" He that will a good Edge win. Must Forge thick and Grind thin."
Thirdly, the heat for tempering or letting down. Between the extreme conditions of hard and soft steel there are many intermediate grades, the common index for which is the oxidation of the brightened surface, and it is quite sufficient for practice. These tints, and their respective approximate temperatures, were tabulated by Mr. Stodart.
Very pale straw yellow ....
Tools for metal.
A shade of darker yellow .
Darker straw yellow ....
Tools for wood and screw taps, etc.
Still darker straw yellow .
A brown yellow . . .
Hatchets, chipping chi-sela, and other percua-sive tools, saws, etc.
A yellow, tinged slightly with purple .
Light purple . . .
Dark purple . . .
Dark blue . . .
Paler blue . . .
Too soft for the above purposes.
Still paler blue . . .
Still paler blue, with a tinge of green .
* Mr. Jordan informs me that in making the magnets for Fox's dipping Needles, which are about ten inches long, one fourth of an inch wide, and the two hundredth respectively reach their boiling temperatures and are evaporated in the gaseous form.
The first tint arrives at about 430° F., but it is only seen by-comparison with a piece of steel not heated: the tempering colours differ slightly with the various qualities of steel.*
The heat for tempering being moderate, it is often supplied by the part of the tool not requiring to be hardened, and which is not therefore cooled in the water. The workman first hastily tries with a file whether the work is hard; he then partially brightens it at a few parts with a piece of grindstone or an emery stick, that he may be enabled to watch for the required colour; which attained, the work is usually cooled in any convenient manner, lest the body of the tool should continue to supply heat. But when, on the contrary, the colour does not otherwise appear, partial recurrence is had to the mode in which the work was heated, as the flame of the candle, or the surface of the clear fire applied if possible, a little below the part where the colour is to be observed, that it may not be soiled by the smoke.
A very convenient and general manner of tempering small objects, is to heat to redness a few inches of the end of a flat bar of iron about two feet long; it is laid across the anvil, or fixed by its cold extremity in the vice; and the work is placed on that part of its surface which is found by trial to be of the suitable temperature, by gradually sliding the work towards the heated extremity. In this manner many tools may be tempered at once, those at the hot part being pushed off into a vessel of water or oil, as they severally show the required colour, but it requires dexterity and quickness in thus managing many pieces.
Vessels containing oil or fusible alloys carefully heated to the required temperatures have also been used, and I shall have to describe a method called "blazing off" resorted to for many articles such as springs and saws, by heating them over the naked fire until the oil, wax, or composition in which they have been hardened ignites; this can only occur when they part of an inch thick, this precaution entirely failed; and the needles assumed all sorts of distortions when released from between the stiff bars within which they were hardened. The plan was eventually abandoned, and the magnets were heated in the ordinary way within an iron tube, and were set straight with the hammer after being let down to a deep orange or brown colour. Steel however is in the best condition for the formation of good permanent magnets when perfectly hard. * The knife edges, for Captain Rater's experimental pendulum, were very carefully hardened and tempered in a bath heated to 430 deg.; being then found too soft they were re-hardened, and tempered, at only the heat of boiling water, after which they were considered admirably suited to their purpose.
The period of letting down the works is also commonly d for correcting, by means of the hammer, those distortions which so commonly occur in hardening; this is done upon the anvil, either with the thin pane of an ordinary hammer, or else with a hack-hammer, a tool terminating at each end in an obtuse chisel edge which requires continual repair on the grindstone.
The blows are given on the hollow side of the work, and at right angles to the length of the curve; they elongate the concave side, and gradually restore it to a plane surface, when the blows are distributed consistently with the positions of the erroneous parts. The hack-hammer unavoidably injures the surface of the work, but the blows should not be violent, as they are then also more prone to break the work, the liability to which is materially lessened when it is kept at or near the tempering heat, and the edge of the hack-hammer is slightly rounded.