BRONZE, in its strictest sense, is a solid solution of tin in copper. As generally used the term signifies any alloy consisting of propor tions of copper and tin, varying according to the purpose desired, to which lead, zinc, alumi num, and silver also, are sometimes added for the purpose of giving greater brilliancy to the compound, or rendering it more fusible, the zinc being introduced in its combination with copper as brass. In some of the modern so called bronzes, brass is used instead of tin; these are then nothing more than forms of brass, with very large proportions of copper. The true bronzes in general use vary the pro portions of copper to tin from 5 to 1 to 24 to 1 (by weight). These alloys are not often used in the arts as they are difficult and costly to finish either with tool or machine. Mixtures of these bronzes with lead, zinc, nickel, phos phorus and aluminum are abundantly strong and much easier to work. Where the propor tion is larger in tin, the color of the bronze is whitish or gray, and as the copper component is increased the color passes through shades of bluish red and reddish yellow to the brick red of pure copper. It is claimed by some founders that a chemical union of copper and tin is effected only in one or two definite proportions, and that mixtures in other proportions are defective, particularly so for engine work where they are subjected to fluids at high pres sures — in which cases they develop porosity. The addition of a small amount of lead, how ever, seems to do away with this difficulty. The uses of bronze in the arts, as defined by the proportion of the tin component, are as follows: With 5 per cent tin, soft gun-metal castings; with 7 per cent tin, mathematical in struments; with 8% per cent tin, toothed gear wheels; with 8 to 12 per cent tin, soft bronze statues, medals, coins and cannon; with 12 to 15 per cent tin, machine and engine bearings; with 16 per cent tin, soft-toned bells; with 18 to 20 per cent tin, gongs, house bells and cym bals; with 22 per cent tin, school bells; with 24 per cent tin, church bells and fire bells; with 33 per cent tin, the so-called speculum metal for telescope mirrors and surgical instruments.
Bronze was used by the ancient Assyrians and Egyptians. Layard brought many ornaments a other articles of this metal from Assyria. It has been found, on examination, that the bronze weapons of the Greeks and Romans were of the best composition for securing the greatest density in the alloy, and the cutting edges were brought to the highest point of tenacity by hammering. For ornamental articles, zinc and lead are frequently added. These two metals are usually contained in the bronzes of France along with the copper and tin. Japanese bronzes frequently contain silver and gold (sometimes to the exclusion of tin) and also lead. A greenish color has been imparted to ancient bronzes by oxidization. This has been imitated in modern bronzes by chemical treat ment. The heat-black finish on bronze, brass and copper is obtained by the use of two solu tions: (1) Nitrate of copper, 1 oz.; water, 1 oz.; (2) nitrate of silver, 1 oz., water, 1 oz. The solutions are mixed for application in the proportions: water, 3 oz.; copper solution, 2 oz.; silver solution, 1 oz. The surface of the bronze to be treated may first be submitted to a sand blast to roughen it. The bronze is heated on a hot iron plate, or in an oven, to about 250° F., and the mixed solution is ap plied with a cotton swab or soft brush. The coating dries greenish, and should be allowed to stand till it is a uniform black. Then it is permitted to cool and is brushed over with metal brushes. It will then appear a brownish black. To produce a dead black, a solution of 2 oz. liver of sulphur to the gallon of water is swabbed over it. When dry it is brushed with bristle brushes and finished with a wax polish or with lacquer. Bronzing upon brass and copper to produce artistic texture and color effects is accomplished by varied manipulations of the electroplating processes to deposit an actual bronze coating upon the objects treated.