hardening copper, as already stated; the alloys are capable of taking a high polish; they present a beautiful metallic lustre, and with their moderate melting points, and fluidity when melted, form excellent alloys for casting. In certain proportions copper-tin alloys emit a beautifully clear sound when struck, the quality of which may be modified by slightly altering the composition of the mixture. Certain varieties of bronze containing, in addition to copper and tin, zinc, lead, manganese, iron, silicon, or phosphorus, are now largely manufactured for machine and engineering purposes. The great feature of modern bronzes is the substitution of triple and quadruple alloys for the old dual alloys. French bronzes nearly always contain the four metals, copper, tin, lead, and zinc, and in some cases small quantities of nickel, arsenic, antimony, and sulphur. Each of these elements exerts an influence on bronze in proportion to the amount present, and if such influence is prejudicial for certain uses, care must be taken in the selection of the metals employed for admixture. Impure copper is by no means a rarity in commerce, and may contain ingredients fatal to the properties of certain varieties of bronze. The difficulty of preparing alloys of definite composition is increased when scrap is remelted with new metal, unless great care is taken to keep scrap of a given quality separate from other varieties; such old metal is also liable to contain iron and other foreign metals mechanically mixed with it. Zinc in small quantity added to copper and tin has often a beneficial influence, as in casting, for instance, the metal runs thinner, fills up the moulds, and is freer from pin-holes. The zinc probably acts favourably in uniting with any oxygen which may be present, forming oxide of zinc. If the addition of zinc much exceeds what is required for this purpose, the alloy will be weaker, although harder, and the colour will more or less resemble that of brass. For this reason the amount of zinc should not exceed 2 per cent when high tenacity and elasticity are desired as important factors in the alloy. Lead alloys very imperfectly with bronze, showing a great tendency to liquate out on cooling, the greater portion being found in the lower part of the casting. A small quantity of lead is said to make the alloy more malleable and denser. The peculiar patina of a velvety black colour found on old Chinese bronzes is probably due to the presence of lead. Iron, in certain amounts, affects the properties of bronze very beneficially. It hardens the alloy and increases its resistance to wear in cases where the bronze is subjected to considerable friction, as in machinery bearings. Such alloys are paler in colour and more difficult to melt than with copper and tin alone. In small quantity iron increases the tenacity of bronze. In 1858 Parker noticed that the addition of phosphorus during the melting together of copper and tin improved the physical properties of bronze in some respects, and this addition was eventually introduced into bronze manufacture with very successful results. The action of phosphorus in phosphor-bronze is to exert a refining influence on the mixed metals, rather than to form a definite alloy of copper, tin, and phosphorus, since many samples of phosphor-bronze of excellent quality contain but the merest traces of phosphorus. During the melting of copper and tin a certain amount of oxides is formed, which, being soluble in the molten metals, exerts a weakening influence on the alloy by preventing that intimate union of the constituent metals which is necessary to give the strength, toughness, and durability for which some varieties of bronze are noted. Phosphorus has a strong affinity for oxygen, and when brought in contact with metallic oxides, such as those of tin and copper, reduces them, forming oxide of phosphorus. Now this oxide has an acid character, and readily unites with metallic oxides, which are generally basic, to form a fusible slag. This slag, being lighter than the metal and very fusible, floats on the surface and may be readily removed. If the requisite amount of phosphorus be added for the above purpose, the oxygen will be completely removed; if any excess of the required quantity be added, such phosphorus will unite with the alloy, and may become a source of weakness instead of strength. Some metallurgists have thought that the beneficial action of phosphorus is due to its combination with the copper and tin, but such is not probably the case, since, if more than a small quantity be added, the metal is hardened at the expense of toughness; but the alloy still possesses considerable tenacity, and, for special purposes, may be useful. Also, as mentioned above, chemical analysis proves that the strongest bronzes contain only minute quantities of phosphorus. Montefiori-Levi and Künzel, who introduced phosphor-bronze as a material to be used in construction in 1871, state that, besides the deoxidising influence of phosphorus on metals, it performs another very important function. In many copper-tin alloys the copper forms the only crystallised constituent, tin crystallising with great difficulty; and the alloy, in consequence of the different physical condition of the two metals, is not as solid as it would be if both the components were crystallised. Phosphorus has the power of imparting to tin a crystalline nature, which enables it to form with copper a more intimate union, and thus produce a more homogeneous alloy. If more phosphorus be used than is necessary for the purpose of deoxidation of the metals, the resulting body may be considered an alloy of crystallised phosphor-tin with copper. The question of producing various qualities of phosphorbronze depends not so much upon the quantity of phosphorus as upon the correct proportioning of the various ingredients. The alloys are generally prepared by adding a specially prepared phosphor-copper or phosphor-tin (both these metals. being sometimes used at the same time) to the bulk of the copper to be treated (see also p. 192). § 64. Phosphor-copper may be prepared in a variety of ways. (1) By dropping phosphorus upon molten copper in a crucible an alloy rich in phosphorus is obtained, forming an extremely hard, steel-gray, fusible compound. (2) By reducing phosphate of copper with charcoal, or charcoal and carbonate of soda. (3) By heating a mixture of 4 parts bone-ash, 1 part charcoal, and 2 parts granulated copper at a moderate temperature. The melted phosphide of copper separates on the bottom of the crucible, and is stated to contain 14 per cent of phosphorus. (4) By adding phosphorus to copper-sulphate solution and boiling. The precipitate is dried, melted, and cast into ingots. When of good quality and in proper condition it is quite black. (5) Copperphosphide is easily prepared by adding to a crucible 14 parts sand, 18 parts bone-ash, 4 parts powdered coal, 4 parts sodium carbonate, and 4 parts powdered glass; the whole being intimately mixed with 9 parts granulated copper. A lid is then luted on and the crucible exposed to a strong heat. The sand acts on the bone-ash, forming silicate of lime. The liberated phosphoric acid is reduced by the coal, and the phosphorus thus set free unites with the copper. (6) Montefiori-Levi and Künzel prepare phosphor-copper by putting sticks of phosphorus into crucibles containing molten copper. To avoid a too ready combustion the sticks of phosphorus are previously coated with a firm layer of copper, by placing them in a solution of copper sulphate. (7) By strongly heating in a crucible an intimate mixture of boneash, copper oxide, and charcoal, phosphor-copper is produced. 65. Phosphor-tin.-(1) When finely divided tin is heated in the vapour of phosphorus, a silvery-white, very brittle phosphide is obtained, containing about 21 per cent of phosphorus. (2) When phosphorus is dropped into molten tin combination takes place with the formation of a white phosphide, containing about 15 per cent of phosphorus. (3) By placing a bar of zinc in an aqueous solution of chloride of tin, a spongy mass of metallic tin is obtained; by placing this moist tin on the top of sticks of phosphorus in a crucible, pressing down tightly, and then exposing to a gentle heat until the flames of burning phosphorus cease, a crystalline mass of phosphor-tin is obtained. M The following excellent plan is adopted in some works for the manufacture of phosphor-copper and phosphor-tin. B Scale of 16 d In a cast-iron crucible A, Fig. 27, is placed the requisite quantity of phosphorus, then the top crucible B is tightly joined to A by means of screw clamps d d. The molten metal is poured into B and runs through the opening c on to the phosphorus. The vaporised phosphorus can only escape by passing through the molten metal, and is thus almost completely absorbed. § 66. Very small quantities of sulphur, arsenic, and antimony render bronze brittle, per cent being sufficient to modify its properties. 1 10 The physical properties of bronze depend upon the composition, mode of manufacture, mechanical treatment, and rate of cooling after heating. 1 Riche has examined a series of copper-tin alloys with regard to fusibility, liquation, and changes of density resulting from certain operations. The alloys having the chemical formulæ SnCu, and SnCu are the only ones which melt and solidify without decomposition, and their melting points lie between 600° and 700° C.; all other alloys of tin and copper undergo liquation at the moment of solidification. The several alloys, in quantities of 500 to 700 grammes, were fused for ten hours in tubular moulds, and the top and bottom portions of the castings were analysed. Another portion of each of the melted alloys was stirred during solidification, and the portion which last remained fluid was poured off and likewise analysed. The following table gives the results: 1 Ann. Chim. Phys. (4) vol. xxx. p. 351. |