BRASS

BRASS, in Chemistry and Manufactures, an alloy of copper and zinc. This name, however, has not been exclusively applied to the alloy of these metals; for the gun-metal, which has been also called brass, is an alloy of copper with tin. The same alloy, with more tin, is used in machinery, and is preferred to the alloy of copper and zinc, on account of its greater hardness. Different kinds. It appears from the analysis of the brass of the ancients, that it was an alloy of copper and tin. A small portion of tin gives to copper great hardness, and renders it capable of bearing much greater resistance. A larger portion of tin gives increased hardness, but is less fitted to bear a straining resistance, on account of its brittleness. Its elasticity is very great, which fits it for bells. In this state it is called bell-metal; with a still greater proportion of tin, it forms an alloy employed for the mirrors of reflecting telescopes. The alloy of copper with tin is easily distinguished from that with zinc from the agreeable colour of the latter, which varies with the proportions of the metals. Pinchbeck has the least proportion of zinc. Common brass has more zinc, and the gold-coloured alloy called Prince's metal, contains a still greater proportion of zinc. An alloy of copper with a very large proportion of zinc is used for the common white metal buttons. Proportions of Alloys. These various alloys of copper with tin and zinc forming the different kinds of brass, are to be considered as chemical compounds, and, of course, governed by the same laws of definite proportions which belong to the more conspicuous compounds. On these principles, which cannot be doubted, we have an unerring rule for uniting these and other metals in the best proportions, the weights of their atoms being previously known. See the article ATOMIC THEORY in this Supplement. The weight of the atom of copper being 8, tin 7.35, and zinc 4; the following table will exhibit the proportions of the various alloys, expressed in atoms, and their proportions by weight, the third column pointing out the colour and character of the resulting compound. CZ and T are to represent the atoms of the metals respectively. COMPOUNDS OF ZINC WITH COPPER. Atoms. Proportions by Weight. Character and Colour of the Compounds. C+Z 1 to 2 The best proportions for common brass. C+2Z 1 to 1 The alloy called Prince's Metal, of a beautiful gold colour. C+3Z 2 to 3 Of a paler yellow, very little malleable. C+4Z 1 to 2 Still of a lighter colour, and not malleable. C+5Z 2 to 5 Yellowish white and brittle. C+6Z 1 to 3 Very brittle, nearly white. 2C+Z 4 to 1 A very malleable brass used in watch-work. 3C+Z 6 to 1 An alloy much harder than copper and inclining to its colour. COMPOUNDS OF TIN WITH COPPER. Atoms. Proportions by Weight. Character and Colour of the Compounds. T+C 11 to 12 A very brittle and rather white alloy. 2T+C 11 to 6 Still more brittle and more white. 3T+C 11 to 4 Very white, used for speculums. 4T+C 11 to 3 Coarse-grained and too brittle for any purpose. T+2C 11 to 24 A yellowish alloy, very hard and sonorous. T+3C 11 to 36 Bell metal. T+4C 11 to 48 A very hard alloy used for some culinary vessels. T+5C 11 to 60 Softer but not malleable. T+6C 11 to 72 Still increases in softness and of a yellower colour. T+7C 11 to 84 Used for some purposes in machinery. T+8C 11 to 96 An alloy used for cannon. T+9C 11 to 108 More common for cannon and machinery, and used for bronze statues. Hitherto the proportions of these alloys have depended upon the caprice of workmen, obtained by numerous trials; and what confirms the law of definite proportions, is proved by the necessity of adhering to such fixed proportions, ascertained by trials. By attending to the stages of composition pointed out in the above table, the most striking and proper compounds will be produced, without the trouble of trying. Any proportions intermediate will, doubtless, be marked by defective colour, irregular crystallization, or imperfect malleability, in such as are expected to be so. Although the most direct way of forming these different kinds of brass is by immediately combining the metals together, one of them, which is most properly called brass, was manufactured long before zinc, one of its component parts, was known in its metallic form. The ore of the latter metal was cemented with sheets of copper, charcoal being present. The zinc was formed and united with the copper, without becoming visible in a distinct form. The same method is still practised for making brass, which we are about to describe. The materials used in making brass are, copper in Brass: small rounded masses produced by passing the melt-making metal through an appropriate vessel into water, in which state it is called shot copper, and calamine, an ore of zinc. This latter substance is a carbonate of zinc, often containing some oxyd of iron, which gives it a reddish appearance. As it is chiefly found with lead, the lumps frequently contain more or less galena, which requires to be separated by the same means employed for purifying lead ore. The calamine is first reduced to powder, and the lead is then separated by washing. When the calamine is separated, reduced to powder, and sifted, it is heated upon the hearth of a reverberatory furnace. This expels the volatile matter, which is principally water and carbonic acid. What remains is principally oxyd of zinc, abounding with some earthy matter, and probably much carbonic acid, which is not all expelled by the heat. The calamine thus prepared, charcoal powder, and copper, are the materials to be operated upon. The proportion in which they are mixed together, are equal weights of copper and prepared calamine, and \frac{1}{10} their weight of powdered charcoal. This mixture, intimately blended, is compressed into a crucible of the form of fig. 3. Plate XXXVI.* One of these crucibles holds about 100 lbs. of brass, when the process is finished; but as this consists of the pure copper and zinc, the pot, when charged, will contain of copper 66.3 lbs., of calamine 63. lbs., and of charcoal powder 13 lbs. When the crucible is filled, the contents should be covered with a mixture of clay sand and horse-dung, in order to defend the metals and charcoal from the action of the air. When this covering is strictly attended to, less charcoal powder might be employed, and a larger dose of the other ingredients might be put in its place, but it is generally the most defective part of the process. Having charged the pots, we will now describe the furnace which has to receive them. Fig. 1. Plate XXXVI.* is a plan of the furnace. The part AB is taken at the level EF, showing the opening into the furnace on the ground floor at a and b: c and d are horizontal flues leading to the chimney, f, and can be cut off from the same by the dampers seen in the dark part of the flue. CD, in the same figure, is a plan on the level GH, where the pots rest upon the cast iron plate on bottom x, y. Fig. 2. is an elevation and section of the same furnace. AB shows a front view of the pyramidal chimney, and the archway opening into it. CD is a section of the same, through the middle of the fire-place II. R, P, Q, is a vaulted passage going across the building, and open at both ends, for the admission of air, which passes through the openings in the arch, through the fires. The bottom of the furnace is not a common grate, but a thick plate of cast metal, perforated with holes for the air to pass through; one hole being between each pot, as they are seen arranged in fig. 1. at I, I, and also in the section at x, y. When the pots are placed upon the plate, the fire is not placed immediately upon them, as it would not only derange them, but it would displace the covering. To prevent this, the pots are first covered by some dried heath, or common brambles. This lying on the pots, defends them for a time, when the fuel is thrown in. By the time the brambles are consumed, the coal will have coked upon the pots, and will act as a defence for the rest of the process. The fire is kept up from twelve to twenty hours at the Cheule Brass Works in Staffordshire, where these drawings were taken from. They cast twice in the twenty-four hours. The melted brass, after the refuse is skimmed off, is cast into ingots, if sold for melting over again, and into plates, if intended to be rolled into sheets, or made into wire. The plates are cast between large blocks of Cornwall stone. The lower stone is fixed, and the face made even and smooth, by filling up the recesses of the ruff stone with fine sand. The upper stone is similarly prepared, and is suspended over the fixed one. The height and breadth of the place to receive the metal is limited by iron bars laid on the lower stone. The upper stone is then laid down upon the bars. The lower stone is a little longer than the upper one, and projects to the front. Being a little higher in that part, it forms a lip, or mouth-piece, to pour the metal into. The flat sides of the cast plate are therefore bounded by the surface of the stones, and the edges of the same by the bars above-mentioned. The ingot moulds are recesses in blocks of cast-iron, open on one side. The most certain and correct method of forming brass and the other compounds expressed in the table above-given, is by immediately uniting the metals in given weights. It should, however, be observed, that it will be found difficult to introduce zinc into melted copper. The best way of uniting it with copper, in the first instance, will be to introduce the copper in thin slips to the melted zinc, till the alloy requires a tolerable heat to fuse it, and then to unite this alloy with the melted copper. (T.)