Popular Technology Volume II Part 21

IRON.

1. The properties which iron possesses in its various forms, render it the most useful of all the metals. The toughness of _malleable iron_ renders it applicable to purposes, where great strength is required, while its difficult fusibility, and property of softening by heat, so as to admit of forging and welding, cause it to be easily wrought.

2. Cast iron, from its cheapness, and from the facility with which its form may be changed, is made the material of numerous structures.

_Steel_, which is the most important compound of iron, exceeds all other metals in hardness and tenacity; and hence it is particularly adapted to the fabrication of cutting instruments.

3. Iron was discovered, and applied to the purposes of the arts, at a very early period. Tubal-Cain, who was the seventh generation from Adam, "was an instructer of every artificer in bra.s.s and iron." Noah must have used much of this metal in the construction of the ark, and, of course, he must have transmitted a knowledge of it to his posterity.

4. Nevertheless, the mode of separating it from the various substances with which it is usually combined, was but imperfectly understood by the ancients; and their use of it was, most likely, confined chiefly to the limited quant.i.ty found in a state nearly pure. Gold, silver, copper, and tin, are more easily reduced to a state in which they are available in the arts. They were, therefore, often used in ancient times, for purposes to which iron would have been more applicable.

This was the case especially with copper and tin.

5. Fifteen distinct kinds of iron ore, have been discovered by mineralogists; but of these, not more than four have been employed in making iron. There are, however, several varieties of the latter kind, all of which are cla.s.sed by the smelters of iron under the general denomination of _bog_ and _mountain_ or _hard_ ores.

6. The former has much of the appearance of red, brown, or yellowish earth, and is found in beds from one to six feet thick, and in size from one fourth of a rood to twenty acres. The mountain, or hard ore, to a superficial observer, differs but little in its appearance from common rocks or stones. It is found in regular strata in hills and mountains, or in detached ma.s.ses of various sizes, and in hilly land from two to eight feet below the surface.

7. The bog-ore is supposed to be a deposite from water which has pa.s.sed over the hard ore. This is evidently the case in hilly countries, where both kinds occur. Some _iron-masters_ use the bog; some, the hard; and others, both kinds together. In this particular, they are governed by the ore, or ores, which may exist in their vicinity.

8. The apparatus in which the ore is smelted, is called a _blast-furnace_, which is a large pyramidal stack, built of hewn stone or brick, from twenty to sixty feet in height, with a cavity of a proportionate size. In shape, this cavity is near that of an egg, with the largest end at the bottom. It is lined with fire-brick or stone, capable of resisting an intense heat.

9. Below this cavity is placed the _hearth_, which is composed of four or five large coa.r.s.e sandstones, split out of a solid rock, and chiselled so as to suit each other exactly. These form a cavity for the reception of the iron and dross, when melted above. The hearth requires to be removed at the end of every _blast_, which is usually continued from six to ten months in succession, unless accidentally interrupted.

10. The preparation for a blast, consists princ.i.p.ally in providing charcoal and ore. The wood for the former is cut in the winter and spring, and charred and brought to the furnace during the spring, summer, and autumn. What is not used during the time of hauling, is stocked in coal-houses, provided for the purpose.

11. The wood is charred in the following manner. It is first piled in heaps of a spherical form, and covered with leaves and dirt. The fire is applied to the wood, at the top, and when it has been sufficiently ignited, the pit is covered in; but, to support combustion, several air-holes are left near the ground. The _colliers_ are obliged to watch the pit night and day, lest, by the caving in of the dirt, too much air be admitted, and the wood be thereby consumed to ashes.

12. When the wood has been reduced to charcoal, the fire is partially extinguished by closing the air-holes. The coals are _drawn_ from the pit with an iron-toothed rake, and, while this is performed, the dust mingles with them, and smothers the fire which may yet remain. Wood is also charred in kilns made of brick.

13. The hard ore is dug by _miners_, or, as they are commonly denominated, _ore-diggers_. In the prosecution of their labor, they sometimes follow a vein into a hill or mountain. When the ore is found in strata or lumps near the surface, they dig down to it. This kind of ore commonly contains sulphur and a.r.s.enic, and to free it from those substances, and to render it less compact, it is roasted in kilns, with refuse charcoal, which is too fine to be used for any other purpose. It is then broken to a suitable fineness with a hammer, or in a crushing mill. The bog-ore seldom needs any reduction.

14. Every preparation having been made, the furnace is gradually heated with charcoal, and by degrees filled to the top, when a small quant.i.ty of the ore is thrown on, and the blast is applied at the bottom near the hearth. The blast is supplied by means of one or two cylindrical bellows, the piston of which is moved by steam or water power.

15. The coal is measured in baskets, holding about one bushel and a half, and the ore, in boxes holding about one peck. Six baskets of coal, and as many boxes of ore as the furnace can carry, is called a _half charge_, which is renewed as it may be necessary to keep the furnace full. With every charge is also thrown in one box of limestone.

16. The limestone is used as a flux, to aid in the fusion of the ore, and to separate its earthy portions from the iron. The iron sinks by its specific gravity, to the bottom of the hearth, and the earthy portions, now converted into gla.s.s by the action of the limestone and heat, also sink, and float upon the liquid iron. This sc.u.m, or, as it is usually called, scoria, slag, or cinder, is occasionally removed by instruments made for the purpose.

17. When the hearth has become full of iron, the metal is let out, at one corner of it, into a bed of sand, called a _pig-bed_, which is from twenty to thirty feet in length, and five or six in width. One concave channel, called _the sow_, extends the whole length of the bed, from which forty or fifty smaller ones, called _pig-moulds_, extend at right angles. The metal, when cast in these moulds, is called _pig-iron_, and the ma.s.ses of iron, _pigs_.

18. _Pig-iron_, or, as it is sometimes called, _crude iron_, being saturated with carbon and oxygen, and containing also a portion of scoria, is too brittle for any other purpose than castings. Many of these, such as stoves, grates, mill-irons, plough-irons, and kitchen utensils, are commonly manufactured at blast furnaces, and in many cases nearly all the iron is used for these purposes. In such cases, the metal is taken in a liquid state, from the hearth, in ladles.

19. In Great Britain and Ireland, and perhaps in some other parts of Europe, iron-ore is smelted with _c.o.ke_, a fuel which bears the same relation to pit-coal, that charcoal does to wood. It is obtained by heating or baking the coal in a sort of oven or kiln, by which it becomes charred. During the process, a sort of bituminous tar is disengaged from it, which is carefully preserved, and applied to many useful purposes.

THE IRON-FOUNDER.

1. The appellation of _founder_ is given to the superintendent of a blast-furnace, and likewise to those persons who make castings either of iron or any other metal. In every case, the term is qualified by a word prefixed, indicating the metal in which he operates, or the kind of castings which he may make; as _bra.s.s_-founder, _iron_-founder, or _bell_-founder. But whatsoever may be the material in which he operates, or the kind of castings which he may produce, his work is performed on the same general principle.

2. The sand most generally employed by the founder is _loam_, which possesses a sufficient proportion of argillaceous matter, to render it moderately cohesive, when damp. The moulds are formed by burying in the sand, wooden or metallic patterns, having the exact shape of the respective articles to be cast. To exemplify the general manner of forming moulds, we will explain the process of forming one for the _spider_, a very common kitchen utensil.

3. The pattern is laid upon a plain board, which in this application is called a _follow board_, and surrounded with a frame called a _flask_, three or four inches deep. This is filled with sand, and consolidated with rammers, and by treading it with the feet. Three wooden patterns for the legs are next buried in the sand, and a hole is made for pouring in the metal.

4. One side of the mould having been thus formed, the flask, with its contents, is turned over, and, the follow board having been removed, another flask is applied to the first, and filled with sand in the same manner. The two flasks are then taken apart, and the main pattern, together with those for the legs, removed. The whole operation is finished by again closing the flasks.

5. The mode of proceeding in forming moulds for different articles, is varied, of course, to suit their conformation. The pattern is often composed of several pieces, and the number and form of the flasks are also varied accordingly. Cannon-b.a.l.l.s are sometimes cast in moulds of iron; and to prevent the melted metal from adhering to them, the inside is covered with pulverized black lead.

6. Rollers for flattening iron are also cast in iron moulds. This method is called _chill-casting_, and has for its object the hardening of the surface of the metal, by the sudden reduction of the temperature, which takes place in consequence of the great power of the mould, as a conductor of heat. These rollers are afterwards turned in a powerful lathe.

7. Several _moulders_ work together in one foundery, and, when they have completed a sufficient number of moulds, they fill them with the liquid metal. The metal for small articles is dipped from the hearth or crucible of the furnace with iron ladles defended on every side with a thin coating of clay mortar, and poured thence into the moulds.

But in casting articles requiring a great amount of iron, such as cannon, and some parts of the machinery for steam engines, the iron is transferred to the moulds, in a continued stream, through a channel leading from the bottom of the crucible. In such cases, the moulds are constructed in a pit dug in the earth near the furnace. Large ladles full of iron are, in some founderies, emptied into the moulds by the aid of huge cranes.

8. Although the moulders have their distinct work to perform, yet they often a.s.sist each other in lifting heavy flasks, and in all cases, in filling the moulds. The latter operation is very laborious; but the exertion is continued but a short time, since the moulds, constructed during a whole day, can be filled in ten or fifteen minutes.

9. Iron-founderies are usually located in or near large cities or towns, and are supplied with crude iron, or pig metal, from the blast furnaces in the interior. The metal is fused either with charcoal or with pit coal. In the former case, an artificial blast is necessary to ignite the fuel; but in the latter, this object is often effected in air furnaces, which are so constructed that a sufficient current of air is obtained directly from the atmosphere.

10. The practice of making castings of iron is comparatively modern; those of the ancients were made of bra.s.s, and other alloys of copper.

Until the beginning of the last century, iron was but little applied in this way. This use of it, however, has extended so rapidly, that cast iron is now the material of almost every kind of machinery, as well as that of innumerable implements of common application. Even bridges and rail-roads have been constructed of cast iron.

THE BAR IRON MAKER.

1. Bar-iron is manufactured from pig-iron, from _blooms_, and directly from the ore; the process is consequently varied in conformity with the state of the material on which it is commenced.

2. In producing bar-iron from pigs, the latter are melted in a furnace similar to a smith's forge, with a sloping cavity ten or twelve inches below, where the blast-pipe is admitted. This hearth is filled with charcoal and dross, or scoria; and upon these is laid the metal and more coal. After the coal has become well ignited, the blast is applied. The iron soon begins to melt, and as it liquefies, it runs into the cavity or hearth below. Here, being out of the reach of the blast, it soon becomes solid.

3. It is then taken out, and fused again in the same manner, and afterwards a third time. After the third heat, when the iron has become solid enough to bear beating, it is slightly hammered with a sledge, to free it from the adhering scoria. It is then returned to the furnace; but, being placed out of the reach of the blast, it soon becomes sufficiently compact to bear the _tilt-hammer_.

4. With this instrument, the iron is beaten, until the ma.s.s has been considerably extended, when it is cut into several pieces, which, by repeated beating and forging, are extended into bars, as we see them for sale. The tilt-hammer weighs from six to twelve hundred pounds, and is most commonly moved by water power.

5. For manufacturing bar-iron directly from the ore, the furnace is similar in its construction to the one just described, and the operations throughout are very similar. A fire is first made upon the hearth with charcoal; and, when the fuel has become well ignited, a quant.i.ty of ore is thrown upon it, and the ore and the fuel are renewed as occasion may require. As the iron melts, and separates from the earthy portions of the ore, it sinks to the bottom of the hearth.

The scoria is let off occasionally, through holes made for the purpose. When iron enough has acc.u.mulated to make a _loop_, as the ma.s.s is called, it is taken out, and forged into bars under the tilt-hammer.

6. This way of making bar-iron is denominated the _method of the Catalan forge_, and is by far the cheapest and most expeditious. It is in general use in all the southern countries of Europe, and it is beginning to be extensively practised in the United States. When a Catalan forge is employed in making _blooms_, it is called a _bloomery_.

7. The blooms are about eighteen inches long, and four in diameter.

They are formed under the tilt-hammer, and differ in substance from bar-iron in nothing, except that, having been imperfectly forged, the fibres of the metal are not fully extended, nor firmly united. The blooms are manufactured in the interior of the country, where wood is abundant, and sold by the ton, frequently, in the cities, to be converted into bar or sheet iron.

8. These blooms are converted into bar-iron, by first heating them in an air-furnace, by means of stone coal, and then pa.s.sing them between chill cast iron rollers. The rollers are filled with grooves, which gradually decrease in size from one side to the other. When the iron has pa.s.sed through these, the bloom of eighteen inches in length, has become extended to nearly as many feet. The bar thus formed, having been cut into four pieces, the process is finished by welding them together laterally, and again pa.s.sing them between another set of rollers, by which they are brought to the form in which they are to remain.

9. Blooms are also laminated into two sheets, on the same principle, between smooth rollers, which are screwed nearer to each other every time the bloom is pa.s.sed between them. Very thin plates, like those which are tinned for the tin-plate workers, are repeatedly doubled, and pa.s.sed between the rollers, so that in the thinnest plates, sixteen thicknesses are rolled together, oil being interposed to prevent their cohesion. The last rollings are performed while the metal is cold.

10. Rolled plates of iron are frequently cut into rods and narrow strips. This operation is performed by means of elevated angular rings upon rollers, which are so situated that they act reciprocally upon each other, and cut like shears. These rings are separately made, so that they can be removed for the purpose of sharpening them, when necessary. The mills in which the operations of rolling and slitting iron are performed, are called rolling and slitting mills.

THE WIRE DRAWER.

1. Iron is reduced to the form of wire by drawing rods of it through conical holes in a steel plate. To prepare the metal for the operation of drawing, it is subjected to the action of the hammer, or to that of rollers, until it has been reduced to a rod sufficiently small to be forced through the largest hole. The best wire is produced from rods formed by the method first mentioned.

2. Various machines are employed to overcome the resistance of the plate to the pa.s.sage of the wire. In general, the wire is held by pinchers, near the end, and as fast as it is drawn through the plate, it is wound upon a roller, by the action of a wheel and axle, or other power. Sometimes, a rack and pinion are employed for this purpose, and sometimes a lever, which acts at intervals, and which takes fresh hold of the wire every time the force is applied.