American Bronze Fine Art Foundry

Perhaps a little more than you really wanted to know about bronze.

Bronze was developed about 3500 BC by the ancient Sumerians in the Tigris-Euphrates Valley. Historians are unsure how this alloy was discovered, but believe that bronze may have first been made accidentally when rocks rich in ores of copper and tin were used to build campfire rings (enclosures for preventing fires from spreading). As fire heated these stones, the metals may have mixed, forming bronze. This theory is supported by the fact that bronze was not developed in North America, where natural tin and copper ores are rarely found in the same rocks.

Around 3000 BC, bronze-making spread to Persia, where bronze objects such as ornaments, weapons, and chariot fittings have been found. Bronzes appeared in both Egypt and China around 2000 BC. The earliest bronze castings (objects made by pouring liquid metal into molds) were made in sand; later, clay and stone molds were used. Zinc, lead, and silver were added to bronze alloys by Greek and Roman metalworkers for use in tools, weapons, coins, and art objects.

During the Renaissance, a series of cultural movements that occurred in Europe in the 14th, 15th, and 16th centuries, bronze was used to make guns, and artists such as Michelangelo and Benvenuto Cellini used bronze for sculpting. The most accurate and most commonly used method of bronze casting is the lost-wax method.

Today, bronze is used for making products ranging from household items such as doorknobs, drawer handles, and clocks to industrial products such as engine parts, bearings, and wire. Bronze is made by heating and mixing the molten metal constituents. When the molten mixture is poured into a mold and begins to harden, the bronze expands and fills the entire mold. Once the bronze has cooled, it shrinks slightly and can easily be removed from the mold.

Bronze is a metal compound containing copper and other elements. The term bronze was originally applied to an alloy of copper containing tin, but the term is now used to describe a variety of copper-rich material, including aluminum bronze, manganese bronze, and silicon bronze.

Bronze is stronger and harder than any other common alloy except steel. It does not easily break under stress, is corrosion resistant, and is easy to form into finished shapes by molding, casting, or machining.

The strongest bronze alloys contain tin and a small amount of lead. Tin, silicon (see end of this article for specifics on silicon), or aluminum is often added to bronze to improve its resistance. ilicon bronze alloys is used today in art castings. As bronze weathers, a brown or green film forms on the surface. This film inhibits corrosion. For example, many bronze statues erected hundreds of years ago show little sign of corrosion. Bronzes have a low melting point, a characteristic that makes them useful for brazing—that is, for joining two pieces of metal. When used as brazing material, bronze is heated above 430°C (800°F), but not above the melting point of the metals being joined. The molten bronze fuses to the other metals, forming a solid joint after cooling.

Lead is often added to make bronze easier to machine. Silicon bronze is machined into piston rings and screening, and because of its resistance to chemical corrosion it is also used to make chemical containers and in the the making of bronze art. Manganese bronze is used for valve stems and welding rods. Aluminum bronzes are used in engine parts and in marine hardware.

Bronze containing 10 percent or more tin is most often rolled or drawn into wires, sheets, and pipes. Tin bronze, in a powdered form, is sintered (heated without being melted), pressed into a solid mass, saturated with oil, and used to make self-lubricating bearings.

In metallurgy bronze is an alloy of copper, tin, zinc, phosphorus, and sometimes small amounts of other elements. Bronzes are harder than brasses. Most are produced by melting the copper and adding the desired amounts of tin, zinc, and other substances. The properties of the alloy depend on the proportions of its components.

Bronze is used for coins, medals, steam fittings, and gunmetal and was formerly employed for cannon. Because of its particularly echoing quality, bell metal, containing from 20% to 24% tin, is used for casting bells.

Essentially, to produce a bronze casting, the alloy is heated to at least 1,700 degrees F. at which it melts, and then poured into some type of fireproof mold of the original work.

Bronze is of exceptional historical interest and still finds wide applications. The proportions of copper and tin varied widely (from 67 to 95 percent in surviving artifacts), but, by the Middle Ages in Europe, certain proportions were known to yield specific properties. An alloy described in an 11th-century Greek manuscript in the library of St. Mark's, Venice, cites a proportion of one pound copper to two ounces of tin (8 to 1), approximately that used for bronze gunmetal in later times. Some modern bronzes contain no tin at all, substituting other metals such as aluminum, manganese, and even zinc

Bronze is harder than copper as a result of alloying that metal with tin or other metals. Bronze is also more fusible (i.e., more readily melted) and is hence easier to cast. It is also harder than pure iron and far more resistant to corrosion. The substitution of iron for bronze in tools and weapons from about 1000bc was the result of iron's abundance compared to copper and tin rather than any inherent advantages of iron.

Bronze in the Ancient World

In both Europe and the Middle East, bronze was mainly used for weapons and cutting tools—swords, spears, arrowheads, shields, adzes, and axes—although bowls and cauldrons were also made from bronze. During the 1st millennium, bronze was especially prized in Greece and later in Rome for sumptuous and elegant furnishings, such as tripods, bed and table frames, small oil lamps, and tall lampstands, often elaborately decorated with raised animal or leaf decoration.

Chinese Bronzes

In China bronze appears to have been used almost exclusively for bells, mirrors, and vessels in a variety of prescribed forms for distinct functions in religious rites, as well as for weapons and for the decoration of horse trappings and chariots. This first Bronze Age in China lasted from about 1800 to the end of the Qin dynasty (221-206BC. The bronze ritual vessels are especially admired for the nobility of their forms and the vigor of their abstract linear decoration. The decoration consists of highly conventionalized and attenuated masks and mythical monster forms, such as dragons. These vessels were cast from molds prepared with the decoration cut and incised on the inner face, resulting in equivalent projections on the cast vessel.>

European Bronzes

After bronze was superseded by iron for weapons, it remained in use in Europe as an artist's medium. Greek bronze statues, vases, and wine vessels, sometimes of large size and elaborately gilded, were greatly admired in Rome. The wandering tribes who gradually superseded Roman power in Europe (including Italy), also appreciated bronze, but used it more often for portable items such as shields and bowls as well as for buckles and brooches (often inlaid with colored stones or opaque enamel). In church furnishings, bronze continued to be used for larger pieces, such as candlesticks, baptismal fonts, and coffers. Perhaps the most famous bronze sculptures of the Renaissance are Lorenzo Ghiberti's sumptuously ornamental gilded bronze doors—the Gates of Paradise (1425-1452)—for the Baptistery at Florence, consisting of ten self-contained rectangular panels of biblical scenes cast in high relief. Many other Renaissance artists used this medium for smaller cast figure sculptures, often inspired by antique works of the classical era; this prime use for bronze has persisted to the present day.

In the 18th and 19th centuries, and especially in France, gilt bronze attachments—called ormolu—in the form of projecting and richly decorated cast mounts on edgings, drawers, and feet, were added to luxury furniture.

African Bronzes

In Nigeria, between the 14th and 16th centuries, cast bronze sculptures of extreme refinement were made at Benin in a highly developed artistic convention unrelated to European styles.

Silicon Bronze

Silicon, symbol Si, semimetallic element that is the second most common element on earth, after oxygen. The atomic number of silicon is 14. Silicon is in group 14 (or IVa) of the periodic table. It was first isolated from its compounds in 1823 by the Swedish chemist Baron Jons Jakob Berzelius.

Silicon is prepared as a brown amorphous powder or as gray-black crystals. It is obtained by heating silica, or silicon dioxide (SiO2), with a reducing agent, such as carbon or magnesium, in an electric furnace. Crystalline silicon has a hardness of 7, compared to 5 to 7 for glass. Silicon melts at about 1410° C (about 2570° F), boils at about 2355° C (about 4271° F), and has a specific gravity of 2.33.

Silicon constitutes about 28 percent of the earth's crust. It does not occur in the free, elemental state, but is found in the form of silicon dioxide and in the form of complex silicates. Silicon-containing minerals constitute nearly 40 percent of all common minerals, including more than 90 percent of igneous-rock-forming minerals. The mineral quartz, varieties of quartz (such as chrysoprase, onyx, flint, and jasper), and the minerals cristobalite and tridymite are the naturally occurring crystal forms of silica. Silicon dioxide is the principal constituent of sand. The silicates (such as the complex aluminum, calcium, and magnesium silicates) are the chief constituents of clays, soils, and rocks in the form of feldspars, amphiboles, micas, and zeolites, and of semiprecious stones, such as garnet, zircon, topaz, and tourmaline.

Silicon is used in the steel industry as a constituent of silicon-steel alloys. Silicon steel, which contains from 2.5 to 4 percent silicon, is used in making the cores of electrical transformers because the alloy exhibits low magnetism. Silicon is also used as an alloy in copper, brass, and bronze.

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