Brass is harder than copper, and therefore better adapted to resist wear and tear. It acts well under the influence of a percussive force, as in the process of stamping, provided it is suitably annealed at proper intervals, in order to counteract the effects of local hardening, due to the compression of the particles into what may be termed unnatural positions. During the ordinary process of annealing the metal becomes coated with a scale of oxide, by union with the oxygen of the air, which oxide requires to be removed at each stage. This is done by dipping the metal in aquafortis, or dilute sulphuric acid, then scouring with sand if necessary, and finally well rinsing in water. The melting point of brass is less than the mean of the melting points of its constituents, and this moderate melting point is of the utmost importance in the case of remelting, for casting or other purposes. The melting point of zinc is very much below that of copper, and when an alloy of these metals is strongly heated, the zinc volatilises, while the copper remains for the most part fixed, so that the loss of zinc will be considerable if the temperature is raised too high, and the operation of melting unduly prolonged; moreover, the affinity of zinc for oxygen is far greater than that of copper, and if air is freely admitted a considerable portion of the zinc will be oxidised, so that the metal should be excluded from the air as much as possible, by covering it with a layer of charcoal, or other substance, which has no chemical action upon the metal. The easy fusibility of brass, and its fluidity when melted, render it valuable for casting, as it is capable of receiving very fine impressions from the mould. Cast brass is generally more or less crystalline, which is very pronounced in the brittle varieties. The formation of copper-zinc alloys is generally attended with contraction, which attains its maximum in the alloys CuZn, and CuZn,, containing 39.2 and 32.6 per cent of copper respectively. These alloys are brittle, and exhibit none of the characteristic properties of the constituent metals. The density of brass is increased by mechanical treatment, but this effect is partly annulled by sudden, and still more by slow cooling.1 In mixing brass for casting, old copper from wornout articles, scrap brass, etc., are frequently used, and as these often contain injurious ingredients, which modify the properties of the alloy, great care should be exercised in the selection. For some kinds of casting this is not important, but when the metal is required for rolling into sheet, or drawing into wire, or for making the best kinds of brass tubing, the use of impure metal is often fatal, entailing a considerable amount of expense on account of waste, and much annoyance to those concerned. The most common impurities, as already stated, are lead, tin, iron, and arsenic, all of which harden the metal and tend to make it brittle. For brass intended for filing and turning, 1 to 2 per cent of lead is added, in order to prevent the unpleasant fouling of the tools in working. Brass containing lead should be very thoroughly mixed before pouring, and the cast metal should be cooled as quickly as is expedient, otherwise the lead separates out in the lower portion of the casting, producing unsightly spots. A little tin is often an advantage in brass; it renders the metal more easily fusible, less brittle, somewhat sounder, and enables it to take a better polish. A little iron considerably increases the hardness of brass, lightens the colour, and such metal is more easily tarnished by the atmosphere than brass free from iron. When an ingot of ordinary brass is broken while hot, its fracture is coarsely fibrous, but when broken while cold, it should be finely granular. When the fracture of a cast ingot of certain metals is fibrous, the directions of the fibres will be at right angles to the cooling surface. In the case of a sphere, the fibres will have the direction of radii; and in the case of a square, two diagonals will be plainly visible on the transverse fracture, formed by the points of junction of the internal extremities of the fibres.2 Mr. F. H. Storer 3 Riche, Ann. Chim. Phys. (4), xxx. 2 Percy's Metallurgy, p. 608. 3 Mem. of Amer. Acad. 1860 (8), p. 35. states "that the tendency to shoot into fibres extends from alloys containing 57 or 58 per cent of copper down to those containing 43 to 44 per cent, where it gradually disappears. The inclination to form fibres is strongest in those alloys which contain nearly equal atomic proportions of copper and zinc, being less clearly marked as one recedes in either direction from this point, until a stringy texture, analogous to that of copper, is reached on one hand, and the peculiar pastiness of zinc on the other. In preparing crystals, this pastiness manifests itself decidedly in the alloys immediately below those which are fibrous, becoming more strongly marked as the alloys are richer in zinc. The fracture of these white alloys is for the most part vitreous or glassy." Brass is occasionally obtained in crystals. Storer prepared the most perfect individual crystals from brasier's solder, which consists of equal parts by weight, of copper and zinc, and occurs in the state of coarse powder, produced by heating the alloy to a sufficient degree and pounding it in a mortar while hot. The alloy containing 5 to 6 per cent of zinc was found to crystallise remarkably well. It will be seen from the above remarks that the crystalline condition of copper-zinc alloys does not depend on an excess of zinc, as might be presupposed from the highly crystalline character of zinc. As before stated, zinc becomes malleable when worked at a temperature of from 100° to 150° C., but at higher temperatures it again becomes brittle. It is assumed that the brittleness is intimately connected with the crystalline condition. Kalischer examined different varieties of sheet brass having the following composition : Nos. I and II were crystalline, No. III showed traces of crystallisation, and No. IV did not become crystalline even In the above specimens no crystallisation could be detected. The tensile strength of copper-zinc alloys is widely divergent; the greatest strength, according to Mallet, is obtained in the alloy containing 32.85 per cent copper, but this is very probably a mistake, and the greatest tenacity is in alloys containing upwards of 50 per cent copper. § 24. The following table of the composition and properties of copper-zinc alloys is taken from Mr. Mallet's table and Karsten's observations 1 : 1 Percy's Metallurgy, p. 611. |