Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/508

Rh 486 MINT which has to be guaranteed, but in the formation of an alloy composed of precious and base metals in definite pro portions. The accuracy of the &quot; standard fineness &quot; of the alloy after melting must be absolutely ascertained ; the alloy must be protected during manufacture against a change of standard, and finally its correctness must be verified after it has been converted into coin. The precious metals are weighed on entering the mint, as well as during various stages in the manufacture of coin. The finished coins are also weighed in bulk before they are issued to the public. The operations incidental to the coinage of bronze and silver differ from those described in relation to gold in some unimportant details only; and the weight and composition of the bronze coins are not so carefully guarded as is the case with gold and silver. Subjoined are the details of the operations involved in the conversion of bullion into coin at the British mint. After being assayed and weighed in the manner already described Melting the bullion is taken to the melting-house, where the details of treat- tha ment for silver and gold respectively differ somewhat. (The sub- metal, sequent operations are nearly identical for both metals.) The silver melting-house (see fig. 1) contains eight furnaces, of the kind shown at A fig. 2, the part of the furnace containing the crucibles being below the lids B, B. Crucibles of cast iron were formerly employed, but these were replaced in 1853 by wrought iron pots, which have since 1870 been in turn abandoned in favour of crucibles made of a mixture of clay and graphite, each crucible being capable of containing about 3000 oz. Such crucibles are very generally adopted throughout the Indian and Continental mints, but the form and dimensions given to them vary. The fuel employed in England is coke, about 75 tt&amp;gt; of which are re quired to melt 3000 oz. of standard silver. Sufficient draught is afforded by the flue C and by a chimney about 35 feet high which communicates with it. The silver and copper are melted together ; FIG. 1. Royal Mint, Tower Hill, London. Plan showing the Operative Department as rearranged in 1881-82. and before the metal is poured into moulds it is stirred with an iron rod having a flattened end. The surface of the molten metal is covered with a layer of charcoal to prevent oxidation of the copper. The crucible with its contents is then removed from the furnace by the aid of a crane and tongs &quot;W, and is placed in a cradle M, which can be tilted by means of a handle D. By the interven tion of toothed wheels E, F, G, H, and K acting on a rack the handle turns the crucible on the fulcrum formed by a spindle, so that the contents of the crucible may be poured into the moulds N mounted on a carriage OP, running on rails Q, Q. The moulds now in use in London are of such dimensions as to enable bars to be cast 12 inches long and f inch thick. The width of the bars varies, according to the coin to be produced, from If to 2 inches. When the metal has solidified in the moulds it is removed, and the bars are trimmed by the aid of a revolving circular file, their ends being cut off and returned to the melting pot. Portions of metal are then cut from certain of the bars, and sent to the assay department. The bars are weighed before they pass to the subse quent operations of coinage, in order that the amount of metal re tained by the crucibles or carried into the flues may be ascertained. Gold bullion is melted in a similar way, but the crucibles are smaller, and contain only 1200 oz. Their contents are poured by- hand into moulds, one end of the tongs by which the crucible is grasped being supported by a chain and suspended from the roof. 1 In many Continental mints it is very generally the practice to leave the crucible containing the precious metals in the furnace, and to pour the contents into the moulds by the aid of small ladles of wrought iron lined with clay. It has been pointed out in GOLD (vol. x. p. 751) that minute quantities of certain metals render standard gold extremely brittle and unfit for coinage. If either the gold bullion or the copper used as an alloying metal should be impure, brittle bars will be the result. Should this prove to be the case, the bars are re- 1 A new form of furnace devised by M. A. Piat of Paris has recently been introduced. In these furnaces the portion which contains the crucible may be detached from the flue, so as to admit of the molten metal being poured into moulds without removing the crucible from the incandescent fuel. Four of such furnaces have been fitted up in the gold melting-house, but have not as yet been used for gold melt ing ; in the melting of silver and bronze, however, they are known to effect considerable economy in labour, fuel, and crucibles.