels.


RESISTANCE AND THERMITE WELDING


In resistance welding, heat is obtained from the resistance of metal to the flow of an electric current. Electrodes are clamped on each side of the parts to be welded, the parts are subjected to great pressure, and a heavy current is applied briefly. The point where the two metalin welding breaks or seams in heavy iron and steel sections. It is also used in the welding of rail for railroad tracks meet creates resistance to the flow of current. This resistance causes heat, which melts the metals and creates the weld. Resistance welding is extensively employed in many fields of sheet metal or wire manufacturing and is particularly adaptable to repetitive welds made by automatic or semiautomatic machines.


In thermite welding, heat is generated by the chemical reaction that results when a mixture of aluminum powder and iron oxide, known as thermite, is ignited. The aluminum unites with the oxygen and generates heat, releasing liquid steel from the iron. The liquid steel serves as filler metal for the weld. Thermite welding is employed chiefly


SAFETY MEASURES WHEN WORKING WITH METAL


A brief outline of the safety recommendations for arc welding and the cutting processes is as follows


1. Keep the working area and floor clean and clear of electrode stubs, scraps of metal and carelessly disposed tools.


. See that the cable connections are tight and that they do not become hot.


. Never look at an electric arc with the naked eye.


4. Never weld while wearing wet gloves or shoes.


5. Never leave the electrode holder on a table or on the table or in contact with a grounded metallic surface.


6. Operate arc welding machines and equipment only in clean dry locations.


7. Always work in a ventilated area


8. Goggles with suitable lenses and protective clothing are recommended as protection against harmful rays, as well as against flying sparks, splattering metals and hot metal.


BIBLIOGRAPHY


• Blackburn, Peter, Chemistry; Pan Pacific Publications Ltd, 181.


• Brumbaugh, Jame .E., Welder’s Guide and Handbook, D. B. Taraporevala Sons & Co. Pvt. Ltd.


• Frisch, Susan, Metal; Watson-Guptill Publications, 18.


• Midgley, Barry, Sculpture, Modelling and Ceramics Techniques and Materials; The Apple Press, 186.


• Prescott, Christopher .N., Chemistry; Federal Publication Ltd, 18.





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