In the large iron works, it is usual to roast or calcine the ores of iron, pre viously to their fusion ; as well for the pur. pose of expelling suiphureous or arseni cal parts, as to render them more easily broken intofragments of a convenient size for melting. The mineral is melted or run down in large furnaces, from sixteen to thirty feet high ; and variously shaped, either conical or elliptical, according to the opinion of the iron-master. Near the bottom of the furnace is an aperture for the insertion of the pipe of large bellows, worked by water or steam, or of other machines for producing a current of air ; and there are also holes at proper parts of the edifice, to be occasionally opened, to permit the scoria and the metal to flow out, as the process may require. Charcoal, or coke, with lighted brush wood, is first throw n in ; and when the whole inside of the furnace has acquired a strong ignition, the ore is thrown in by small quantities at a time, with more of the fuel, and commonly a portion of lime stone, as a flux : the ore gradually sub sides into the hottest part of the furnace, where it becomes fused; the earthy part being converted into a kind of glass, while the metallic part is reduced by the coal, and falls through the vitreous mat ter to the lowest place. The quantity of fuel, the additions, and the heat, must he regulated, in order to obtain iron of any desired quality ; and this quality must likewise, in the first product, he necessa rily different, accordingto the nature of the parts which compose the ore.
The iron which is obtained from the smelting furnaces is not pure, and may be distinguished into three states: white crude iron, which is brilliant in its frac ture, and exhibits a crystallized texture, more brittle than the other kinds, not at all malleable, and so hard as perfectly to withstand the file : grey crude iron, which exhibits a granulated and dull tex ture when broken ; this substance is not so hard and brittle as the former, and is used in the fabrication of artillery, and other articles which require to be bored, turned, or repaired: and black cast iron, which is still rougher in its fracture ; its parts adhere together less perfectly than those of the grey crude iron; this is usually fused again with the white crude iron.
Whenever crude iron,esp ecially the grey sort, is used again in contact with air, it emits sparkles, loses weight, and becomes less brittle. In order to convert it into malleable iron it is placed on a hearth, in the midst of charcoal, urged by the wind of two pair of bellows. As soon as it becomes fused, a workman continually stirs it with a long iron instrument. Dur ing the course of several hours it be comes gradually less fusible, and assumes the consistence of paste. In this state it is carried to a large hammer, the repeat ed blows of which drive out all the parts that still partake of the nature of crude iron so much as to retain the fluid state. By repeated heating and hammering, more of the fusible iron is forced out ; and the remainder, being malleable, is formed into a bar or other form for sale. Crude iron loses upwards of one fourth of its weight in the process of refining ; sometimes, indeed, one half.
Purified, or bar iron, is soft, ductile, flexible, malleable, and possesses all the qualities which have been enumerated under this article as belonging exclusive ly to iron. When a bar of iron is broken, its texture appears fibrous ; a property which depends upon the mechanical ac tion of the hammer while the metal is cold. Ignition destroys this fibrous tex
ture, and renders the iron more uniform throughout ; but hammering restores it.
If the' purest malleable iron be bedded in pounded charcoal, in a covered cruci ble, and kept for a certain number of hours in a strong red heat, (which time must be longer or shorter, according.,to the greater or less thickness of the bars of iron) it is found that by this opera tion, which is called cementation, the iron has gained a small addition of weight, amounting to about the hundred and fif tieth, or the two hundredth part, and is remarkably changed in its properties. It is much more brittle and fusible than be fore. Its surface is commonly blistered when it conies out of the crucible ; and it requires to be forged to bring.its parts together into a firm and continuous state. This cemented iron is called steel. It may be welded like bar iron, if it have not been fused or over-cemented ; but its most useful and advantageous property is that of becoming extremely hard when ignited and plunged into cold water. The hardness produced is greater in propor tion as the steel is hotter, and the water colder. The colours which appear on • the surface of steel slowly heated are yel lowish•hite, yellow, gold colour, pur ple, violet, deep blue; after which the ignition takes place. These signs direct the artist in tempering or reducing the hardness of steel to any determinate ' standard. If steel be too hard, it will not be proper for tools which are intend ed to have a fine edge, because it will be so brittle than the edge will soon become notched ; if it be too soft, it is evident that the edge will bend or turn. Some • artists ignite their tools, and plunge them into cold water ; after which they bright. en the surface of the steel upon a stone : the tool being then laid upon charcoal, or upon the surface of melted lead, or , placed in the flame of a candle, gradually acquires the desired colour ; at which in- " stant they plunge it into water. If a hard temper be desired, the piece is dipped again, and stirred about in the cold water as soon as the yellow tinge appears. If the purple appear before the dipping, the temper will be fit for gravers, and tools used in working upon metals ; if dipped while blue, it will be proper for springs, and for instruments used in the cutting of soft substances, such as cork, leather, and the like ; but if the last pale colour be waited for, the hardness of the steel will scarcely exceed that of iron. When soft steel is heated to any one of these colours, and then plunged into wa- • ter, it does not acquire nearly so great a degree of hardness as if previously made quite hard, and then reduced by tem pering. The degree of ignition required to harden steel is different in the dif ferent kinds. The best kinds require only a low red heat. The harder the steel, the more coarse and granulated its • fracture will be ; and as this is not com pletely remedied by the subsequent tem pering, it is advisable to employ the least heat capable of affording the requisite hardness. It is a circumstance worthy of remark, that steel has a less specific gra vity when hardened than when soft ; but there are no circumstances upon which a probable connection between these two properties, namely, the increased hard ness and the diminished specific gravity, can be made out.