their carbon, whereby they are nearly reduced to the condition

of

pure malleable iron, but without the fibre which is due to the hammering and rolling employed at the forge.

The malleable iron-castings are made from the rich Cumberland iron, and are at first as brittle as glass or hardened steel; they are enclosed in iron boxes of suitable size, and surrounded with pounded iron-stone, or some of the metallic oxides, as the scales from the iron forge, or with common lime, and various other absorbents of carbon, together or separate. The cases, which are sometimes as large as barrels, are luted, rolled into the ovens or furnaces, and submitted to a good heat for about five days, and are then allowed to cool very gradually within the fur

naces.

The time and other circumstances determine the depth of the effect; thin pieces become malleable entirely through, they are then readily bent, and may be slightly forged; cast-iron nails and tacks thus treated admit of being clenched; thicker pieces retain a central portion of cast-iron, but in a softened state, and not brittle as at first; on sawing them through, the skin or coat of soft iron is perfectly distinct from the remainder.

The mode is particularly useful for thin articles that can be more economically and correctly cast, than wrought at the forge, as bridle-bits, snuffers, parts of locks, culinary and other vessels, pokers and tongs, &c.; many of which are subsequently casehardened and polished, as will be explained, but malleable castiron should never be used for cutting-tools.

SECT. VI.-CASE-HARDENING WROUGHT AND CAST IRON.

THE property of hardening is not possessed by pure malleable iron; but I have now to explain a rapid and partial process of cementation, by which wrought-iron is first converted exteriorly into steel, and is subsequently hardened to that particular depth; leaving the central parts in their original condition of soft fibrous iron. The process is very consistently called casehardening, and is of great importance in the mechanical arts, as the pieces combine the economy, strength, and internal flexibility of iron, with a thin casing of steel; which although admirable as an armour of defence from wear or deterioration as regards the surface, is unfit for the formation of cutting edges or tools, owing

MODES OF CONDUCTING THE PROCESS.

261

to the entire absence of hammering, subsequent to the cementation with the carbon. Cast-iron obtains in like manner a coating of steel, which surrounds the peculiar shape the metal may have assumed in the iron-foundry and workshop.

The principal agents used for case-hardening are animal matters, as the hoofs, horns, bones, and skins of animals; these are nearly alike in chemical constitution, they are mostly charred and coarsely pounded, some persons also mix a little common salt with some of the above; the works should be surrounded on all sides with a layer from half an inch to an inch thick.

The methods pursued by different individuals do not greatly differ; for example, the gunsmith inserts the iron work of the gun-lock, in a sheet-iron case in the midst of bone-dust, (often not burned,) the lid of the box is tied on with iron wire, and the joint is luted with clay; it is then heated to redness as quickly as possible, and retained at that heat from half an hour to an hour, and the contents are quickly immersed in cold water. The objects sought are a steely exterior, and a clean surface covered with the pretty mottled tints, apparently caused by oxidation from the partial admission of the air.

Some of the malleable iron castings, such as snuffers, &c., are case-hardened to admit a better polish; it is usually done with burnt bone-dust, and at a dull red heat; they remain in the fire about two or three hours, and should be immersed in oil, as it does not render them quite so brittle as when plunged into water. It must be remembered they are sometimes changed throughout - their substance into an inferior kind of steel, by a process that should in such instances be called cementation, and not casehardening, consequently they will not endure violence.

The mechanician and engineer, use horns, hoofs, bone-dust, and leather, and allow the period to extend from two to eight hours, most generally four or five; sometimes for its greater penetration, the process is repeated a second time with new carbonaceous materials. Some open the box and immerse the work in water direct from the furnace; others, with the view to preserve a better surface, allow the box to cool without being opened, and harden the pieces with the open fire as a subsequent operation; the carbon once added, the work may be annealed and hardened much the same as ordinary steel.

When the case-hardening is required to terminate at any

particular part, as a shoulder, &c., the object is left with a band or projection, the work is allowed to cool without being immersed in water, the band is turned off, and the work when hardened in the open fire is only effected so far as the original cemented surface remains. This ingenious method was introduced by Mr. Roberts, of Manchester, who considers the success of the casehardening process to depend on the gentle application of the heat; and that, by proper management not to overheat the work, it may be made to penetrate three-eighths of an inch in four or five hours.

A new substance for the case-hardening process, but containing the same elements as those more commonly employed, has of late years been added, namely, the prussiate of potash, (a salt consisting of two atoms of carbon and one of nitrogen,) which is made from a variety of animal matters.

It is a new application without any change of principle; the time occupied in this steelifying process, is sometimes only minutes instead of hours and days; as for example when iron is heated in the open fire to a dull red, and the prussiate is either sprinkled upon it or rubbed on in the lump, it is returned to the fire for a few minutes and immersed in water; but the process is then exceedingly superficial, and it may if needful be limited to any particular part upon which alone the prussiate is applied. The effect by many is thought to be partial or in spots, as if the salt refused to act uniformly; in the same manner that water only moistens a greasy surface in places.

The prussiate of potash has been used for case-hardening the bearings of wrought-iron shafts, but this seems scarcely worth the doing it has been also employed with the view of giving an additional and extreme, although superficial hardness to steel, as in Jones's axletrees, Perkins's engraved steel plates, &c. ; but I have only heard of one individual who has encased work with this salt, it was for case-hardening the iron rollers and side plates of glaziers' vices employed for milling window-lead.

In the general way, the conversion of the iron into steel, by case-hardening, is quite superficial, and does not exceed the sixteenth of an inch; if made to extend to one quarter or three eighths of an inch in depth, to say the least it would be generally useless, as the object is to obtain durability of surface, with strength of interior, and this would disproportionately

CONCLUDING REMARKS ON HARDENING, ETC.

263

The steel obtained in this

encroach on the strong iron within. adventitious manner is not equal in strength to that converted and hammered in the usual way, and if sent in so deeply the provision for wear would far exceed that which is required.

Let us compare the case-hardening process with the usual conversion of steel. The latter requires a period of about seven days, and a very pure carbon, namely wood charcoal, of which a minute portion only is absorbed ; and it being a simple body, when the access of air is prevented by the proper security of the troughs, the bulk of the charcoal remains unconsumed, and is reserved for future use, as it has undergone no change. The hasty and partial process of cementation is produced in a period commonly less than as many hours with the animal charcoal, or than as many minutes with the prussiate of potash; but all these are compound bodies, (which contain cyanogen, a body consisting of carbon and nitrogen,) and are never used a second time, but on the contrary the process is often repeated with another dose. It would be therefore, an interesting inquiry for the chemist, as to whether the cyanogen is absorbed after the same manner as carbon in ordinary steel, (and which in Mackintosh's patent process was driven through the crucible in the form of carbonic acid gas, and is stated to be absorbed at the rate of one-thirtieth of an inch in depth, each hour;) or whether the nitrogen assists in any way in hastening the admission of the carbon, by some as yet untraced affinity or decomposition*.

This hasty supposition will apply less easily to cast-iron, which contains from three to seven times as much carbon as steel, and although not always hardened by simple immersion, is constantly under the influence of the case-hardening process; unless we adopt the supposition, that the carbon in cast-iron which is mixed with the metal in the shape of cinder in the blast furnace, when all is in a fluid state, is in a less refined union than that instilled in a more aëriform condition in the acts of cementation and case-hardening+.

It may happen that the carbon is not essential, as the Indian steel or wootz is stated to contain alumine and silex; and manganese is used in Heath's and Vicker's patents.

It would have been an easy task to have multiplied the examples and remarks upon this curious subject, and it has already far, very far, exceeded the intended limits. The reader will find much useful matter upon the same in Lardner's Cyclopedia in the volume on Iron and Steel, and in Gill's Technological Repository, in

CHAPTER XIII.

THE METALS AND ALLOYS MOST COMMONLY USED.

THE thirteen metals before referred to have now to be considered, namely, Antimony, Bismuth, Copper, Gold, Lead, Mercury, Nickel, Palladium, Platinum, Rhodium, Silver, Tin, and Zinc. Unlike iron and steel, they do not admit of being hardened beyond that degree which may be produced by simple mechanical means, such as hammering, rolling, &c., neither, (with the exception of platinum,) do they submit to the process of welding.

On the other hand, their fusibility offers an easy means of uniting and combining many of these metals with great readiness, either singly, or in mixtures of two or several kinds, which are called alloys. By the process of founding, any required form may be given to the fusible metals and alloys; their malleability and ductility are also turned to most useful and varied accounts; and by partial fusion neighbouring metallic surfaces may be united, sometimes per se, but more generally by the interposition of a still more fusible metal or alloy called solder.

I intend therefore, to commence with a brief notice of the physical characters and principal uses of the thirteen metals before named, and of their more important alloys. Tables of the cohesive force and of the general properties of metals are next added to avoid the occasional necessity for reference to other works.

The tables will be followed by some remarks on alloys, which as regards their utility in the arts, may be almost considered as so many distinct metals: this will naturally lead to the processes of melting, mixing and casting the metals; a general notice and explanation of many works, taking their origin in the malleable and ductile properties, will then follow; and the consideration of the metals, and of materials from the three kingdoms, will be concluded by a descriptive account of the modes of soldering.

each of which the subjects are perhaps treated in a more practical manner than in many other works in which they are referred to. The Journal of the Royal Institution, vol. ix. p. 319 to 333, contains much curious information upon alloys of steel with silver, rhodium, platinum, nickel, &c.