In giving the requisite degree of hardness to cutting instruments of steel, two distinct processes are employed; first, hardening, and afterwards tempering. The hardening is effected by heating the steel to a cherry-red, and immediately plunging it into cold water; by this process, the steel becomes so hard as to resist, or turn the edge of the hardest file; likewise so brittle as to be useless for most purposes, but particularly for cutting instruments. To adapt the steel to the latter, the second process, called tempering, is resorted to, which is a species of annealing, or softening, or as the workmen term it, letting down, to the degree of hardness which is necessary for the peculiar purpose for which it is designed; for in proportion as the edge is harder than is required, is its liability to break in use. It is a remarkable fact, that the greatest part of our ordinary cutlery is hardened by a process which, although well known to be a very defective one, is persisted in from the force of habit, or from an indisposition in the workman to make any changes in the customary routine of executing work, in which the remunerating prices are very low.

We allude in particular to the practice (which, we are informed, is general at Sheffield,) of hardening direct from the anvil; that is, the articles are hardened with the scale on the surface, which is produced by the act of forging. The scale varies in thickness, according to the degree of heat the steel received in forging; it is also a very bad conductor of heat, consequently the transition from heat to cold (by which the effect of hardening is produced) is not so sudden in one piece of steel as in another; and some are scarcely hardened at all, owing to the temperature of the water having scarcely penetrated through the scale. Hence, when such articles are afterwards tempered or "lowered," there are not two alike in temper out of a great number; a fact which can hardly have escaped the observation of any man who has shaved for several years. Instead, therefore, of the customary mode of hardening the blade direct from the anvil, it has been recommended by an experienced manufacturer, that the blades be passed from the anvil to the grindstone. "A slight application of the stone," he observes, "will remove the whole of the scale or coating, and the razor will then be properly prepared to undergo the operation of hardening with advantage.

It will be easily ascertained that steel in this state heats in the fire with great regularity, and that when immersed, the obstacles being removed to the immediate action of the water on the body of the steel, the latter becomes equally hard from one extremity to the other. To this may be added, that as the lowest possible heat at which steel becomes hard is indubitably the best, the mode here recommended will be found the only one by which the process of hardening can be effected with a less portion than is or can be required in any other way."

Considerable difficulty has been experienced in giving to articles about to be hardened a perfectly uniform degree of heat in an ordinary fire; and one of the best means of obviating it is probably that which has been published by Mr. Nicholson, of his adoption, and which he had for some time previous, for justifiable reasons, kept secret: this was to employ a bath of melted lead, heated to moderate redness, and well stirred; into this the piece was plunged for a few seconds, until, when brought near to the surface, that part did not appear less luminous than the rest. The piece was then speedily stirred in the bath, suddenly drawn out, and plunged into a large body of water. Instead of employing simple water as the cooling medium, a variety of salts added to it have been at different times recommended and boasted of. Not long ago, mercury was cried up, and just now it is the fashion to extol a current of air, grounded, we believe, on the report of travellers, that the sabres of Damascus are hardened by cleaving the north wind with them.

Although it is probable that improvements in the present mode of hardening may be discovered, we think it is improbable that they will be found in that fluid which is an inferior conductor of heat, and that cannot be applied with equal uniformity to water.

A mode of tempering instruments of hardened steel was invented by Mr. Hartley, in 1789, for which he obtained a patent; and we have never yet heard of a better. Mr. Hartley's plan was to immerse the articles in a bath of oil, heated to a regulated temperature, and measured by a thermometer. This was certainly a very great improvement, both in point of precision and dispatch, on the common method of heating the instrument over a flame till a certain colour, produced by a film of oxide, appears on its surface. These colours are -

At 430° Fah. a very faint yellow, - for lancets.

450° „ a pale straw colour, - for razors, and surgeons* instruments. 470° „ a full yellow, - for penknives.

At 490* Fahr, a brown colour, - for scissors and chisels for catting old iron.

510° „ a brown, with purple spots, - for axes and plane irons.

530° „ a purple, - for table-knives and large shears.

550° ,, a bright blue, - for swords, watch springs, truss springs, and bell springs.

560° „ a full blue, for small fine saws, daggers, etc.

600° „ a dark blue, verging on black, - is the softest of all the gradations, when the metal becomes fit only for hand and pit-saws, which must be soft, that their teeth may bear sharpening by the file, and bending or " setting." If the steel be heated still further, it becomes perfectly soft. When tools having a thick back and thin edge, like penknives, are to be tempered, they are sometimes placed with their backs in a plate of hot iron, or on hot sand; otherwise they would become too soft at their cutting edges before their backs would be sufficiently heated. It is evident that baths of any of the soft metals, whose fusible points are above those required for tempering, may be used instead of oil; and alloys of those metals might be so proportioned as to obtain points of fusion at the exact degree of heat required. In these cases, however, to prevent oxidation, it would be necessary to keep the fluid metal covered with grease, and it would be advisable not to omit the use of a thermometer. Mr. Gill, in the Technological Repository, has recommended several compositions for hardening and tempering steel, to which work we must refer the reader for the formulae and processes.

We do not insert them here, because they are, for the most part, apparently unsuited to operations on the great scale, although they are certainly, in many respects, well deserving the attention of engineers. We shall, however, avail ourselves of his instructions in the following article.