Bronze. An alloy of copper with additional metals of zinc and/or tin in small amount, infrequently with other metals as trace or impurities. Bronze is the world's most popular alloy for coins and medals, irrespective of how they are made. Bronze ranks high in coinability, its cost is fairly inexpensive, it is ductile, easy to work with, excellent for casting, it is ideal for engraving or to hammer for repoussé, it is resistant to tarnish, yet has a beauty of its own and can take an attractive patina finish, allowed to tone on its own, or be electroplated. If the most ideal coin and medal composition were to be invented today, it would have many of the properties of bronze.Man has used bronze for centuries, since the bronze age, as early as 3500 BC, when metal objects were first cast. While it was not the first coinage metal (a natural alloy of gold and silver, electrum, was), bronze was indeed used for ancient coins, as early as 450 BC. Bronze is still universally used for coins today, particularly low valued coins. The first medals (by Pisanello in 1438) were cast in bronze, and – to no one's surprise – bronze is still used for medals of all kinds today. It is a historical, time honored, universal, attractive, efficient medallic composition.Types of bronze alloys. The properties of bronze alloys change slightly with the amount of alloy, yet a wide range of mixtures are suitable for coin and medal purposes. Bronze alloys can be both struck and cast and the color of bronze in just-struck or as-cast state (before toning) ranges from copper red to brass yellow to nickel-silver gray. Although the color gradation is gradual, bronze with a composition of 85% or more copper would have a red hue; alloys from 85 to 72% copper fall in the area of brass and have the typical brass yellow color; alloys with copper less than 72% copper are gray-to-white depending upon the other alloy components.The attached table lists most of the bronze alloys for coinage metals, and for both struck and cast bronze medals. Nickel-silver – with its high degree of copper – is listed as well as other "white" metals for comparison.United States cents were struck in coinage bronze prior to World War II. In 1943 a zinc-coated steel composition was used; in 1944-46 spent cartridge brass shells from World War II were reclaimed by melting (with pure copper added for proper alloy). Because of the rise in price of tin, this metal was eliminated entirely in coinage bronze beginning in 1962. Since 1982 a copper-clad zinc composition has replaced coinage bronze. Other countries throughout the world have had similar histories utilizing bronze for their minor denomination coins.Over 40 different bronze formulas have been used for statuary bronze, the metal used for casting statues; in fact so versatile is bronze that any of these alloys are suitable for casting. Studies of bronze statues of the past show each foundry or sculptor used his or their own formulation, it is a decision of personal preference rather than customor metallurgical requirements. A somewhat standard casting bronze is 90 copper, 7 tin and 3 zinc. Medals, of course, have been both cast and struck in statuary bronze compositions.The ease of obtaining commercial bronze is due to its being widely used in modern industry. Ninety-ten bronze is a standard "on the shelf" alloy that has wide uses and is often available in strip or sheet stock that can be blanked as needed. You could imagine a dozen different formulations in a dozen gauges and a dozen different forms could lead to tremendous inventory problems for metal suppliers. Commercial bronze is an all-purpose bronze alloy widely used for modern coins and medals.Bronze versus brass. The amount of zinc in a copper alloy determines a range of qualities in addition to color differences. Less than 15% zinc in a copper alloy is bronze; more than 15% zinc the alloy is brass. The greater the amount of zinc the easier the alloy is to strike. Coinability and malleability increase with greater zinc as the hardness lessens. The less zinc the harder the alloy. As mentioned, color changes from copper red to brass yellow with greater zinc in the alloy. Brass yellow is often chosen because of its close resemblance to gold and is fairly permanent. It does not tone rapidly as pure copper does.All bronze alloys ideal for plating. Every bronze alloy, and, in fact, copper itself, is ideal to be plated. Gold, silver (and other metals) may be electroplated to any bronze alloy. So ideal is this composition for plating, that some terms reveal this: gilt bronze and gilding metal, for example, are copper bronze alloys that have been goldplated. Bronze is easily worked to the final shape desired, it can then be given any finish, any color, any patina, any plating required. (Your plated tableware, for instance, is struck in bronze, then silverplated.)Bronze patinas. All copper alloys can be given a patina; the amount of the zinc in the bronze alloy affects the final patina color only slightly. In fact original incrusted patina occurred on bronze. The earliest patina work was on bronze statues. All patina work is based on their easy adaptability to bronze alloy – virtually any color can be applied to a bronze metal. See patina. 'Corinthian Bronze' – How To Make Bronze More Splendid! In 1913 coin dealer Thomas Elder issued a series of plaques of famous people which he commissioned sculptor Jeno Juszko to model. They included U.S. presidents, world rulers, poets, authors, musicians, and composers. In all he had 31 different renowned people commemorated in what he called "Medallions of The Immortals." He issued these in three sizes (13-, 7- and 4-inch diameters) and had them cast by Metal Products Company of New York City. But in his sales literature, calling them "bronze" wasn't good enough. Instead Elder touted them as cast in "Corinthian Bronze." Corinthian bronze is a bronze containing a little gold and silver in its composition. It harkens back to the days when the ancient city of Corinth was sacked and burned where the bronze recovered was said to contain the city's gold and silver supply. Frugal Elder probably tossed a cheap coin of each in the molten bronze pot before the caster filled the molds of his "Immortals." [Table: Types of Bronze and Brass Alloys] Types of Bronze and Brass Alloys Percent Copper Zinc TinBronze or Brass Alloy Cu Zn Sn Other NotesMedal bronze. . . . . . . . . 92-97 0-2 1-8 -- Exact formulations notexclusive for medals.Coinage bronze. . . . . . . . 95 1 4 -- Also called French bronze.Modern coinage bronze . . 95 5 -- -- Most malleable bronze.Phosphor bronze . . . . . . 90-95.5 -- 4.3-10 T-0.2P Trace phosphor.Statuary bronze (standard). . 90 3 7 -- Best alloy for fine castings.Commercial bronze . . . . . . 90 10 -- -- Easily available.Gunmetal . . . . . . . . . . . . . 88-90 0-4 8-10 -- Strongest bronze alloy.Red brass . . . . . . . . . . 85-90 10-15 -- -- Rich low brass, pinchbeck.Jeweler's bronze. . . . . . . 88 12 -- -- Actually red brass.. Engraver's brass. . . . . . . 85 15 -- -- Red brass; ideal for engraving.Medal brass . . . . . . . . . 84 16 -- -- For brass "gold" color.Oriental "bronze" . . . . . . 84 1 5 10 Pb High lead content.Tombac. . . . . . . . . . . . 82-99 1-18 -- -- Also called Mannheim gold,Dutch metal.Gilding metal . . . . . . . . 80-90 10-20 -- -- Not unattractive in naturalstate, but usually plated.Low brass . . . . . . . . . . 80 20 -- -- Strong yellow color.Tin brass . . . . . . . . . . 79 20 1 -- One percent tin content.Nickel brass. . . . . . . . . 79 20 -- 1 NI One percent nickel content.Bell metal. . . . . . . . . . 78-80 -- 20-22 -- For bell casting.Copper nickel . . . . . . . . 75-80 -- -- 20-25 Ni British cupro-nickel; alsocalled nickel-bronze or coinage nickel metal.Bath metal. . . . . . . . . . 75 24.7 -- 0.3 Ag Wood's metal; very soft, poorwearing quality.Cartridge brass . . . . . . . 67-70 30-33 -- Pb, Fe Trace of lead; iron 0.07% max.Yellow brass. . . . . . . . . 67 33 -- -- Widely called oroide or goldene.Speculum metal. . . . . . . . 67 -- 33 -- Takes a high polish. Sometimes found with minute arsenic, antimony or zinc to improve whitenessand reflectiveness.Nickel-silver . . . . . . . . 66-72 10-24 -- 10-18 Ni Formerly German-silver;expensive, high nickel cost.Muntz metal . . . . . . . . . 59-62 38-41 -- Pb Lead 0.6% maximum.White brass . . . . . . . . . 51 49-65 -- -- Gray-to-white color.
excerpted with permission from
An Encyclopedia of Coin and Medal Technology
For Artists, Makers, Collectors and Curators
COMPILED AND WRITTEN BY D. WAYNE JOHNSON
Roger W. Burdette, Editor