The blanks are next softened by annealing, and are then thoroughly cleaned before being passed to the coining presses. The blanks thus cleaned are charged into a hopper at one end of the furnace and conveyed towards the other end by a revolving Archimedean screw. In a few minutes they are raised to a dull red heat and become blackened by the formation on the surface of a film of oxide of copper. They leave the furnace by falling through an aperture into water. The oxide of copper is removed by solution in hot dilute sulphuric acid, sometimes with the addition of bichromate, 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." The blanks are washed in water to remove all trace of acid and are then dried.
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.
Modern coining presses are improved forms of the lever press invented by Uhlhorn in 1839 which replaced screw presses. One of the London mint presses is shown in fig. 2. The blanks are fed by hand or by an automatic feeder into the machine which performs the rest of the operations. A blank is carried forward by a moving slide and placed exactly on the lower die by iron fingers, the "layer-on." This imitates the action of the human finger and thumb which it has superseded. The collar encircles the blank and the upper die is brought down and pressed upon it. By means of continued revolution the machine next lifts the upper die, and the lower die rises simultaneously and pushes the coin out of the collar. Lastly the layer-on pushes off the finished coin (which falls down a tube into a bowl) and places another blank on the die. Coining presses strike about 'co coins a minute.
The dies consist of hard carbon steel, and the process of making them is complicated. The original design of the artist, in the form of a large plaque of plaster of Paris, is reproduced in relief in iron or bronze by casting or in nickel by electro-deposition. An exact copy of the plaque in miniature, of the diameter of the coin to be produced, is made by a reducing machine.
The machine has two adjustable heads and on these are mounted the model or plaque and a block of soft steel. In front of them is a balanced arm, pivoted at one end, and carrying both the tracer point, which presses against the model, and the revolving cutter. The model and steel block are rotated synchronously and at the same time the free end of the balanced arm is gradually raised. The result is that the tracer and cutter are made to travel in fine spirals over all parts of the model and steel block respectively, converting the block into the "reduction" punch.
The old way of making the original steel copy of the artist's design was by hand engraving (see NumismAncs), a method still in use for slight changes in the design, as for example a change of date, and for all purposes in some mints.
Dies strike about 50,000 coins before they are worn out.
Gold and silver coins are examined individually by eye on the overlooking machine, upon which the coins are spread on a travelling belt, arranged so that both faces of the coins are exposed in succession. Each coin, except ing those of smaller denomination, is then tested by ringing and finally weighed separately by being passed over delicate automatic balances. Automatic balances for weighing single coins were in troduced at the Bank of England in
In the London mint both light and heavy coins are returned to the melting pot. The proportion of rejected gold coin varies with the quality of the bullion, and may exceed io%. The percentage of rejected silver is usually less than I %.
Periodical examinations of the coins issued by the mint have been made from very early times in England by persons appointed by the Crown. Specimens are se lected from the finished coins and are put into a box or "pyx." At intervals these coins are weighed and assayed by a skilled jury and the results reported to the Government. The trial is now held annually by the Company of Goldsmiths. (See MONEY.) BIBLIOGRAPHY.—Rogers Ruding, Annals of the Coinage of Great Britain (1840) ; F. Lenormant, La Monnaie dans l'Antiquite (1878) ; Sir W. C. Roberts-Austen, "Alloys used in Coinage" (loans. Soc. of Arts, London, 1884) ; Annual Reports of the Deputy Master of the Mint (London, 187o onwards) ; Annual Reports of the Director of the (United States) Mint (Washington). (T. K. R.; H. W. L. E.)