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furnaces heated by gaseous fuel have been made during the last thirty years, notably through the labours of Messrs. Siemens. By the use of gas-producers fuel may be used which is too inferior for ordinary furnaces, and a higher temperature attained by the combustion of the gas formed, than by using solid fuel directly. A gas-producer is generally a somewhat rectangular chamber, in which fuel is burnt for the formation of carbonic oxide as already explained. Siemens's producer, Fig. 14, is lined with fire-brick; the side A is formed of iron plates lined with fire-bricks, and has a step-grate B, with wrought-iron bars C. The fuel is charged

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through the hopper D. The gas passes up the pipe E, which is cased with iron, and issues into a horizontal wrought-iron pipe, which conveys it to the regenerator, where it is highly heated before entering the furnace in which it is burnt, thus producing a much higher temperature. The waste heat of the furnace is not lost, but utilised in heating the regenerators. The arrangement of a Siemens's crucible furnace is shown in Fig. 15, where the pots are heated by the combustion of hot gas and hot air. The regenerators are chambers of open refractory brickwork, built in pairs, two pairs being required for each furnace; each pair being used alternately for absorbing the heat of the gaseous products from the furnace, and heating the gas and air required for combustion. By means of a reversing valve, the waste gases

When the waste gases

pass to the right or left pair at will. are passing down through the right pair, the cold air and producer gas are passing up through the left pair, the direction being reversed when sufficient heat has been absorbed. It will be observed that the combustible gas and air enter the furnace alternately at the right and left, according to which pair of regenerators is being used for heating them. By using gas for melting metals a neutral or nonoxidising flame can be obtained, which prevents loss of metal by oxidation, a manifest advantage in cases where it is necessary to preserve the composition of an alloy intact, and should be of great advantage in the manufacture of such alloys as brass.

Reverberatory furnaces are only used for the manufacture of brass where large quantities require to be cast, as in the case of ingots of yellow metal used for ships' sheathing, etc. The furnace commonly employed, Figs. 16, 17, is capable

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of holding about a ton of metal. It is somewhat rectangular in shape, with flat sloping bed, which inclines from the bridges and back towards the front, or working door. The bed is formed of fire-brick carefully set edgeways, or a well rammed sand bottom is used.

It is advisable that the copper should be melted first in

an atmosphere of flame, so as to exclude the air as much as possible. Then the scrap and zinc, previously heated, are introduced, and the mixture thoroughly stirred as rapidly as possible, to prevent loss of zinc. Another plan, which finds favour with some manufacturers, is to melt the copper in

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the furnace, tap it into a large iron ladle, and add the heated scrap and zinc to the ladle, so that the metal may be poured into the moulds immediately after the necessary stirring required to mix the contents is effected.

The alloys of copper and zinc are easily formed, but when the zinc is added to the copper in the furnace the damper of the chimney should be nearly closed, and the fire should not be too brisk, since by having too high a temperature at this stage much zinc will be wasted. Moreover, when the metals are thoroughly mixed the surface should be covered with charcoal or sand, especially when it is necessary to raise the temperature before tapping. When the metal is ready for tapping the tap-hole is opened with an iron bar, and the metal run into a ladle. The surface of the metal is covered with charcoal, which keeps in the heat, and preserves the metal from oxidation by the air. The temperature of molten brass and bronze becomes rapidly lowered,

and in order to produce sound castings, no time should be lost in pouring the metal into the moulds. All currents of air should be guarded against, and all openings tending to produce them should be closed during the time of casting.

§ 52. Condensation of Zinc Fume.-In the melting or

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manufacture of brass, the zinc, being of a volatile character, is to some extent volatilised, as before mentioned, especially when a reverberatory furnace is used, so that it becomes advisable to adopt some means for its recovery. The following plan is employed in some works. A syphon

flue, Fig. 18, is placed between the furnaces and the chimney, and at the bottom of the flue is placed a tank about 3 feet wide by 4 feet long, as shown at A. This tank is filled with water, and when the heated gases from the furnaces pass over the water, a vapour arises, which causes the zinc fume to condense and settle down into the tank. Zinc fume consists largely of zinc oxide, which is periodically removed and sold to zinc smelters. A second tank is sometimes placed near the top of the syphon-flue, as shown at B, and from this tank a spray of water is allowed to continually drop, thus assisting in the condensation of zinc fume. The tank A is oval in plan, and the end projects outside the flue, and is fitted with an air-tight iron lid for cleaning purposes.

CASTING OF BRASS

§ 53. Great care and skill are required in casting brass after the alloying has taken place, as the success of the operation depends upon the discrimination displayed at this juncture. However perfectly the metal may be made in the furnace, the whole will be vitiated by an unsound or spilly casting, if the brass is required for sheet or wire. Two different modes of casting may be distinguished, viz. ingot casting, and plate or strip casting. In the former method the metal is poured into moulds producing brick - shaped ingots, which are to be remelted for ordinary castings or for further mixing. In the latter method the metal is poured into flat closed moulds, producing a strip or plate of metal to be rolled into sheets or otherwise.

§ 54. Plate or Strip Casting.-The moulds for strip casting are made of iron, and consist of two halves fastened together by a ring and wedge, so arranged as to be easily detachable from each other. The plan and section, Figs. 19, 20, will show the construction without further explanation.

The sizes of the mould vary considerably, depending upon the dimensions of the sheet or wire strip desired.

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