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

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

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.

The following are the running sizes of the strips for sheet: 31, 4, 4, 5, 6, 7, 8, 9, 10, and 12 inches wide, 18

FIG. 19.

to 28 inches long, and ğ to

inches thick. The ordinary sizes of the strips for wire are 3 inches wide, 11 inches thick, and 7 feet to 7 feet 6 inches long.

In order to obtain per

fect castings the metal must be poured at the proper temperature; for if the metal is too hot a porous casting will result, and if too cold the mould will be imperfectly filled and the metal non-coherent in parts. Two grave defects are liable to occur in castings, arising from different

FIG. 20.

When metals are

causes, viz. blowholes and spillyness. melted in ordinary crucibles and furnaces a certain amount of air penetrates the metal, and certain gases, such as hydrogen and carbonic oxide, which are generated during the process. As the metal cools these gases are gradually liberated, and this evolution is greatly facilitated by mechanical agitation, which is usually effected by vigorously stirring with an iron rod. If the metal be poured at too high a temperature into a closed mould, some gases will be retained in the metal, forming cavities, and producing a honeycombed appearance, termed by the workmen " 'spuey metal." This is especially the case in a large mass, when the surface is rapidly solidified, while the interior remains in the molten state; the means of escape for gases from the interior being thus effectually cut off. For this reason it is a disadvantage to cool

a casting too rapidly. The mould is therefore heated previous to running in the metal. The best plan of heating a mould is to have a plate of iron, which loosely fills the cavity of the mould, made redhot in a furnace, then placed inside the mould until the necessary temperature is attained. The iron plate is then withdrawn, the two halves of the mould detached, the interior surfaces oiled, and then powdered charcoal dusted on to prevent the metal sticking to the mould. Some coating material is absolutely necessary, but it is a frequent cause of defective casting, producing both unsoundness and spillyness. Now air is unavoidably carried unto the mould with the molten metal, and the oxygen unites with the carbon present to form the gas carbonic oxide; also hydrogen gas is generated by chemical change; if, therefore, such gases are not liberated before the metal solidifies, they will produce cavities in the metal. It is advisable then to use charcoal dust and oil very sparingly. The author is informed that a much better plan of blacking moulds is to use a mixture of resin and lard-oil. Three parts of resin are melted with one part of the best lard-oil, forming a viscous mass of about the consistence of treacle ; this is applied to the surface of the hot mould by means of an ordinary paint brush. By the use of this composition much better and more uniformly sound castings are obtained.

A spilly casting is produced by the admixture of impurities in the metal, causing imperfect cohesion. This may be caused by loose particles of metal in the unalloyed state, but a more common cause is the presence of charcoal or dross, run into the mould along with the metal; or charcoal and dirt detached from the sides of the mould, when oil and charcoal are used as a preventative for sticking of the metal, as stated above. Great care should always be given to properly skimming the metal, and seeing that no dirt is carried into the mould. The metal should be poured in a clear and uninterrupted stream, otherwise serious flaws may occur, and the casting be rendered useless. It was formerly