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Brass (Alloy)

1911 Encyclopedia Britannica

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BRASS (0. Eng. braes ), an alloy consisting mainly if not exclusively of copper and zinc; in its older use the term was applied rather to alloys of copper and tin, now known as bronze. Thus the brass of the Bible was probably bronze, and so also was much of the brass of later times, until the distinction between zinc and tin became clearly recognized. The Latin word aes signifies either pure copper or bronze, not brass, but the Romans comprehended a brass compound of copper and zinc under the term orichalcum or aurichalcum, into which Pliny states that copper was converted by the aid of cadmia (a mineral of zinc) .

In England there is good evidence of the manufacture of brass with zinc at the end of the 16th century, for Queen Elizabeth by patent granted to William Humfrey and Christopher Schutz the exclusive right of working calamine and making brass. This right subsequently devolved upon a body called the "Governors, Assistants and Societies of the City of London of and for the Mineral and Battery Works," which continued to exercise its functions down to the year 1710.

When a small percentage of zinc is present, the colour of brass is reddish, as in tombac or red brass, which contains about Io%. With about 20% the colour becomes more yellow, and a series of metals is obtained which simulate gold more or less closely; such are Dutch metal, Mannheim gold, similor and pinchbeck, the last deriving its name from a London clockmaker, Christopher Pinchbeck, who invented it in 1732. Ordinary brass contains about 30% of zinc, and when 40% is present, as in Muntz, yellow or patent metal (invented by G. F. Muntz in 1832), the colour becomes a full yellow. When the proportion of zinc is largely increased the colour becomes silver-white and finally grey. The limit of elasticity increases with the percentage of zinc, as also does the amount of elongation before fracture, the maximum occurring with 30%. The tenacity increases with the proportion of zinc up to a maximum with 45%; then it decreases rapidly, and with 50% the metals are fragile. By varying the proportion between 30 and 43% a series of alloys may be prepared presenting very varied properties. The most malleable of the series has an elongation of about 60%, with a tensile strength of 17.5 tons per sq. in. Increase in the proportion of zinc gives higher tensile strength, accompanied, however, by a smaller percentage of elongation and a materially increased tendency to produce unsound castings. The quality of copperzinc alloys is improved by the addition of a small quantity of iron, a fact of which advantage is taken in the production of Aich's metal and delta metal. Of the latter there are several varieties, modified in composition to suit different purposes. Some of them possess high tensile strength and ductility. They are remarkably resistant to corrosion by sea-water, and are well suited for screw-propellers as well as for pump-plungers, pistons and glands. Heated to a dull red delta metal becomes malleable and can be worked under the hammer, press or stamps. By such treatment an ultimate tensile strength of 30 tons per sq. in. may be obtained, with an elongation of 32% in 2 in. and a contraction of area of 30%.

In the arts brass is a most important and widely used alloy. As compared with copper its superior hardness makes it wear better, while being more fusible it can be cast with greater facility. It is readily drawn into fine wire, and formed into rolled sheets and rods which are machined into a huge number of useful and ornamental articles. It is susceptible of a fine polish, but tarnishes with exposure to the air; the brilliancy of the surface can, however, be preserved if the metal is thoroughly cleansed by "dipping" in nitric acid and "lacquered" with a coating of varnish consisting of seed-lac dissolved in spirit.