THE MAKING OF STEEL DIES

For stamping coins or medals, for impressing banknote plates, and copper cylinders for calico printing, is an art of much importance. It requires considerable skill, time and expense, to make such dies; all of which may be lost by imperfect material, or mismanagement in hardening or tempering. The first requisite to success is the selection of the steel. Cast-steel is in all cases the best; and it should be cast-steel which has been manufactured at a low heat, well-cemented, and made of the best materials. All steel, without an exception, contains veins of unequal hardness. Natural steel is the worst in this respect; blistered and shear are not much better; and even the best cast-steel is not exempt from this characteristic. These veins are generally the cause of cracks. The steel, before it is selected for these operations, is carefully washed over with dilute nitric acid, or aquafortis, which causes the damask veins or spots to appear at the surface. Steel for dies should be entirely free of such veins, and more particularly of cracks and ash-holes; for detecting which latter, a lens is required.

In cautiously and slowly tempering steel, the hard

veins and spots may be concealed, especially if it has been tempered in charcoal; but they will appear again in heating and forging the steel. These veins are less apparent in hardened steel, and would, in fact, be of but little consequence to the engraver, were it not for their greater liability to crack and fly than uniformly grained steel. Very much depends upon the die-sinker; he can spoil the best steel by faulty work; that is, by overheating, or heating too often. Steel generally, and particularly this kind of steel, ought to be forged by the lowest possible heat- as little as it can be done with, and no more. After the steel has been selected and forged into rolls, or dies of the desired form, it is annealed. The common way of annealing is to imbed the steel in coarse charcoal powder, in a crucible or iron pot, heat it to a cherry-red heat, and let the fire slowly go out, while the steel is in it. Animal coal is frequently substituted for charcoal, or mixed with it; but one is as good as the other: the time which the steel remains in the fire is generally too short for the mixture to act upon it. This annealing is of the utmost consequence in the subsequent engraving operation, and also in hardening, and ought to be extended to the proper period; six, or even twelve hours, are not sufficient to anneal steel to perfection.

A low heat for twenty-four hours, or even twice that time, is not too much.

When dies are engraved, they are next hardened ; but as the face of the engraving is to be faithfully preserved, it is protected by being covered over with a mixture of lamp-black and linseed oil. The whole is then imbedded in charcoal powder, in a pot, as in annealing, and finally plunged into cold spring-water, where it is rapidly moved about; or it may be cooled under a current of water.

As such dies will not safely bear twice hardening, the heat by which that particular kind of steel assumes its greatest hardness is to be ascertained by experiments upon a piece cut from the bar; the die is then subjected to that heat. Dies and heavy bodies of steel are naturally exposed to cracks in hardening, resulting from its expansion. The interior of a body of steel cannot shrink as much as the exterior, because it is protected by the surface steel. Nor can the hardening be of the same degree in the interior as at the surface.

For the reasons we have given, we may conclude that all round bodies of steel are more or less fractured at the periphery; and experience, under all circumstances, will prove the correctness of this conclusion.

To prevent breakage as the result of these cracks, steel is to be tempered as soon as possible after hardening, taking care that no impurities of any kind are in the water, which might fill the invisible crevices. Round bodies, such as dies and similar articles, may be tempered by fitting a wrought-iron ring around them, first heating the ring to redness, and inserting the die or other object in it; the ring, in cooling, will firmly compress the die, and secure it against subsequent flying. When the die thus inserted receives its proper temper, which is indicated by the colour, it is thrown into cold water, or water of 60° or 80°, and cooled. After tempering, the die is boiled in water for some hours, and suffered to cool slowly in the water. This process increases its tenacity considerably, and makes the hardening and strain more uniform throughout the body of the steel.

The liability of dies and other engraved steel instruments to break in hardening, or oftentimes hours after hardening, is rather a serious matter; for it may cause great loss to an artist. Every kind of -steel is not liable to shrinkage, and consequently less liable to breaking. Steel containing much carbon is more liable to crack than where it is of a less carbon

iferous quality. The practice of imbedding steel in

animal or wood-charcoal, is therefore not judicious when steel is saturated with carbon, as is the case with the not-welding cast-steel. Steel with hard and soft spots or veins is also more liable to breakage than uniform steel. The latter steel generally contains less carbon than other steel of the same hardness; slow tempering in hard charcoal will make it more uniform, and be a guard against cracks. Crude German steel does not shrink, and, if moderately heated and hardened, will not crack; but if heated to such an extent as to acquire its full degree of hardness, it becomes very brittle. The steel made of this crude material shrinks and cracks, though not so much as cast-steel; still, it never assumes that uniform hardness and tenacity which characterize the last-named variety.

A number of plans have been devised to avert the danger of breaking dies, matrices and die-rollers, in hardening them; but there is nothing better or more safe than slow and careful annealing, gentle heat in hardening, clear hard spring-water, and time and patience in tempering. The roller-dies for bank-note plates, and copper calico-printing rollers-an invention of the late Jacob Perkins, of Massachusettsare hardened in this simple manner, the often very delicate engraving being protected by a chalk paste,