Page:The American Cyclopædia (1879) Volume III.djvu/321

 BRONZING 315 mixture stirred until it is homogeneous. It is then turned into the mould as quickly as pos- sible, and when the exterior is sufficiently solidified the casting is uncovered, in order, by hastening the setting of the interior, to prevent as much as possible the formation of strata of unequal composition, which is liable to occur from the great difference in the fusibility of the metals, that of copper being above 2,200 F., while tin melts at 442. In casting cannon, where there is a great mass of metal, such stratification is liable to occur, the parts which solidify first being richer in copper. This dif- ficulty is largely overcome by the addition of about' one tenth per cent, of phosphorus, by which the gram of the bronze is also improved. After cooling, if it is again raised to a tem- perature of about 1,000 and allowed to cool slowly, its toughness of texture will be im- proved. The dark olive hue which bronze ac- quires by exposure is hastened by the applica- tion of oxidizing washes, and different shades may be given according to the chemical quali- ties of the wash employed. Some extract the tin from the surface, and leave the copper in excess, and others remove the copper and leave the tin most prominent. -Among the ancient Egyptians, Greeks, and Romans, the manufac- ture of bronze articles was very extensively car- ried on. In Greece especially the taste for statuary in this material was cultivated to an unparalleled extent. The wealth of some cities was estimated by the number of their statues. In Athens alone no fewer than 3,000 statues have been found, and in Rhodes, at Olympia, and in Delphi many more. The famous colossuses were cast of this alloy. The names of many of the ancient artists are still celebrated, and their groups of statuary continue to be our models. The alloy was employed by them for purposes to which we apply the harder metals, as in some periods for their arms and armor, medals, and even their surgical instruments, a set of which was discovered at Pompeii. By them it was regarded as a sacred metal, and endowed with a mysterious power of driving away evil spirits. The laws were inscribed on tables of bronze, and upon bronze coins alone were placed the words moneta sacra. The Phoenicians were the first known workers of it ; they made it into plates, which were nailed together ; and they also cast it solid, and cored. The Athe- nian sculptor Myron employed it of a pale color and unknown composition, in the 5th century B. 0. The Cojinthian bronze is supposed to have been suggested by the accidental fusing of metals at the burning of Corinth, 146 B. 0. ; it was of three colors, white, yellow, and the last not known. Tho antique liver-colored cinque-cents, and the Florentine bronze, are of the same shade, approaching a dull reddish brown. Aluminum bronze is composed of 90 parts of copper and 10 of aluminum. It re- sembles gold, and is used in ornaments. HKOIV/I.VG, the process of covering articles ef wood, clay, plaster, metals, ivory, &c., with compositions which give to them the appear- ance of bronze. These compositions vary in their ingredients, and the process also, with the articles to be coated. An application is first made of size or oil varnish, into which when nearly dry a metallic powder is rubbed, or this may be previously mixed with the var- nish. This powder is most commonly a prepa- ration called gold powder, prepared as follows: Gold leaf is ground together with honey upon a stone. When thoroughly mixed, and the particles of gold completely reduced, the prepa- ration is stirred up in water, and washed until the honey is entirely removed. The gold which settles is then collected upon filtering paper and dried. Another variety of powder, called aurum mosaicum or musivitm, is prepared in the following manner : A pound of tin, melted in a crucible, is amalgamated with half its weight of pure mercury. When the amalgam is cold, it is reduced to powder, and ground with Ib. of sal ammoniac and 7 oz. of sul- phur. On subliming this mixture in a matrass, the tin remains at the bottom of the vessel in a flaky golden powder, which is the aurum mosaicum. A shade of rod is given to this when desired, by adding a small portion of red lead. Copper powder is obtained for the same purpose by the precipitation of the metal from its solution in nitric or sulphuric acid, by means of pieces of metallic iron. The copper deposits itself upon these, from which it may be brushed oft' in powder, care being taken to exclude it from the action of the air as it is washed in water, or better in alcohol. It is used either alone or mixed with pulverized bone ash. The preparation called gold size is also used in bronzing. It is made by boiling 4 oz. of pow- dered gum anim6 and a pound of linseed oil, the gum being gradually added, and stirred into the oil while this is heated. The boiling is continued till the mixture becomes thicker than tar. This is then to be strained through a coarse cloth. When applied, vermilion is added to render it opaque, and a convenient consistency is given to it with oil of turpentine. After being applied, it is allowed to dry very nearly, and when it has become sufficiently hard the powder selected is rubbed over the work with a piece of soft leather wrapped round the finger; or the application is bet- ter made with a soft camel's hair pencil, with which, when quite dry, the loose pow- der is brushed away. If gold size is not to be used, the powders may be mixed in gum water and laid on with a brush. Vinegar is often applied to brass castings to give them the green bronze color, sometimes in combination with sal ammoniac and sometimes with common salt. Coins and medals are sometimes bronzed with a solution of verdigris and sal ammoniac in vinegar, which is afterward diluted with water and boiled, and applied while hot. It is said that the Chinese bronze copper vessels by taking 2 parts of verdigris, 2 of cinnabar, 5 of sal ammoniac, and 5 of alum in powder, making a