CHAPTER III

BRONZE

§ 63. The term "bronze" will be applied in all alloys consisting chiefly of copper and tin. have been known from very remote times, and th of the mixture of copper and tin appears to hav the first discoveries of the metallurgist. In various kinds were fabricated from these alloys, were made with a keen cutting edge, harder t almost rivalling that of steel. The bronzes of varied considerably in the proportions of the for in the main copper and tin only were used, the purposes for which they were intended. other ingredients were added, either purposely given effect, or it may be, in some cases, that than copper and tin were present as accidental This would arise from the use of impure met largely from ores of copper or tin associated wit which is often the case.

Of late years very great attention has been the study of copper-tin alloys, and those propor constituents which have been found by experie the greatest strength and the keenest cutting ed same as those used by the Greeks and Roman weapons of war and of the chase. The effect wa by causing the bronze to undergo a process of han well as a method of hardening, by heating and sl

Many ancient coins were made of bronze, containing in some cases lead, zinc, or iron. The following table will show the composition of some ancient bronzes :

It will be observed from an examination of the foregoing table that the principal constituent of bronze is, in all cases copper, the other components being added to harden of otherwise modify its properties, according to the purpose for which the alloy is intended. Tin has the property o

hardening copper, as already stated; the all

of taking a high polish; they present a bea lustre, and with their moderate melting point when melted, form excellent alloys for castin proportions copper-tin alloys emit a beautiful when struck, the quality of which may be slightly altering the composition of the mixt varieties of bronze containing, in addition to c zinc, lead, manganese, iron, silicon, or phosph largely manufactured for machine and engineer

The great feature of modern bronzes is th of triple and quadruple alloys for the old French bronzes nearly always contain the copper, tin, lead, and zinc, and in some cases sm of nickel, arsenic, antimony, and sulphur. E elements exerts an influence on bronze in prop amount present, and if such influence is pr certain uses, care must be taken in the sele metals employed for admixture. Impure cop means a rarity in commerce, and may contain fatal to the properties of certain varieties of b difficulty of preparing alloys of definite co increased when scrap is remelted with new n great care is taken to keep scrap of a given qua. from other varieties; such old metal is also liabl iron and other foreign metals mechanically mixed

Zinc in small quantity added to copper and t a beneficial influence, as in casting, for instance runs thinner, fills up the moulds, and is freer from The zinc probably acts favourably in uniting with which may be present, forming oxide of zinc. If t of zinc much exceeds what is required for this p alloy will be weaker, although harder, and the more or less resemble that of brass. For this amount of zinc should not exceed 2 per cent tenacity and elasticity are desired as important fac 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, s will unite with the alloy, and may become a ness instead of strength. Some metallurgist that the beneficial action of phosphorus is due tion with the copper and tin, but such is not p since, if more than a small quantity be adde hardened at the expense of toughness; but possesses considerable tenacity, and, for sp may be useful. Also, as mentioned above, ch proves that the strongest bronzes contaiquantities of phosphorus. Montefiori-Levi an introduced phosphor-bronze as a material construction in 1871, state that, besides t influence of phosphorus on metals, it performs important function. In many copper-tin allo forms the only crystallised constituent, tin cry great difficulty; and the alloy, in consequence physical condition of the two metals, is not would be if both the components were crysta phorus has the power of imparting to tin nature, which enables it to form with co intimate union, and thus produce a more homog

If more phosphorus be used than is neces purpose of deoxidation of the metals, the resulti be considered an alloy of crystallised phosphor-tim The question of producing various qualities bronze depends not so much upon the quantity o as upon the correct proportioning of the various The alloys are generally prepared by adding prepared phosphor-copper or phosphor-tin (both being sometimes used at the same time) to the copper to be treated (see also p. 192).

§ 64. Phosphor-copper may be prepared in ways. (1) By dropping phosphorus upon molte a crucible an alloy rich in phosphorus is obtain an extremely hard, steel-gray, fusible compound