When a piece of work will be improved by having its exterior hardened and tempered, with the interior left softer, it may be heated in melted lead, the latter being covered with charcoal to prevent its oxidation. It is an excellent plan to heat the steel in some flux. The Waltham Watch Co. heat their Hair-springs. in melted glass. The Pratt & Whitney Co. heat their taps in a mixture of equal quantities of cyanide of potash and salt. The Morse Twist - Drill Co. use a similar mixture. The object of heating in these mixtures is to prevent the loss of carbon in the steel, which is of great consequence in small or slight articles.

When a tool requires to be tempered at and near the cutting edge only, and it is desirable to leave the other part or parts soft, the tempering is performed by heating the steel for some little distance back from the cutting edge, and then immersing the cutting edge and about half the rest of the steel, which is heated to red - heat, in the water until it is cold; then withdraw the tool, and brighten the surface which has been immersed by rubbing it with a piece of soft stone (such as a piece of worn - out grindstone) or a piece of coarse emery cloth, the object of brightening the surface being to cause the colours to show themselves distinctly to indicate the state of the steel. The instant this operation has been performed, the brightened surface should be lightly brushed by switching the finger rapidly over it; for unless this is done, the colours appearing will be false colours. A piece of waste. or other material may of course be .used in place of the hand. The heat of that part of the tool which has not been immersed will become imparted to that portion which was hardened, and, by the deepening of the colours, denote the point of time at which it is necessary to again immerse the tool and quench it altogether cold.

The operation of the first dipping requires some little judgment and care; for if the tool is dipped a certain distance, and held in that position without being moved till the end dipped is cold, and the tempering process is pro - , ceeded with, the colours from yellow to green will appear in a narrow band, and it will be impossible to directly perceive when the cutting edge is at the exact shade of colour required; then, again, the breadth of metal of any one degree of colour will be so small that once grinding the tool will remove it and give us a cutting edge having a different degree - of temper or of hardness. The first dipping should be performed thus: Lower the tool vertically into the water to about one - third of the distance to which it is red - hot, hold it still for about sufficient time to cool the end immersed, then suddenly plunge it another third of the distance to which it is heated red, and withdraw it before it has had time to become more than half cooled. By this means the body of metal between the cutting edge and the part behind, which is still red - hot, will be sufficiently long to cause the variation in the temperature of the tool end to be extended in a broad band, so that the band of yellow will extend some little distance before it deepens into a red; hence it will be easy to ascertain when the precise degree of colour and of temper is obtained, when the tool may be entirely quenched.

A further advantage to the credit of this plan of dipping is that the required degree of hardness will vary but very little in consequence of grinding the tool; and if the operation is carefully performed, the tool can be so tempered that, by the time the tool has lost the required degree of temper from being ground back, it will also require re - forging or re - forming. The distance a tool requires to be heated and dipped at the first dipping, and the distance to which the transient dipping should be performed, vary so much with the substance of the metal, that no rule can be given, more than to say that the heating should be to redness for a distance of about 3 times the diameter of the steel. A great deal of the cracking occurring during hardening arises from improper dipping. Cylindrical. or square bar pieces should be dipped endwise. Blades should be dipped vertically and edgewise, with the length of the blade horizontal. The steel should be lowered vertically in the water, and for hardening should be held quite still near the bottom of the tank. The thick side should enter the water first.

The water should be heated to prevent liability to crack; a difference of 40° or 50° F. makes no practical difference in the effectiveness of the quenching water, provided its temperature is at least 20° F. above freezing - point. Soft water is better for tempering than hard water, and the water improves by age, provided it is kept clean, which is an important element. Water at 200°. F. (93° C.) will harden, and will reduce the liability of the work to crack, but it should contain 1 lb. salt per gal. in solution. (Joshua Rose, Scient. Amer.)