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by the number 2, being in the descending scale, half-gallons, quarts, pints, half-pints, and gills; and in the ascending scale, pecks, half-bushels, and bushels. And that, for greater precision, the last, or measure of eight gallons, shall contain at the mean pressure of the atmosphere at the level of the sea, eighty pounds of distilled water at its maximum of density.
On the Manufacture of Dies for Medals and Coinage. By W. T. Brande, F.R.S., Prof. Chem. R. I., &c.
[The subject of this discourse is not, I apprehend, of any general interest, but it has, as far as I know, been no where particularly described, if we except a brief notice respecting it annexed to Mr. Mushet's valuable paper on the art of coinage, printed in the supplement to the Encyclopædia Britannica. I therefore thought it worthy the attention of the members of the Royal Institution, and trust that the following outline will not be unacceptable to the readers of the Journal.]
THE first circumstance that claims particular attention in the manufacture of dies is the selection of the best kind of steel for the purpose, and this must in some measure be left to the experience of the die-forger, who, if well skilled in his art, will be able to form a tolerably correct judgment of the fitness of the metal for the purpose, by the manner in which it works upon the anvil. It should be rather fine-grained than otherwise, and above all things perfectly even and uniform in its texture, and free from spots and patches finer or coarser than the general mass. But the very fine and uniform steel, with a silky fracture, which is so much esteemed for some of the purposes of cutlery, is unfit for our present purpose, from the extreme facility with which it acquires great hardness by pressure, and its liability to cracks and flaws. The very coarse grained, or highly crystalline, steel, is also equally objectionable; it acquires fissures under the die-press, and seldom admits of being equally and properly hardened. The object, therefore, is to select a steel of a medium quality as to fineness of texture, not too easily acted upon by dilute sulphuric acid, and exhibit
ing an uniform texture, when its surface is washed over with a little aqua-fortis, by which its freedom from pins of iron, and other irregularities of composition, is sufficiently indicated.
The best kind of steel being thus selected, and properly forged into the rough die, it is softened by very careful annealing, and in that state, having been smoothed externally, and brought to a table in the turning lathe, it is delivered to the
The process of annealing the die consists in heating it to a bright cherry red, and suffering it to cool gradually, which is best effected by bedding it in a crucible or iron pot of coarselypowdered charcoal, that of animal substances being generally preferred. In this operation it is sometimes supposed that the die, or at least its superficial parts, becomes super-carbonized, or highly-converted, steel, as it is sometimes called; but experience does not justify such an opinion, and I believe the composition of the die is scarcely, certainly not materially, affected by the process, for it does not remain long enough in the fire for the purpose.
The engraver usually commences his iabours by working out the device with small steel tools, in intaglio; he rarely begins in relief (though this is sometimes done); and having ultimately completed his design, and satisfied himself of its general effect and correctness, by impressions in clay, and dabs, or casts,† in soft metal, the die is ready for the important operation of hardening, which, from various causes, a few of which I shall enumerate, is a process of much risk and difficulty; for should any accident now occur, the labour of many months may be scriously injured, or even rendered quite useless.
The process of hardening soft steel is in itself very simple,
The art of forging dies requires much practice and experience, not only as to the choice of steel, but as to the manual part of the operation. They should be forged at a high heat, and great care should be taken to give a perfect and dense texture to the upper part of the die.
Type metal is usually employed for this purpose. The die is warmed, and placed in a convenient box, in which the melted metal may be splashed, or dabbed, upon it. The impression is often extremely clear and perfect, and exceeds in effect those which are taken in wax or plaister,
though not very easily explained upon mechanical or chemical principles. We know by experience, that it is a property of this highly valuable substance, to become excessively hard, if heated and suddenly cooled; if, therefore, we heat a bar of soft malleable and ductile steel red hot, and then suddenly quench it in a large quantity of cold water, it not only becomes hard, but fragile and brittle. But as a die is a mass of steel of considerable dimensions, this hardening is an operation attended by many and peculiar difficulties, more especially as we have at the same time to attend to the careful preservation of the engraving. This is effected by covering the engraved face of the die with a protecting paste, composed of fixed oil of any kind, thickened with powdered charcoal: some persons add pipe-clay, others use a pulp of garlic, but pure lamp-black and linseed oil answer the purpose perfectly. This is thinly spread upon the work of the die, which, if requisite, may be further defended by an iron ring; the die is then placed with its face downwards in a crucible, and completely surrounded by powdered charcoal. It is heated to a proper temperature, that is, about cherry red, and in that state is taken out with proper tongs, and plunged into a cistern of cold water, of such dimensions as not to become materially increased in temperature; here it is rapidly moved about, until all noise cease, and then left in the water till quite cool. In this process it should produce a bubbling and hissing noise; if it pipes and sings, we may generally apprehend a crack or fissure.
I have found no process answer better than the above simple and common mode of hardening dies, though I have given others repeated and fair trials. It has been proposed to keep up currents and eddies of cold water in the hardening cistern, by means of delivery-pipes, coming from a height; and to subject the hot die, with its face uppermost, to a sudden and copious current of water, let upon it from a large pipe, supplied from a high cistern; but these means have not in any way proved more successful, either in saving the die, or in giving it any good qualities. It will be recollected, from the form of the die, that it is necessarily only, as it were, case-hardened, the hardest strata being outside, and the softer ones within, which envelope a core, something in the manner of the successive
coats of an onion; an arrangement which we sometimes have an opportunity of seeing displayed in dies which have been smashed by a violent blow.
The hardening having been effected, and the die being for the time safe, some further steps may be taken for its protection; one of these consists in a very mild kind of tempering, consisting in putting it into water gradually raised to the boiling point, till heated throughout, and then suffering it gradually to cool. This operation renders the die less apt to crack in very cold weather. A great safeguard is also obtained by thrusting the cold die into a red-hot iron ring, which just fits it in that state, and which, by contracting as it cools, keeps its parts together under considerable pressure, preventing the spreading of external cracks and fissures, and often enabling us to employ a split die for obtaining punches, which would break to pieces without the protecting ring.
If the die has been successfully hardened, and the protecting paste has done its duty, by preserving the face from all injury and oxydizement, or burning, as it is usually called, it is now to be cleaned and polished, and in this state constitutes what is technically termed a MATRIX: it may of course be used as a source of medals, coins, or impressions, but it is not usually thus employed, for fear of accidents happening to it in the coining press, and because the artist has seldom perfected his work upon it in this state. It is, therefore, resorted to for the purpose of furnishing a PUNCH, or a steel impression in relief. For this purpose a proper block of steel is selected, of the same quality, and with the same precautions as before, and being carefully annealed, or softened, is turned like the matrix, perfectly true and flat at the bottom, and obtusely conical at top. In this state, its conical surface is carefully compressed by powerful and proper machinery upon the matrix, which being very hard, soon allows it to receive the commencement of an impression; but in thus receiving the impression, it becomes itself so hard by condensation of texture, as to require during the operation to be repeatedly annealed, or softened, otherwise it would split into small superficial fissures, or would injure the matrix. Much practical skill is therefore required in taking this impression, and the punch, at each
annealing, must be carefully protected, so that the work may not be injured.
Thus, after repeated blows in the die-press, and frequent annealing, the impression from the matrix is at length perfected, or completely up, and having been touched up by the engraver, is turned, hardened, and collared, as the matrix, of which it is now a complete impression in relief, and, as we have before said, is called a punch.
This punch becomes an inexhaustible source of dies, without further reference to the original matrix; for now by impressing upon it plugs of soft steel, and by pursuing with them an exactly similar operation to that by which the punch itself was obtained, we procure impressions from it to any amount, which of course are fac-similes of the matrix, and these dies being turned, hardened, polished, and, if necessary, tempered, are employed for the purposes of coinage.
The distinction between striking medals, and common coin, is very essential, and the work upon the dies accordingly adjusted to each. Medals are usually in very high relief, and the effect is produced by a succession of blows; and as the metal in which they are struck, be it gold, silver, or copper, acquires considerable hardness at each stroke of the press, they are repeatedly annealed during the progress of bringing them up. In a beautiful medal, which, as well as the dies for it, I have had lent me by Mr. Wyon, and which he has just completed for the Royal Naval College, the obverse represents a head of the King, in very bold relief; it required thirty blows of a very powerful press to complete the impression, and it was necessary to anneal each medal after every third blow, so that they went ten times into the fire for that purpose. In striking a coin or medal, the lateral spread of the metal, which otherwise would ooze out as it were from between the dies, is prevented by the application of a steel collar, accurately turned to the dimensions of the dies, and which, when left plain, gives to the edge of the piece a finished and polished appearance; it is sometimes grooved, or milled, or otherwise ornamented, and occasionally lettered, in which case it is made in three separate and moveable pieces, confined by a ring, into which they are most accurately fitted, and so adjusted that the metal may be forced into the