The temper greatly depends on the quantity of carbon that is in the steel. This the practical man soon finds out, and he tempers or draws down the tool accordingly. In Switzerland, razors, pocket - knives, etc, made from English cast - steel, are tempered by immersing the blades at a dark cherry - red into a bath composed of 4 parts finely powdered yellow rosin, 2 of fish - oil, to which is added, in a very hot state,.. 1 of melted tallovr, and allowing them to cool perfectly; after which they are heated without wiping them, and hardened in water in the ordinary way. The blades hardened by this process are found to be more uniformly tempered than by any other, making it possible to produce an exceedingly fine cutting edge. (8) By melting together about 1 gal. spermaceti oil, 2 lb. tallow, and 1/4 lb. wax, a mixture is obtained very convenient for tempering any kind of steel articles of small size. Adding 1 lb. rosin, it is used for the tempering of larger articles. The addition of rosin must be made with care, for an excess of this material renders the steel too hard and brittle. After several months' use the mass loses its energy; it must then be wholly renewed, taking care to thoroughly cleanse the bottom of the tub which contained it.

Another mixture, of which practice likewise has proved the efficacy, consists of 20 gal. spermaceti oil, 20 lb. tallow, 10 gal. ox - foot oil, 1 lb. pitch, and 3 lb. rosin. The pitch and rosin are melted together, then the 3 other materials are successively added, and the whole is heated in an iron pot till all the water is evaporated. This is ascertained when the mass takes fire at the approach of a burning chip of wood; the flame is put out by hermetically shutting the pot with a cover. The tempering is effected in both cases as follows:- Saw - blades, for instance, are heated in special ovens, and when they have reached the temperature, they are dipped .in the mass contained in tubs arranged side by side. For a continuous manufacture a certain number of tubs are used, so as to allow the mass time for cooling during the progress of the operation. As soon as the blade is cooled, it is withdrawn from the bath and cleaned with a piece of leather, so that there remains still on it a thin layer of grease. It is then passed over a coke fire till the grease catches fire and burns with a clear smoke. In this way the blade acquires elasticity. If it is desired very hard, a part only of the grease is allowed to be burnt; the more softness is desired the more the burning is completed.

For springs, the flame is left to burn itself out. If the objects are of various forms and sizes, the burning is repeated on the several parts till all are deemed equally tempered. The blades are finished by hammering and heating them again on a clear coke fire till they return to a straw - yellow hue. The coloration is then taken away by - washing in dilute hydrochloric acid, and afterwards in plenty of water.

(9) Some mechanics attach much importance to a hardening pickle, but probably failure comes as often by injury in heating the article as by hardening and tempering. An evenly - distributed heat of the proper temperature is absolutely requisite to success, and this it is not always possible to assure by heating in an open fire. One portion of the article is liable to be overheated, while another portion is underheated; judging of the amount of heat by colour, is not always to be trusted; a dark corner or a cloudy day changes the conditions from a light shop and a sunny day sufficiently to make a great and telling difference in the amount of heat judged by right.

A perfectly . cliable method of heating for - hardening is by means of the lead bath. It is an easy matter to keep in the shop a crucible or iron pot of lead to be used as occasion demands. The article to be heated for hardening will not suffer when in the lead bath, even if not closely washed, as is necessary at the open fire; the melted lead cannot pass to a degree of heat injurious to the steel. But one condition must be strictly observed - the lead must be pure and clean; it is best to buy the mercantile pig for this purpose.

In order to harden well, it is necessary to heat the article through and through. If the piece is of unusual thickness, as a tap or reamer of 3 in. or more in diameter, it is better to drill a hole through it from end to end, so that the heating can be even and the hardening be equal. A tap of 4 in. diameter broke squarely Across in the hardening. It was of solid steel. The drilling of an inch hole from end to end was practised, and a large number of the same size taps were hardened without a failure. (Scic.t. Amer.)

(10) Firstly, the steel must only be heated to dark red, which is the temperature at which a film of soot burns off.

Secondly, the heated article must be carefully protected from oxidation; hence a' flame rich in carbon must be used, and the immersion be done as quickly as possible, so as not to keep it long in the air.

Thirdly, water used for hardening must be free from alkalies and lime carbonate. (Reichel, Zeit. f. Inatrumenten - kunde.)

(11) A great many different articles require to be hardened. The first will doubtless be a cutting - tool of some kind. Take, for example, a set of tools for metal - turning - say, gravers, chisels, and parting - tools. The tools having been filed up and prepared, a clear fire at a small forge will be the best means of heating them. The tools are held by the tang in a pair of small forge - tongs, and the blade must be heated slowly until it is at low redness. Care must be taken not to overheat the steel, or its cutting properties will deteriorate, and in that case it will be perfectly useless as a cutting - tool. When the steel is made to the desired heat, it must be plunged into cold water, which will render it perfectly hard, and if it were used in this state, the point, or cutting - edge, would immediately break off. It depends a great deal upon the material that is to be turned what heat will be the best to make the tool before dipping or plunging it; and another plan, used in some cases, is to take the extreme chill off the water. The hardening having been finished in a satisfactory way, proceed to temper it and make it fit for use.