Bronze

Fragment of a bronze portrait of Marco Aurelio

Bronze is any alloy of copper and tin, in which the first is the base and the second appears in a proportion of 3% to 20%. May include other metals.

The alloys made up of copper and zinc are properly called brass; however, since copper is now often alloyed with tin and zinc at the same time, in layman's everyday language the difference between bronze and brass is rather imprecise.

Bronze was the first major alloy made by man and gives its name to the Bronze Age. For millennia it was the basic alloy for the manufacture of weapons and utensils, and goldsmiths of all ages have used it in jewelry, medals and sculpture. Coins minted with bronze alloys played a relevant role in trade. It was also used for weapons.

It is worth noting among its current applications its use in mechanical parts resistant to friction and corrosion, in good quality musical instruments such as bells, gongs, accompaniment cymbals, saxophones, and in the manufacture of strings for harps, guitars and pianos

Etymology

The term "bronze" probably derives from the Persian berenj ("brass"). Other versions relate it to the Latin aes brundisium ("Brindisi ore") after the old port of Brundisium. It is believed that the alloy may have been shipped to this port, and from there it was distributed throughout the Roman Empire.

History

Earring-knife of the Bronze Age from Ukraine

The introduction of bronze was significant in any civilization that used it, constituting the most innovative alloy in the technological history of humanity. Tools, weapons, and various construction materials such as mosaics and decorative plates achieved greater hardness and durability than their predecessors in stone or chalcopyritic copper.^{[citation needed]}

The technique consisted of mixing copper ore —usually chalcopyrite or malachite— with tin ore (cassiterite) in a charcoal-fired furnace. The carbon from the charcoal reduced the ores to copper and tin which were melted and alloyed with 5 to 10% by weight of tin. The metallurgical knowledge of the manufacture of bronze gave rise in different civilizations to the so-called Bronze Age.^{[citation required]}

Initially, the natural impurities of arsenic made it possible to obtain a superior natural alloy, called arsenic bronze. This alloy, with no less than 2% arsenic, was used during the Bronze Age for the manufacture of weapons and tools, taking into account that the other component, tin, was not common in many regions, and had to be imported from distant places.

Polished bronze plate

The presence of arsenic makes this alloy highly toxic, since it produces —among other pathological effects— muscle atrophy and loss of reflexes.

Copper statue of the pharaoh Pepy I. Century XXIII a. C.

The oldest known tin-based alloys date from the fourth millennium BCE. C. in Susa (present-day Iran) and other archaeological sites in Luristan and Mesopotamia.^{[citation required]}

Although copper and tin can be easily alloyed, mixed mines are rarely found, although there are a few exceptions in ancient deposits in Iran and Thailand. The regular forging of bronze has always involved the tin trade. In fact, some archaeologists suspect that one of the triggers for the Iron Age, with the subsequent progressive replacement of bronze in the most important applications, was due to some serious interruption in the trade of that mineral around 1200 BC. C., coinciding with the great migrations of the Mediterranean. The main source of tin in Europe was Great Britain, which has important deposits in Cornwall. It is known that the Phoenicians already reached its shores with goods from the Mediterranean to exchange them for tin.^{[citation required]}

In Ancient Egypt, most of the metallic elements that were made consisted of copper alloys with arsenic, tin, gold and silver. In New Kingdom tombs, or in the Karnak temple, there are bas-reliefs showing a foundry dating from the 15th century BC. C.^{[citation required]}

In the case of classical Greece, known for its marble sculptural tradition, it is known that they also developed advanced bronze casting techniques, as evidenced by the Riace bronzes, originating in the 5th century BCE. C.^{[citation required]}

Bronzes of the Chola Dynasty (India), approx. of the X century

In India, the artistic heyday of the Chola Dynasty produced remarkable sculptures between the 10th and 11th centuries AD, representing the various forms of the god Shiva and other deities.^{[citation needed ]}

The civilizations of pre-Hispanic America all knew the use of bronze alloys, although many utensils and tools continued to be made of stone. Objects made of binary alloys of copper-silver, copper-tin, lead-copper, and even unusual brass alloys have been found. It is believed that, at least in the pre-Columbian Andes, the earliest bronze work was done during the Wari Empire. In Inca times, bronze was exclusively reserved for the Inca nobility and high command of the army. It was processed in places like Viña del Cerro and Curamba. Already in colonial times, the most important foundries were in Peru and Cuba, dedicated mainly to the manufacture of bells and cannons.

Inca bronze knife

Bronze remained in use because quality steel did not become widely available until many centuries later, with improvements in casting techniques in the early Middle Ages in Europe, when cheaper and stronger steel became available, eclipsing bronze. bronze in many applications.^{[citation needed]}

Properties

Excepting steel, bronze alloys are superior to iron in almost all applications. Due to its high specific heat, the highest of all solids, it is used in heat transfer applications.

Comparison between bronzes and steels

Although they develop a patina, they do not oxidize below the surface, they are more brittle and have a lower melting point. They are approximately 10% heavier than steel, with the exception of those made of aluminum or silica. They are also less rigid, therefore in elastic applications such as springs they accumulate less energy than similar pieces of steel. They resist corrosion, even that of marine origin, the metallic fatigue threshold is lower, and they are better conductors of heat and electricity.^{[citation needed]}

Another differential characteristic of bronze alloys compared to steel is the absence of sparks when struck against hard surfaces. This property has been used to manufacture hammers, mallets, adjustable wrenches and other tools for use in explosive atmospheres or in the presence of flammable gases.

Versatility

Copper and its alloys have a wide variety of uses as a result of the versatility of their mechanical, physical, and chemical properties. Take into account, for example, the electrical conductivity of pure copper, the excellent malleability of ammunition cartridges made of brass, the low friction of copper-lead alloys, the sonority of bell bronze, and the corrosion resistance of most of its alloys.^{[citation needed]}

Physical properties

Data for an average alloy with 89% copper and 11% tin:

• Density: 8,90 g/cm3
• Merge point: from 830 to 1020 °C
• Ebullition point: from 2230 to 2420 °C
• Temperature coefficient: 0.0006 K^-1
• Electrical resistivity: from 14 to 16 μΩ/cm
• Thermal expansion coefficient: between 20 and 100 °C → 17.00 x 10^-6 K^-1
• Thermal conductivity at 23 °C: from 42 to 50 W^m-1

Mechanical properties

• Elongation: ≤65%
• Dureza Brinell: from 70 to 300
• elasticity module: from 80 to 115 GPa
• Cizalla resistance: from 230 to 490 MPa
• Traction resistance: from 300 to 900 MPa

Main alloys

Basic Types

Bronze Key

The basic alloy of bronze contains about 88% copper and 12% tin. “Alpha” bronze is the solid mixture of tin in copper. The alpha bronze alloy with 4 to 5% tin is used to mint coins and to make springs, turbines, and cutting tools.

In many countries, brass, which contains 90% copper and 10% zinc, but no tin, is incorrectly called “commercial bronze”. It is harder than copper, and has a similar ductility. It is used in screws and wires.

Arsenical bronze

The alloy of copper with arsenic is the first bronze used by man. It is a whitish alloy, very hard and brittle. It is manufactured in a proportion of 70% copper and 30% arsenic, although it is possible to cast bronzes with arsenic percentages of up to 47.5%. In these cases, the result is a shiny, gray material that is red-fusible and unaffected by boiling water.

Merely exposing arsenical bronze to air produces a dark patina. This circumstance, and the high toxicity of arsenic made it an alloy rarely used, especially after the discovery of alpaca, German silver or white bronze, known since ancient times in China and manufactured in Germany since the end of the 18th century.

Sun bronze

The so-called sun bronze (in German; Sonnenbronze) is an alloy used in jewelry, tenacious, ductile and very hard, which melts at temperatures close to those of copper (1357 °C) and is made up of 60% cobalt.

Cuproaluminum

Cuproaluminum is a type of bronze, similar in color to gold, in which aluminum is the primary alloying metal that is added to copper. A variety of aluminum bronzes, of different compositions, have found industrial use.

Bronze for firearms

Heavy kidney of the Dardanelos, used by the Turks on the site of Constantinople (1453)

From the discovery of gunpowder, a cannon bronze composed of 90 to 91% copper and 9 to 10% tin was used, a proportion commonly called "ordinary bronze." These weapons were known in China as early as the 11th century BCE. C., and in Europe they were used from the 13th century for both cannons and falconets.

By the 15th century, the artillery of the Ottoman Empire had large bronze bombards. Built in two pieces, with a total length of 5.20 m and a weight of 16.8 tons, they launched 300 kg bullets at a distance of up to 1,600 meters. Difficult to operate, with a firing capacity of no more than 15 shots a day, they were used at the siege of Constantinople in 1453.

Bronze for bells

La Tsar Kólokol (chuckles: "bell of the bells"), is the largest bronze bell that is preserved. Founded in 1733, it was never used and displayed in the Moscow Kremlin.

Bell casting is generally brittle: new castings range in color from dark ash to grayish white, with yellowish-red or even bluish-red tones in alloys with higher copper content.^{[ citation required]}

The higher proportion of copper produces lower and deeper tones at equal mass, while the addition of tin, iron or zinc produces higher tones. In order to obtain a more crystalline structure and produce variations in sonority, smelters have also used other metals such as antimony or bismuth in small quantities. ^{[citation required]}

The loudest alloy for making bells is called bell metal, which is 78% copper and 22% tin. It is relatively easy to cast, it has a compact granular structure with a reddish vitreous-conchoidal fracture. This type of bronze was known since ancient times in India to make gongs. Although rare due to its cost, the addition of silver is one of the few that further improves loudness.^{[citation needed]}

Alloys with up to 2% antimony have also been used. In China an alloy with 80% copper and 20% tin was known to make bells, large gongs and timpani. ^{[citation required]}

In England an alloy made up of 80% copper, 10.25% tin, 5.50% zinc and 4.25% lead was used. It has a lower sonority, taking into account that the lead is not homogenized with the alloy.^{[citation required]}

For bells and small instruments, an alloy of 68% copper and 32% tin was frequently used, which results in a brittle material, with an ashy fracture.^{[citation needed]}

For cymbals and gongs various alloys are used ranging from an 80% copper and 20% tin tempered alloy (B20), 88% copper and 12% tin (B12, for example, ZHT Zildjian, Alpha Paiste), and the cheaper B8, which consists of only 8% tin for 92% copper (Example, B8 Sabian, Paiste 201, Zildjian ZBT). Tempering is achieved by reheating the casting and rapidly cooling it.^{[citation needed]}

The largest surviving bell, called Tsar Kólokol, was cast in 1733 by Iván Motorin, commissioned by Empress Anna of Russia, niece of Tsar Peter the Great. With a weight of 216 tons, 6.14 m high and 6.6 in diameter. It was never used as an instrument, as a fire in 1737 destroyed its large wooden supports. Since 1836 it has been exhibited in the Moscow Kremlin.

Kara Kane

Buda Daibutsu bronze statuette, in Kamakura, Japan, is 11.40 m high and 93 tons weight.

The kara-kane ("Chinese metal" in Japanese) is a bronze for bells and traditional jewelry from Japan made up of 60% copper, 24% tin and 9% zinc, with iron and lead aggregates.

Many goldsmiths add small amounts of arsenic and antimony to harden the bronze without losing fusibility, and achieve greater detail in the impression of the molds.

The kara-kane is widely used for crafts and statuary not only for its low melting point, high fluidity and good mold-filling characteristics, but also for its smooth surface that quickly develops a fine patina.

There is a unique variety called seniokuthis, or golden bronze, originating in the Ming dynasty in China, which stands out for its lustrous texture and golden hue. In its manufacture, patina techniques are especially important.

The large Buddha sculptures made by Japanese goldsmiths demonstrate the high technical mastery they possessed and taking into account their large size, most of them had to be cast on site through successive stages.

Alloys that imitate silver

• Tonca Metal: alloy composed of 36% copper, 28% nickel and equal amounts of tin, lead, iron, zinc and antimony. It is a hard metal to melt, unductile, which is used very often.
• Silver mine: manufactured with 57% copper, 40% nickel, 3% tungsten and aluminum traces, has the property of not being attacked by sulfur and has very similar properties to silver.

Alloys with lead

For the manufacture of bearings and other parts subjected to friction, bronze alloys with up to 10% lead are often used, which gives them self-lubricating properties.

The distinctive feature of lead is that it does not form an alloy with copper; hence it is distributed according to the casting technique in the mass of the alloy, without mixing intimately. For this reason, excessive heating of a piece of machinery made from this material can lead to the "bleeding" of lead that becomes apparent as mud or sludge. Recycling these parts is also difficult, because the lead melts and separates from the alloy long before the copper reaches the melting point.

Commercial Alloys

Casting technique

The most widely used method for the artistic casting of bronze is that of «lost wax» or microfusion, which —with various variants— follows the following steps:

1. Original modeling in mud, escayola or other material.
2. Take the main mold, usually in eschayola. Once frigid, the core (original model) is removed.
3. The "negative" mould is filled with wax to produce a "positive" of this material.
4. The wax is coated with a clay mixture. Once dry, it is taken to an oven, where the wax melts and "gets lost".
5. In the case of small objects, the new mold serves directly for the tanning. For larger pieces, it is usual to fill it with a layer of wax that will form a film of the desired thickness for the bronze, and the interior (macho) is filled with refractory material. The whole process requires the placement of aerators, casting channels and various care to obtain a homogeneous strain.
6. Finish that includes smoothing of imperfections, roughness and polishing of the piece.
7. Patina, by applying different acids and heating to soplete to streamline oxidation.

Symbols

• The bronze medals award the third place in sports competitions.
• It represents the eight years of marriage in various Western cultures: Bronze weddings.
• It is the eighth level in the progression of the sports mountain range.