Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/511

Rh MINT 489 It will be evident that the rough classification of the fillets according to their thickness, to which reference has already been made, renders it easy to compensate for slight irregularities in thick ness caused by rolling, by employing cutters of a slightly larger diameter than the standard size for fillets which are too thin. The fillets after the removal of the disks present the perforated appearance shown in fig. 9. The residual metal, called &quot;scissel,&quot; which amounts to from 25 to 30 percent, of the metal operated upon, is returned to the melting-house in bundles weighing 180 oz. It may be mentioned here that all attempts to cut disks or blanks for coinage from the ends of rods or cylinders, and thus to avoid the production of scissel, have hitherto failed. The next operation to which the blanks are submitted varies in different mints. In some, each blank is weighed by hand or by automatic machinery, and each blank that is too heavy is adjusted either to an exact weight or to within the remedy prescribed by law. On the Continent it is very generally the practice to adjust blanks by the aid of a file, or by a machine that removes a tine shaving of metal from the surface of the blank. In mints where mechanical adjustment is adopted there is a tendency to produce &quot; too heavy &quot; blanks in the rolling and cutting departments, as it is impossible to adjust blanks which are too light. In the London mint finished coin alone is weighed, so that the blanks after leaving the cutting room pass directly to an edge-rolling machine, which thickens the edge of each blank* so as to form a rim intended to protect the impression on the finished coin. The operation of edge-rolling is called &quot;marking,&quot; and the method of conducting it varies considerably in different mints. In the Royal Mint the blanks are made to pass in quick succes sion, at the rate of six hundred a minute, between a circular groove in the face of a revolving steel disk and a groove in a fixed block placed parallel to the face of the revolving disk. The groove in the block exactly corresponds to that on the disk ; and, as the distance between the block and the disk is slightly less than the diameter of the blank submitted to the operation, the result is that before the blank escapes from the machine its edge has been thickened. The operation may be varied by admitting the blanks between a groove in the periphery of a revolving wheel and a groove in a segmented block, placed at a distance from the wheel rather less than the diameter of the blank. The wheel and block may be either vertical or horizontal. In some cases the edges of the blanks, at the same time that they are thickened, receive the impression of a legend, or inscription, or an ornamental device. When this is the case the blank is rolled Fig. 10. between two planes, one of which is fixed and bears the device, while the other has a reciprocating motion imparted to it, or the edge of the blank receives the impression, which may be either raised 1 A description of a machine used for the adjustment of blanks will be found in Dingier s Polytechnischet Journal (1882, ccxlv. 61, pi. 6); and some years ago Mr J. II. Napier devised for the Indian mints a beautiful machine which first ascertains how much it is necessary to cut from each blank in order to reduce it to the standard weight, and then removes the necessary amount of metal and no more. The initial cost of such machinery, however, is considerable. In 1849 M. Diereck, director of the mint in Paris, endeavoured to substitute a chemical for a mechanical treatment by submitting the heavy gold blanks to aqua regia, which it was anticipated would bring them within the prescribed limits of accuracy. The results were not satisfactory, and the attempt was abandoned. In 1870 the present chemist of the mint, Professor V. Chandler Roberts, showed that gold alloyed with copper might be removed from heavy blanks with singular regularity by means of a suitable solvent aided by a battery. The blanks are arranged in a frame of wood and submitted to the action of a solution of cyanide of potassium, the heavy blanks forming the dissolving pole of the battery. The process was not used in the London mint, as it became evident that it could not profitably replace the present system, under which finished coins alone are weighed and the manufacture of good coin only is paid for. It was, however, introduced into the Bombay mint in 1870 by the late Mr L. G. Hines, who extended its usefulness by transferring the metal dissolved from the heavy blanks to blanks which are too light, the latter being by this means raised to the prescribed weight. The process has now fairly taken its place as an ordinary operation of coining, and its importance to the mints where it is used may be gathered from the fact that in the Indian mints no less than 1300 tons of silver were converted into coin in one year (1879), so that the saving effected by its Introduction must be considerable. Fig. 11. or sunk, from a collar surrounding the blank in the coining press, as will be afterwards explained. Before passing to the coming press the blanks either of gold or A nneal- silver are annealed. In many mints the object of the heating is ing the not only to soften the blanks before they receive the impression, but blanks, also to produce a film of oxide of copper on their surface. This is attained in various ways. In England gold blanks are placed in cylindrical crucibles of plumbago and covered with a layer of char coal, heated in a reverberatory furnace, and when the blanks reach cherry-redness they are cooled by plunging them in water. The thin film of oxide of copper thus formed on the surface of the gold or silver blanks is readily soluble in dilute sulphuric acid, and the removal of a small portion of the alloying metal in this way con stitutes &quot;blanching&quot; or &quot;pickling&quot; the coin. The method of conducting the operation varies somewhat in different mints, mainly, however, in the strength of the acid used, which varies from 3 to 5 of the hydrometer of Baume. The solution is sometimes heated to 96 to 98 C., while in other cases the blanks are intro duced into the solution while at a red heat. The latter method is, however, objectionable, as a dense layer of pure metal is found at the surface of the blank which is apt to protect the underlying oxide of copper from the action of the acid. The blanks are afterwards washed in pure water and dried either in sawdust or in copper vessels heated by steam jackets. The object of the process is to XVI. 62