1902 Encyclopedia > Bronze


BRONZE is an alloy formed wholly or chiefly of copper and tin, in variable proportions. It has been used from a very early period. Archaeologists distinguish a bronze age in prehistoric times in Western Europe (intermediate between those of stone and iron), characterized by a general use of the alloy for cutting instruments and other objects. The " brass" of the Bible was probably of the nature of bronze. The use of bronze in early times is noticed more particularly below.
The addition of tin to copper gives rise to a product more fusible than copper, and thus better suited for casting. The alloy is also harder and less malleable. The proportions in which copper and tin are combined to make bronze vary according to the object for which the alloy is designed. With about 7 parts copper to 1 part tin, bronze is very hard, brittle, and sonorous, Soft bronze, again, which bears drifting, rolling, and drawing, is generally composed of 16 copper to 1 tin ; while a flexible tenacious alloy, good for nails and bolts, is made of 20 copper to 1 tin. In preparing bronze for statues, bas-reliefs, &c, the qualities chiefly looked at are fusibility and hardness, also readiness to acquire a fine patina on exposure, though it appears this may be acquired by bronzes differing widely in composition. A common statue bronze is formed of copper 80, tin 20. Bell-metal, for large bells, is generally made with about 3 parts copper to 1 part tin; for house bells, 4 copper to 1 tin. The bronze of bells (as of various other objects) sometimes contains a little zinc, lead, &c, in addition to the primary ingredients. The Chinese tain tarns or gongs, are made of bronze forged by the hammer ; they contain about 20 per cent, tin, the rest copper only. The secret of their manu-facture seems to have been revealed by MM. Julien and Champion, who find that a bronze of this nature, though at the common temperature brittle as glass, may, at a dull red heat, be forged and beaten out as easily as soft tin. The speculum metal employed in telescopes is of 2 copper to 1 tin; and on the other hand, with larger proportions of copper, we have an alloy suitable for machinery bearings, and also for medals, 8 copper to 1 tin; another for brass ordnance or bronze cannon, 9 copper to 1 tin; another for mathematical instruments, 12 copper to 1 tin, &c. The hardness and resistance to oxidation of bronze fit it admirably for coins, and many ancient bronze coins have come to us but little deteriorated, though buried for ages in damp soil, or immersed in water. The composition of the present bronze British coinage is (in 100 parts) 95 copper, 4 tin, 1 zinc.
A few years ago some very beautiful Chinese and Japanese bronzes were exhibited in Paris, remarkable chiefly for the dead black colour of their patina. From analyses by M. Morin it appeared that they contained a large proportion of lead, the average composition being copper 80 parts, lead 10, tin 4, zinc 2, and the remaining four parts consisting of iron, nickel, arsenic, silver, and gold. According to M. Christofle, lead is not essential for production of a fine black patina; and it renders the alloy brittle. Bronze can be covered with a black, red, brown, or green patina, as desired, by suitable oxidation or sulphurization.
Some important researches on bronze for field-guns have lately been made by Colonel Uchatius of Vienna; and the steel bronze he produces is said to be quite equal to steel in hardness, homogeneousness, resistance, and other qualities; while it is less affected by atmospheric agency, and less costly. He casts the bronze (which contains 8 per cent, tin, the rest copper) in a cast-iron ingot mould, with a core of wrought-copper 50 mm. in diameter. Then after boring out the hollow ingot to a diameter of 80 mm. he forces through it a series of six conical pistons of hardened steel, slightly larger in diameter than the bore. The interior is then excessively hard and ready for rifling, The hardness, elasticity, and solidity diminish from within outwards. These new bronze guns have been found to bear several hundred discharges successively without the slightest apparent deformation or other injury.
It is only of late years that the changes produced in bronze by addition of phosphorus have been scientifically investigated ; and from experiments by Messrs Montefiore, Kiinzel, Kirkaldy, and others, phosphor bronze is proved to have great superiority to ordinary bronze in tenacity, elasticity, and tensile strength (being to it much what.steel is to wrought iron). The presence of oxides in ordinary bronze accounts for its possessing these qualities in less degree, and phosphorus increases them by reduction of the metal. Phosphor bronze is further greatly improved in tensile strength by being drawn into wire or rolled into sheets; and it resists the action of sea-water much better than copper. Such a substance cannot fail to find many important applications, military, industrial, and domestic. In virtue of its reducing properties, we may add, phosphor bronze can be platinized better than any other metal

The alloy known as aluminium bronze is one endowed with great strength, malleability, and ductility. It is formed of 10 parts aluminium and 90 of copper.
In the melting of ordinary bronze, reverberatory fur-naces have long been used, as rapid fusion is desirable in order to prevent loss of tin, zinc, or lead by oxidation. Bellfounder3 often use dome-topped furnaces, as their alloy does not require so intense a heat for fusion o but there is some waste of material with these. The copper is melted first, and covered with small charcoal or coke ; and the tin is rapidly thrust down to the bottom of the melted mass. After stirring, the alloy is poured into the moulds, in which the cooling should be as rapid as possible. Sometimes pressure is applied during cooling, in order to make the cast free from pores.
In the old method of bronze-casting, known as the cire perdue, wax is first used for the thickness of the statue (between core and mould, which are of baked clay), and is melted and run off before the metal is poured in,—the core and mould being held apart by stays of iron wire. In the present day large works are never cast in one piece, but in several, which are afterwards united by heating and application of fused metal. A model is made in plaster, and a piece mould of Caen sand, about 1| or 2 inches thick, made round it, the sizes of the pieces being determined by the shape and character of the portions they occupy. These pieces are backed with plaster of Paris to about a foot in thickness, with indentations cut in their horizontal thickness, into which the succeeding portion of the mould (its. The mould is then taken to pieces, dried, and rebuilt in the casting-pit. It is then filled with core-composition in a liquid state, and when this is sufficiently hardened, again taken to pieces. The core thus obtained is thoroughly dried, and reduced in size by scraping away as much of the material as would represent the thickness of the -metal to be cast. This done, the mould is again built up over the core, and the pit filled, ifec, as in the other process. The statue is completed after its removal from the mould by cutting off the jets, removing roughnesses where they occur, and giving greater sharpness to the details when necessary.
Statues and various ornamental objects may also be manufactured by the process of electro-deposition from a metallic solution; and some excellent results have been obtained in this way by Oudry, Christofle, Elkington, and others. While the method offers some advantages in regard to cheapness, lightness, &c, of the products, the bronzes thus produced are not so hard and durable as those got by casting, and are thus less suited for exposure.
Bronzing is the process by which a bronze-like surface is produced on objects made of metal, plaster, wood, or other material. It may be done variously. The green bronze colour is sometimes produced on metal with vinegar alone, or dilute nitric acid, or sal-ammoniac. To give an antique appearance to newly made articles of bronze or brass, it is recommended to dissolve three-quarters of an ounce of sal-ammoniac and a drachm and a half of bin-oxalate of potash (salt of sorrel) in a quart of vinegar, moisten a soft rag or brush with the solution, and rub over the clean bright metal till its surface becomes quite dry through the friction. This process should be repeated several times, and the object should bo kept a little warm. With a solution of chloride of platinum (which is, however, an expensive liquid) almost any colour can be imparted to copper, brass, iron, or new bronze, according to the degree of dilution and number of applications. The following solution is suitable for bronzing coins and medals :—Two parts of verdigris and one part of sal-ammoniac are dissolved in vinegar, the solution is boiled, skimmed, and diluted with water till it has only a weak metallic taste, and on further dilution lets fall no precipitate. This solu-tion is made to boil briskly and poured on the objects. These are well washed with clean water and then lacquered.
Objects of cast-iron may be made to assume a fine bronze appearance by being coated with a very thin layer of vegetable oil, and then placed in a drying oven, the tempera-ture being such that oxidation of the iron and decomposi-tion of the oil take place simultaneously. Another mode of bronzing iron (lately recommended by Weiskopf) is with a solution of one part sylvate of silver in twenty parts of oil of lavender. The object is lightly coated with this, and warmed rapidly up to 302° C. For bronzing tin or white metal a solution may be used consisting of 1 oz. sal-ammoniac, -g- oz. alum, and \ oz. arsenic, dissolved in 1 pint of strong vinegar.
A good method of bronzing wood, porcelain, stoneware, composition picture and looking-glass frames, etc., is first to coat the article with a thin solution of water glass, using a soft brush. Bronze powder is then dusted on, and any excess not adherent is knocked off by a few gentle taps. The article is next heated to dry the silicate, and the bronze becomes firmly attached. Bronze powders are pre-pared of many different shades. In Messrs Brandeis's process the alloy used (generally copper-zinc) is laminated into very fine leaves, which are then ground. The powder is washed out and dried, and by revolving in a box, which contains some mineral varnish, the particles receive a coating of the varnish. Bronze powders are also prepared from leaf gold ground with honey on a stone, mosaic gold ground with bone ashes, compounds of tungsten and soda, and in other ways.
As regards cleansing of bronze statues.that have become coated with dirt in large towns, it has been found that a dilute solution of caustic alkalies removes the overlying dirt and allows the green patina to become visible. Where the metal was not originally oxidized, the alkali simply cleanses it and does not promote any formation of green rust. An occasional rubbing with oil (all excess being carefully removed) is also found to preserve a fine bronze surface. The shining brown colour of gun barrels or other arms, is sometimes imparted by first producing a very thin uniform film of oxide or rust on the iron, e.g., with vapour of muriatic acid, and giving a gloss to the surface by rubbing wax over it, or coating it with a shellac varnish. But the most common material for browning is the butter or chloride of antimony, sometimes called bronzing-salt. It is mixed with olive oil and rubbed on the iron, which is slightly heated. A little aquafortis is then rubbed on to quicken the operation ; the barrel is then cleaned, washed with water, dried and polished, either with a steel burnisher, or by rubbing with white wax, or is varnished with a very weak solution of shellac and spirit of wine. (See Ure's Dictionary of Arts, etc.)
Greek and Roman Bronze.
The bronze (Greek, _____; Latin, oss) of classical antiquity consisted chiefly of copper, with an alloy of one or more of the following metals, zinc, tin, lead, and silver, the quantity and the character of the alloy changing as times changed, or as was required for different purposes. Among existing bronze remains the copper is found to vary from 67 to 95 per cent. At present the only valuable results which we possess are derived from the analysis of coins (Von Bibra, Die Bronzen und Kupferlegirungen der alten und ältesten Völker, Erlangen, 1869), from which it appears that for their bronze coins the Greeks adhered to an alloy of copper and tin till 400 B.C., after which time they used also lead with increasing frequency. Silver is rare in their bronze coins. The Bomans also used lead as an alloy in their bronze coins, but gradually reduced the

quantity, and under Caligula, Nero, Vespasian, and Domitian, coined pure copper coins ; afterwards they reverted to the mixture of lead. So far the words x^A/co's and ces may be translated as bronze. Originally, no doubt, ^oA/cos was the name for pure copper. It is so employed by Homer, who calls it ipvOpos (red), al6<m(/ (glittering), <£aevvos (shin-ing), terms which apply only to copper. But instead of its following from this that the process of alloying copper with other metals was not practised then, or was unknown to the poet, the contrary would seem to be the case from the passage (Iliad, xviii. 474) where he describes Hephaestus as throwing into his furnace, copper, tin, silver, and gold, to make the shield of Achilles, so that it is not always possible to know whether when he uses the word XOAKOS he means copper pure or alloyed. Still more difficult is it to make this distinction when we read of the mythical Dactyls of Ida in Crete or the Telchines or Cyclopes being acquainted with the smelting of XOAKOS. It is not, however, likely that later Greek writers, who knew bronze in its true sense, and called it XOAJCOS, would have employed this word without qualification to objects which they had seen unless they had meant it to be taken as bronze. When Pausanias (iii. 17, 6) speaks of a statue, one of the oldest figures he had seen of this material, made of separate pieces fastened together with nails, we understand him to mean literally bronze, the more readily since there exist very early figures and utensils of bronze so made. The earliest employment of bronze for artistic purposes was to hammer it out in thin plates and fasten them together with nails. This process was called sphyrelaton. The next stage was casting, in connection with which the earliest Greek artists of fame are Theodoras and Bhcecus of Samos (Pausanias, viii. 14, 8, and x. 38, 5). It has been supposed that their merit consisted in introducing the process of casting statues hollow, that is, with an inner core of some material which could afterwards be removed and leave the figure light, less costly, and no less durable. There are remains of Assyrian bronze, probably older than the time of Theodorus and Rhcecus, cast with an inner core of iron ; and there is also in the British Museum an early Etruscan statuette from Sessa on the Volturno, with a core of this metal, which from its being split down the side, owing to the expansion of the iron, shows how unserviceable the iron was for this purpose. Obviously the power of casting in bronze, whether solid or hollow, was a very great gain to sculptors, whose models worked in the clay with the rapidity of their inspiration could thus be accurately and at once reproduced. The difficulty and expense of the process must have been against it as compared with marble ; yet it was frequently employed, and in the case of colossal statues it had no rival. Of these the Colossus of Rhodes —a figure of the sun-god Helios, said to have been 70 cubits high—was an example of the utmost that art could do with bronze. It was thrown down by an earthquake after standing fifty-six years. A statue of Zeus at Tarentum by Lysippus was 40 cubits high, and though it could be moved with a touch of the hand, yet it resisted the force of storms by means of a support at the point of the greatest stress. The oldest seat of bronze-founding, at least to any extent, was the island of Delos, and next to that the island of ^Egina, and yet copper does not appear to have been found in either. Between the two there existed a rivalry in the time of the sculptors Myron and Polycletus, of whom the former used the bronze of Delos, the latter that of iEgina. More celebrated than either was the bronze of Corinth, which some believed to have been first obtained by the melting together of statues of ordinary bronze, gold, and silver at the burning of that town. Pliny says that it consisted of gold, silver, and copper, and was considered more precious than silver and little less valuable than gold.
There were three kinds of it—one white, having almost the appearance of silver, in which silver predominated; another yellow, because of the great quantity of gold in it; and a third in which all three metals were equally represented. But the Corinthian bronze was used rather for drinking cups and utensils than for statues. The process of casting statues as given by Pliny was to bring the mass of copper to a liquid state, and then to throw into it a third part of old bronze and \2\ per cent, of plumbum argentarium, i.e., tin and lead in equal parts.
Of the vast number of bronze statues by ancient sculptors nothing beyond a few fragments remain ; but if the colossal bronze head of Venus in the British Museum be taken as a typical example, it will show with what fineness and thin-ness those figures were cast; or, again, as an instance of the quality of Greek bronze we may take the bronzes of Siris, also in the British Museum, on which a very thin plate of bronze will be seen in some parts of the figures beaten out nearly half an inch till it reaches the thinness of "note-paper. Works in relief (Topcupt), whether beaten out or chased, like those just mentioned, or cast, are comparatively rare, though this branch of art was largely practised even by the greatest sculptors. On the other hand, it does not appear to have been carried out by them to the extent in which it is found in Germany and Italy after the beginning of the 11th century,—for instance, in the reliefs on cathedral gates. The temple of Athene Chalkioikos in Sparta, with its walls covered with bronze reliefs, stands out as an exception. By the time of the Byzantine empire, when the power of modelling had declined, and a taste for glittering appearance took its place, the process of ornamenting bronze with reliefs was superseded by inlaying it with silver and other materials. As to the colour of the ancient bronzes little can now be said, because from lying so long in the earth they have become covered with what is technically called a patina, which is generally some shade of green, though sometimes also nearly blue, and at other times drab. This blue colour is very brilliant in bronzes from Herculaneum and Pompeii. A difference of soil very probably makes a different patina, but something may also be due to varieties in the alloy. Perhaps the finest examples of patina arej to be found among the bronze mirrors, in which there seems to have been generally a considerable quantity of silver for the sake of obtaining a highly reflecting surface. It does not appear that the process of gilding bronze was carried to any extent in classical times, unless, perhaps, in the production of finger-rings, of which a considerable number remain. But if larger works in bronze fail, there is an abundance of statuettes, candelabra, mirrors, cistce, and vessels of all kinds—Greek, Roman, and Etruscan. One fact to be noticed is that the great number of bronze mirrors which exist are nearly all Etruscan. A few may be Roman from the Latin inscrip-tions which they bear, and a few also come from Greece. But the general rule of their being Etruscan reminds us of the reputation which the Etruscans enjoyed for the produc-tion of works in bronze, not of high art, but of what might be called industrial art. They were celebrated also for modelling in clay; and that, as Pliny states, was the stage of art which immediately preceded casting in bronze, and went hand in hand with it.
The art of bronze casting, which had sunk with the Byzantine empire, was again revived with great vigour in Germany in the 11th century, from which period are the bronze gate of the cathedral at Hildesheim (1015) and the column decorated with reliefs on the model .of the column of Trajan in Rome (1022). In the 12th century the art spread southward to Italy, and at first was taken up energetically inLower Italy. But though many interesting works of this kind exist also from the 13 th and 14th

centuries, it was not till the 15th that the art obtained its complete mastery under the Florentine artists. In the following century, again, it is found carried with extraordinary skill in Germany at Nuremberg, Augsburg, Munich, and Coburg. Since then, however, the higher order of sculpture in bronze may be said to have reverted to nearly its ancient limits, that is, the production of statues and groups in the round. (See Dr C. Bischoff, Das Kiipfer in der vorchristlichen Zeit, Berlin, 1865; and L. R. v. Fellenberg, Analysen von antiken Bronzen, 1865.)

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