COMMERCIAL VARIETIES OF Bronze

§ 68. Gun-metal.—With regard to this alloy it should be stated that iron and steel have almost entirely superseded it for making guns, yet a consideration of its properties for this purpose may not be out of place in this work. For the construction of ordnance it is necessary to employ a metal which is capable of resisting great and sudden pressures, for it is calculated that in firing an ordinary cannon a pressure of 2000 atmospheres is suddenly developed; and it will readily be seen from this that a metal of peculiar strength and endurance is required for such a purpose. A vast number of experiments have been made to endeavour to arrive at the best proportions of copper and tin to employ in making an alloy for guns, but no uniform standard has been adopted for all countries, each preferring a separate mixture. It should be borne in mind that two alloys of exactly the same chemical composition may have very different properties conferred upon them by different mechanical treatment, and unless we know the manner in which a gun is finally completed, we cannot produce an article of similar properties by simply using the proportions given by chemical analysis. This is probably the reason why, in the guns of different countries and of different periods of history, a uniform standard has not been employed. The following table will illustrate this point :

The proportions best suited for guns vary from 8 to 11 parts copper to 1 of tin; the best of these proportions, according to Karsten, is 9 parts copper to 1 of tin. The alloy of 11 parts copper to 1 of tin appears uniform after sudden cooling to the unassisted sight, but when examined with a lens it appears to be composed of striated faces of a reddish alloy, mixed with a white one. If it be still more rapidly solidified by pouring into thick iron moulds, an alloy is obtained which appears perfectly uniform, even under the lens. When cooled in water, after continued strong ignition, it remains uniform; but if suffered to cool slowly after continued ignition, it becomes variable in composition, like that which has been slowly cooled after fusion. Hence the alloy which is uniform at the melting point, and likewise at a strong red heat, separates into two different alloys when slowly cooled. The large mass of a cannon cannot be cooled, even by moulds which conduct heat well, suddenly enough to prevent the separation of two distinct alloys, the one that is richer in copper solidifying first, while that which is richer in tin containing 82.3 per cent copper and 17.7 per cent tin, partly rises to the top and partly sinks into the mould.

Gun-metal must possess a considerable degree of hardness and elasticity, as in firing the gun the cartridge strikes several times against the sides, and if the metal yields permanently to the pressure thus exerted upon it, the bore gradually loses its cylindrical form after a time, and its accuracy for shooting at a given object is largely destroyed. Moreover, in firing a cannon a large quantity of gas is generated, which has a more or less corrosive action on metal, so that it is advisable to use that alloy which is least

affected by such gases, provided it has the requisite strength and endurance. In short, the properties desired for good gun-metal are that it should be very tenacious, sufficiently hard and elastic to resist distortion, indifferent to the ordinary chemical influences, thinly liquid when melted, and capable of settling down solid in the moulds when cast.

Modern practice in producing gun-metal has resulted in the exclusive use of copper and tin, as combining the above advantages in the highest degree, and although the addition of a third metal may be advantageous in strengthening one particular property, it is injurious to the required properties taken as a whole. The addition of a little phosphorus is useful in special cases, but the quantity added must be very small. In fact the phosphorus is used only as a purifier, and the amount remaining in the bronze is almost infinitesimal, and cannot be reckoned as a factor in the final alloy. In old guns many foreign ingredients are found, such as nickel, cobalt, lead, iron, bismuth, arsenic, etc.; but in many cases the amounts are so small that they may be looked upon as adventitious impurities, and not as purposely introduced into the mixture for alloying. With regard to lead, zinc, and iron, these metals have doubtless been intentionally added in certain cases, with a view to producing a given effect. The other metals mentioned are occasional impurities found in commercial copper, which in former times was much more impure than the varieties obtainable at the present time.

The term "gun-metal," as understood at the present time by many engineers and brassfounders, is not confined exclusively to alloys of copper and tin, as zinc and other metals are very frequently added; and, in fact, the term seems to be applied to any alloy in which copper largely predominates, and which possesses considerable strength and toughness. Several of the triple alloys of copper, tin, and zinc possess many excellent properties, which fit them for parts of machinery and for anti-friction metals. These will be referred to under the head of "Machine Brasses." The following table will show the composition of a few of these alloys :—

No. II is

Nos. III and No. V is hard but

No. I is tough, malleable, and tenacious. hard, somewhat unyielding, and easily broken. IV work well under the file and chisel. somewhat malleable. No. VI is hard and resisting, tough, and works fairly well with the file and chisel. No. VII is hard and easily broken, but may be filed.

The alloys are hard and brittle when the copper is less than 66 per cent of the mixture; and when the copper is reduced to 50 per cent the alloys are extremely hard and brittle. The addition of a little lead improves the above alloys for turning and filing. A sample of so-called "gunmetal," stated by the user to be very strong and durable, and used for crown-wheel escapements, gave on analysis—

An alloy prepared by Mr. Stirling, and tried in the Arsenal of Woolwich, has a resistance to flexion much greater than that of ordinary bronze; it contains

The above alloy is difficult to obtain in a sound and homogeneous state.

BELL-METAL

§ 69. The various alloys used in the manufacture of bells consist essentially of copper and tin, but in some cases other metals are added in small quantity either for cheapness, or to produce a desired quality of sound. The additional

metals chiefly used are zinc, lead, iron, and sometimes bismuth, silver, antimony, and manganese. The following table will show a few of the proportions employed :