Page:EB1911 - Volume 18.djvu/588

 power required for breaking down mint bars amounts to from 25 to 35 h.p. The bars are fed to the rolls by hand. Heavy pinches are applied at first, the space between the rolls being diminished by a hand-screw after each passage of the bars through them. When the bars are nearly to gauge, light pinches are given, the power required by finishing rolls being about 5 h.p. only. The reduction in thickness of the bars is accompanied by a slight increase in their width and a very great increase in their length, so that it is generally necessary to cut partly rolled bars into two parts to keep them of convenient dimensions. By repeated passages through the rolls the bars are hardened, and to facilitate further reduction they are usually softened by annealing before being passed to the finishing rolls. In some mints the fillets are annealed frequently, the fillets for one mark pieces at the Berlin mint, for example, being annealed four times in the course of rolling. In this case the bars are reduced from 5 mm. in thickness to 1 mm. by being passed thirteen times through the rolls. At the Vienna mint the practice has been to anneal silver bars after each passage through the rolls. On the other hand, in the United States mints, the use of very carefully refined metal has made it possible to discontinue the annealing of partly rolled bars. In the Royal Mint silver bars are annealed once during rolling by passing through a Bates & Peard gas furnace. The fillets are placed on an endless chain which moves slowly through the furnace, returning underneath. At each end of the furnace is a trough of water which covers the furnace mouth, so that air is prevented from entering the furnace. The chain dips below the water, then rises into the furnace and passes down into the other trough on its way out. The result is that so long as the fillets are hot they are kept from contact with the air and blackening of the metal is prevented. In some mints the drag-bench or draw-bench is used after the rolls to equalize the thickness of the fillets. The fillet is drawn between two little steel cylinders which do not revolve and held rigidly in position. The principle resembles that used in wire drawing. It was introduced by Sir John Barton at the Royal Mint in 1816 and was abandoned there in 1905. The thickness of the fillets is measured by the gauge-plate shown in fig. 2. When they been reduced to the correct thickness they are examined by the “tryer”, who cuts out one or two blanks from each fillet with a hand machine and weighs them on a delicate balance. If the weight of the blank is slightly below the standard weight, a somewhat larger cutter is used, so that the blanks may be of correct weight. If the blank is too heavy the fillet may of course be passed through rolls again. . 2.—Gauge Plate

Remedy.—The degree of accuracy required is indicated by the “remedy” allowance for weight, which is different for each coin, and is the maximum difference from the standard weight which is allowed by law. In the sovereign it is 0·2 grain or about 1·62 per 1000. As the mean thickness of a sovereign is 0·0466 in. the remedy for weight corresponds to a difference of less than in. in the thickness of the fillet. The remedy for English silver coins varies from 2 grains or 4·58 per 1000 in the case of the crown, to 0·087 grain or 11·97 per 1000 in the case of the silver penny. The remedies for weight on foreign coins are in general greater than those allowed in the British Empire, averaging 2 per 1000 for gold coins. Reference may here be made to the similar working margin allowed in respect of the fineness of gold and silver. In England the remedy for fineness is 2 per 1000 on gold coins and 4 per 1000 on silver coins above and below the legal standard. Thus gold coins would be within the limits if they contained between 914·6 and 918·6 parts of gold per 1000. Remedies are intended to cover accidental variations from the exact standard and are now generally used only in this way. In former times, however advantage was sometimes of the remedy as a means of profit. In the reign of Queen Elizabeth the master of the Mint, finding the allowance under his contract to be insufficient, availed himself of the remedy on the silver coinage, which amounted to 6d. on the pound troy, or about 8·7 per 1000. Cutting Blanks.—The cutting machine used in the Mint is shown in fig. 3. The revolution of an eccentric A causes two short steel cylinders or cutters mounted on a block of iron B, suitably guided, to enter two holes in a plate fixed to the bed of the machine. When the fillet FF is brought above the holes, the cutters descend and force disks of metal through the holes. After each descent of the cutters, the fillet is advanced by small gripping rolls С С′ С″ worked by a ratchet wheel E driven from the shaft which bears the eccentric A. The disks fall down the tube G to a receptacle on the floor. The cutters are so placed as to remove blanks in the manner shown in fig. 4, this arrangement leaving less “scissel” or residual metal than any other. In the case of very large silver coins only one blank is cut in the width of the fillet, but bronze fillets are made wider so that three penny blanks are cut out at each stroke of the machine. The cutting machines at the Mint work at 160 revolutions per minute, so that each of the eleven machines would, be capable of cutting 19,200 blanks in an hour if it could be fed continuously. The scissel, which amounts to about 30% of the metal operated on, is returned in bundles to the melting house.

Marking.—The blanks are then passed to an edge rolling machine, by which they are thickened at the edge so as to form a rim to protect the finished coin from wear. This operation is called marking, because originally the edges were not only thickened but were also marked with an inscription. This is still done in the case of many foreign coins. The letters are sometimes sunk and sometimes raised. Like the graining or “milling” on the edge of many coins, the inscriptions were intended to put a stop to the practice of clipping and filing coins, which was prevalent in the 16th and 17th centuries. They also render the manufacture of counterfeit coin more difficult. At the Royal Mint the blanks are passed between the parallel faces of a revolving steel plate and fixed block. The plate has a circular groove in its face and the block has a corresponding curved groove. The blank passes between these grooves. The distance between the block and the plate is adjusted so as to be slightly less than the diameter of the blank, and the result is that the edge of the blank is thickened and its diameter reduced before it escapes from the machine. About 720 blanks are passed through this machine per minute. In marking machines in some foreign mints the groove is in the periphery of the revolving wheel, and the grooved block is curved (fig. 5).

Annealing and Blanching the Blanks.—The blanks are next softened by annealing, and are then thoroughly cleaned before being passed to the coining presses. In England gold and copper blanks are protected from oxidation, and after their passage through the furnace are merely washed in colanders with water and dried with sawdust in a rotating drum. Silver blanks, however, are passed through rotary gas furnaces in which no attempt is made to exclude the air. The blanks are charged into a hopper at one end of the furnace and conveyed towards the other end by a revolving Archimedean screw. The blanks fall through an aperture after having been heated for a few minutes. They are at a dull red heat and are allowed to cool gradually in the air and become blackened by the formation on the surface of a film of oxide of copper. This is removed by solution in hot dilute sulphuric acid and a layer of pure frosted silver is left on the surface, which appears dead white in colour, and has lost its metallic lustre. The operation is called “blanching.” A similar method was formerly used for gold coins in England and is still employed in some mints. The removal of part of the copper from the blank raises the percentage of silver contained in them and this is allowed for by adding an equivalent amount of copper to the metal when it is melted. The amount of copper removed from silver blanks containing 900 to 925 parts of silver per 1000 is from 0·6 to 1·0 per 1000. The process will probably be abandoned as soon as the tarnishing of the metal during rolling and annealing can be avoided.

Coining Press.—The blanks are converted into coin by receiving an impression from engraved dies. Each blank is placed on the lower of two dies and the upper die is brought down forcibly upon it. The pressure causes the soft metal to flow like a viscous solid, but its lateral escape is prevented by a collar which surrounds the blank while it is being struck. The collar may be plain, or crenated (“milled”), or engraved with some device. In the last case the collar must be made in two or more pieces, as otherwise the coin could not be removed without injury. The collar for striking English crown pieces is made in three sections now that raised lettering is put on the edge of the coin. Sunk letters, such as occur on the edges