Chap. II.

COMPOSITION AND USE OF MINERALS.

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cavity of which is like a square prism lined with fire-bricks, and the smoke is conductedinto a lofty chimney. Cast-steel will not hear more than a cherry red heat withoutbecoming very brittle; it cannot be welded together, but will unite with iron through theintervention of a fine film of vitreous horacic acid, and the latter metal may be plated withcast-steel, by pouring the liquid steel from the crucible upon a bar of iron laid in a mouldwith the upper face polished; the adhesion becomes so perfect, that the two metals may berolled out together, and instruments made of it will have the toughness of iron combinedwith the hardness of steel.

According to Mr. Mushet, carbon combines with iron in the following proportions toform the different carburets :

Soft cast-steelCommon cast-steelWhite cast-ironBlack cast-iron

and the specific gravity of steel varies from 7 *31 to 7’91, which is that of the best ham-mered: so great is the affinity of iron for carbon, that it will absorb it from carburettedhydrogen or coal gas, and thus become converted into steel.

Hardening Steel is performed by putting it into a charcoal fire, and when the metal hasacquired a red heat, it is suddenly plunged into cold water; where these plates are requiredto be hardened, they are plunged into oil and tallow, or bces-wax and resin, as the waterwould render them too brittle, and cause them to crack.

Tempering is effected by again submitting the metal to the action of fire; as the heatincreases it becomes softer, and when the requisite degree is arrived at, it is withdrawn andquenched in cold water. To an experienced workman, the degree of temper is indicatedby the colour; for springs or where elasticity is the object, it is quenched when the colourassumes a fine blue: when a fine edge is required it is brought to a straw colour, whilstthe back of the instrument is left blue. For magnets, the ends of the bar only are broughtto a blue colour, as the harder the whole bar is left the better is the magnetism retained,although communicated with more difficulty in the first instance. The heat required byFahrenheit’s scale to produce

Very pale straw yellow, was ----- 430°

Light purple ------- 530

Dark blue ------- 570

Pale blue - - 590

and for all a free access of oxygen is required.

Alloys of steel with platinum, gold, and nickel, may be made when the heat is sufficientlypowerful.

Founding or Casting of Iron -Iron intended for the foundery receives a high charge of

carbon, whilst the bar or malleable iron must be deprived of it; consequently a differentprocess must be adopted in the manufacture of these two varieties: that intended for thewhitesmith, as already described, is put into a furnace where it is exposed to air and heatonly, without the fuel coming in contact with it; that for the foundery must be remeltedin close contact with the fuel, and excluded almost entirely from the air ; hence as it meltsit takes up an additional quantity of carbon when cast into pigs. There are varieties ofpig-iron; the grey, which is the best, another of a medium quality, and one which is notmuch superior to the forge-iron. The finest soft iron, when struck with a hammer,scarcely yields any sound, like a mass of lead ; its fracture shows little lustre and is coarselygranular, whilst the inferior quality is very brittle, easily broken by the hammer, and gives outa sound like a bell; its fracture is shining and of a silvery whiteness, with no granular appear-ance. Iron which after casting is required to be turned on the lathe, filed, or drilled, shouldbe of the purest quality : where, however, great strength is required, and the casting is to beused as when taken from the mould, the medium quality should be preferred; when it isto be applied to the ram of a pile-driving engine, or for an anvil, the third quality shouldbe selected on account of its hardness. These three varieties of iron owe their qualities tothe carbon, the best containing the greatest proportion; when this is running from thefurnace, a large quantity of carburet of iron floats on its surface, denoting it to be asuperior kind.

The Cupola and Air Furnace _The first is used for small, the latter for large castings ;

the cupola requires a blowing machine of some kind, whilst the air-furnace has a sufficientdraught created by means of a lofty chimney.

The open Sand Casting is applicable to flat plates, where one side is rough or uneven ;the mould is made of sand of a peculiar kind, having its grains of an equal size, and not ofa nature to vitrefy when highly heated; this is mixed with enough loam to give it whenmoistened the property of being moulded into the form required; it must also besufficiently open to allow the escape of air or steam when the hot iron is poured into it.

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