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THE MOST USEFUL METALS.
The most useful metals are iron, lead, copper, tin, and zinc.
73. IRON.-Iron is, of all metals, the most important to mankind. Its uses were long unknown to the human race, the age of iron implements being preceded by the ages of bronze and stone. Pure metallic iron exists only in
very small quantity on the earth's surface, almost entirely occurring in meteoric stones.
The process of obtaining iron from its ores requires an amount of knowledge and skill which the early races of men did not possess. The iron of commerce exists in three different forms, exhibiting very different properties, and having different chemical constitutions: (1) wrought iron; (2) cast-iron; (3) steel.
The first is nearly pure iron; the second is a compound of iron with varying quantities of carbon and silicon; and the third a compound of iron with less carbon than is needed to form cast-iron.
Pure iron in the form of powder may be obtained by heating the oxide moderately in a current of hydrogen. The hydrogen combines with the oxygen to form water, leaving the iron free. It must, however, be kept in an atmosphere of hydrogen, as finely divided iron takes fire spontaneously in the air. Iron has a bright white color, and is remarkably tough, though soft. The pure metal crystallizes in cubes. Iron which has been hammered has, when broken, a granular structure; but it becomes fibrous when the iron is rolled into bars, and the more or less perfect form of the fibre determines to a great extent the toughness and the value of the metal. This fibrous texture of hammered bar-iron undergoes a change when
exposed to long continued vibration, the iron returning to its original crystalline condition. Many accidents have occurred in the breaking of railway axles, owing to this change from the fibrous to the granular texture. Wrought iron melts at a very high temperature ; but, as it becomes soft at a much lower point, it can be easily worked. When hot, it possesses the peculiar property of welding; that is, the power of uniting firmly when two clean surfaces of hot metal are hammered together.
Iron and certain of its compounds are strongly magnetic. The metal loses this power when red-hot, but regains it upon cooling. A solid mass of iron does not oxidize or tarnish in dry air, at the ordinary temperature; but if heated, it oxidizes in black scales. When more strongly heated in the air, or plunged into oxygen gas, it burns with the formation of the same black oxide (11). In pure water, iron does not lose its brilliancy; but, if a trace of carbonic acid is present, and access of air is permitted, the iron soon rusts.
74. Manufacture of Iron. — The oldest method of manufacturing wrought iron was to heat the ore in a blastfurnace with charcoal or coal, and to hammer out the spongy mass of iron thus obtained. This plan can only be economically followed on a small scale and with the purest ores, and has been superseded by a more complicated method, applicable to all kinds of iron-ore. The most important ore is an impure carbonate called clay ironstone. This is first roasted (that is, heated in contact with air) to drive off the carbonic acid, and reduce the iron to an oxide. It is next smelted in a blast-fur
These furnaces (Figure 15) are usually about fifty feet high, and about fifteen feet in diameter in the widest part of the cavity CD. The lowest part, F, is called the crucible, or hearth. I I are the tuyères, or pipes through which air is forced by powerful bellows.
K, K, and M are arched galleries for the convenience of workmen employed about the furnace. When working regularly, the
Fig. 15 furnace is charged from a door at the end of the gallery near the top, first with coal, and then with a mixture of roasted ore and limestone broken into small pieces. As the fuel burns
and the materials gradually sink, fresh supplies of fuel and of ore are added; so that the furnace is kept filled with alternate layers of each.
The oxygen of the air from the bellows combines with the carbon of the fuel, forming carbonic oxide, CO, which rises through the porous mass, and, taking the oxygen from the ore, becomes converted into carbonic acid, CO,. The iron, mixed with the earthy matter of the ore, settles down into the hottest part of the furnace, where both are melted. The iron, being the heavier, sinks to the bottom, where it is drawn off at intervals through a tap-hole in the floor H. The lighter earthy matter, or slag, floats on the surface of the iron, like oil on water, and flows off through an opening above the tymp-stone L. The limestone aids in liquefying the earthy matter, and unites with it to form the slag.
At this stage the iron is cast-iron.
with the amount of carbon and silicon which it contains ; for cast-iron is not a definite chemical compound of these elements with iron. The carbon is found in cast-iron, (1) as scales of graphite, giving rise to mottled cast-iron ; and (2) in combination, forming white cast-iron.
In order to make wrought iron from cast-iron, the latter must undergo the processes of refining and puddling. These consist essentially in burning out the carbon and silicon, by exposing the heated metal to a current of air in a reverberatory furnace. Such a furnace is repre
sented in Figure 16. The ore is put into the hoppers HH, from which it falls into the chamber C, where it is spread out on the bed cc. The fuel is burned on a hearth at A, separated from the ore by the bridge b. The heated gases rising from the burning fuel are reverberated, or reflected, by the arched roof of the furnace, and driven down upon the ore, and then pass off through the flue f. When the ore is sufficiently roasted, it is allowed to fall through openings, dd, into the chamber E. The ore is stirred from time to time, to expose fresh surfaces to the action of the air and the flame. The melted cast-iron becomes first covered with a coat of oxide, and gradually thickens, so as to allow of its being rolled into large lumps or balls. During this process, the
whole of the carbon escapes as carbonic oxide, and the silicon becomes oxidized to silica, which unites with the oxide of iron, and forms a fusible slag. Any phosphorus or sulphur contained in the iron is also oxidized in this process. The ball is then hammered, to give the metal coherence, and to squeeze out the liquid slag, and the mass is afterwards rolled into bars or plates.
-Steel is formed when bars of wrought iron are heated to redness for some time in contact with charcoal. The bar is then found to have become finegrained instead of fibrous; the substance is more malleable and more easily fusible than the original bar-iron, and is found to contain carbon varying in amount from one to two per cent. Steel has several important properties, especially the power of becoming very hard and elastic when quickly cooled, which fits it for the manufacture of edge-tools. These are, however, generally made of bar-steel, which has been previously fused and cast into ingots.
A new and very rapid mode of preparing cast-steel is that known as the Bessemer process. This process consists in burning out all the carbon and silicon in cast-iron, by passing a blast of atmospheric air through the molten metal, and then adding such a quantity of pure cast-iron to the wrought iron thus prepared as is necessary to give carbon enough to convert the whole mass into steel. The melted steel is then at once cast into ingots. In this way six tons of cast-iron can, at one operation, be converted into steel in twenty minutes. The Bessemer steel is now largely manufactured for railway axles and rails, for boiler-plates, and other purposes, for which it is much better fitted than wrought iron, so that this process bids fair to revolutionize the old iron industry. It has already been put into practice on a large scale in England, France, Belgium, Sweden, and India.