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For other uses, see Brass (disambiguation).
Brass die, along with zinc and copper samples.

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.[1]

In comparison, bronze is principally an alloy of copper and tin.[2] Bronze does not necessarily contain tin, and a variety of alloys of copper, including alloys with arsenic, phosphorus, aluminum, manganese, and silicon, are commonly termed "bronze". The term is applied to a variety of brasses and the distinction is largely historical.[3]

Brass is a substitutional alloy. It is used for decoration for its bright gold-like appearance; for applications where low friction is required such as locks, gears, bearings, doorknobs, ammunition, and valves; for plumbing and electrical applications; and extensively in musical instruments such as horns and bells for its acoustic properties. It is also used in zippers. Because it is softer than most other metals in general use, brass is often used in situations where it is important that sparks not be struck, as in fittings and tools around explosive gases.[4]

Brass has a muted yellow color which is somewhat similar to gold. It is relatively resistant to tarnishing, and is often used as decoration and for coins. In antiquity, polished brass was often used as a mirror.

Lead content

To enhance the machinability of brass, lead is often added in concentrations of around 2%. Since lead has a lower melting point than the other constituents of the brass, it tends to migrate towards the grain boundaries in the form of globules as it cools from casting. The pattern the globules form on the surface of the brass increases the available lead surface area which in turn affects the degree of leaching. In addition, cutting operations can smear the lead globules over the surface. These effects can lead to significant lead leaching from brasses of comparatively low lead content.[5]

Silicon is an alternative to lead; however, when silicon is used in a brass alloy, the scrap must never be mixed with leaded brass scrap because of contamination and safety problems.[6]

In October 1999 the California State Attorney General sued 13 key manufacturers and distributors over lead content. In laboratory tests, state researchers found the average brass key, new or old, exceeded the California Proposition 65 limits by an average factor of 19, assuming handling twice a day.[7] In April 2001 manufacturers agreed to reduce lead content to 1.5%, or face a requirement to warn consumers about lead content. Keys plated with other metals are not affected by the settlement, and may continue to use brass alloys with higher percentage of lead content.[8][9]

Also in California, lead-free materials must be used for "each component that comes into contact with the wetted surface of pipes and pipe fittings, plumbing fittings and fixtures." On January 1, 2010, the maximum amount of lead in "lead-free brass" in California was reduced from 4% to 0.25% lead. The common practice of using pipes for electrical grounding is discouraged, as it accelerates lead corrosion.[10][11]

Corrosion-resistant brass for harsh environments

Brass sampling cock with stainless steel handle.

The so called dezincification resistant (DZR or DR) brasses are used where there is a large corrosion risk and where normal brasses do not meet the standards. Applications with high water temperatures, chlorides present or deviating water qualities (soft water) play a role. DZR-brass is excellent in water boiler systems. This brass alloy must be produced with great care, with special attention placed on a balanced composition and proper production temperatures and parameters to avoid long-term failures.

Germicidal/antimicrobial applications

Main article: Antimicrobial copper-alloy touch surfaces.

See also: Antimicrobial properties of copper, Copper alloys in aquaculture

The copper in brass makes brass germicidal. Depending upon the type and concentration of pathogens and the medium they are in, brass kills these microorganisms within a few minutes to eight hours of contact.[12][13][14]

The bactericidal properties of brass have been observed for centuries and were confirmed in the laboratory in 1983.[15] Subsequent experiments by research groups around the world reconfirmed the antimicrobial efficacy of brass, as well as copper and other copper alloys.[16][17][18][19] Extensive structural membrane damage to bacteria was noted after being exposed to copper.

In 2007, U.S. Department of Defense’s Telemedicine and Advanced Technologies Research Center (TATRC) began to study the antimicrobial properties of copper alloys, including four brasses (C87610, C69300, C26000, C46400) in a multi-site clinical hospital trial conducted at the Memorial Sloan-Kettering Cancer Center (New York City), the Medical University of South Carolina, and the Ralph H. Johnson VA Medical Center (South Carolina).[20] Commonly-touched items, such as bed rails, over-the-bed tray tables, chair arms, nurse's call buttons, IV poles, etc. were retrofitted with antimicrobial copper alloys in certain patient rooms (i.e., the “coppered” rooms) in the Intensive Care Unit (ICU). Early results disclosed in 2011 indicate that the coppered rooms demonstrated a 97% reduction in surface pathogens versus the non-coppered rooms. This reduction is the same level achieved by “terminal” cleaning regimens conducted after patients vacate their rooms. Furthermore, of critical importance to health care professionals, the preliminary results indicated that patients in the coppered ICU rooms had a 40.4% lower risk of contracting a hospital acquired infection versus patients in non-coppered ICU rooms.[21] [22][23] The U.S. Department of Defense investigation contract, which is ongoing, will also evaluate the effectiveness of copper alloy touch surfaces to prevent the transfer of microbes to patients and the transfer of microbes from patients to touch surfaces, as well as the potential efficacy of copper-alloy based components to improve indoor air quality.

In the U.S., the Environmental Protection Agency regulates the registration of antimicrobial products. After extensive antimicrobial testing according to the Agency’s stringent test protocols, 355 copper alloys, including many brasses, were found to kill more than 99.9% of methicillin-resistant Staphylococcus aureus (MRSA), E. coli O157:H7, Pseudomonas aeruginosa, Staphylococcus aureus, Enterobacter aerogenes, and vancomycin-resistant Enterococci (VRE) within two hours of contact.[24][25] Normal tarnishing was found to not impair antimicrobial effectiveness.

Antimicrobial tests have also revealed significant reductions of MRSA as well as two strains of epidemic MRSA (EMRSA-1 and EMRSA-16) on brass (C24000 with 80% Cu) at room temperature (22 degrees Celsius) within three hours. Complete kills of the pathogens were observed within 4 ½ hours. These tests were performed under wet exposure conditions. The kill timeframes, while impressive, are nevertheless longer than for pure copper, where kill timeframes ranged between 45 to 90 minutes.[26]

A novel assay that mimics dry bacterial exposure to touch surfaces was developed because this test method is thought to more closely replicate real world touch surface exposure conditions. In these conditions, copper alloy surfaces were found to kill several million Colony Forming Units of Escherichia coli within minutes.[27] This observation, and the fact that kill timeframes shorten as the percentage of copper in an alloy increases, is proof that copper is the ingredient in brass and other copper alloys that kills the microbes.[28]

The mechanisms of antimicrobial action by copper and its alloys, including brass, is a subject of intense and ongoing investigation.[29][30][31] It is believed that the mechanisms are multifaceted and include the following: 1) Potassium or glutamate leakage through the outer membrane of bacteria; 2) Osmotic balance disturbances; 3) Binding to proteins that do not require or utilize copper; 4) Oxidative stress by hydrogen peroxide generation.

Research is being conducted at this time to determine whether brass, copper, and other copper alloys can help to reduce cross contamination in public facilities and reduce the incidence of nosocomial infections (hospital acquired infections) in healthcare facilities.

Also, due to its antimicrobial/algaecidal properties that prevent biofouling, in conjunction with its strong structural and corrosion-resistant benefits for marine environments, brass alloy netting cages are currently being deployed in commercial-scale aquaculture operations in Asia, South America, and the USA. [32][33][34]

Season cracking

Cracking in brass caused by ammonia attack

Brass is susceptible to stress corrosion cracking, especially from ammonia or substances containing or releasing ammonia. The problem is sometimes known as season cracking after it was first discovered in brass cartridge cases used for rifle ammunition during the 1920s in the Indian Army. The problem was caused by high residual stresses from cold forming of the cases during manufacture, together with chemical attack from traces of ammonia in the atmosphere. The cartridges were stored in stables and the ammonia concentration rose during the hot summer months, so initiating brittle cracks. The problem was resolved by annealing the cases, and storing the cartridges elsewhere.

Brass types

  • Admiralty brass contains 30% zinc, and 1% tin which inhibits dezincification in many environments.
  • Aich's alloy typically contains 60.66% copper, 36.58% zinc, 1.02% tin, and 1.74% iron. Designed for use in marine service owing to its corrosion resistance, hardness and toughness. A characteristic application is to the protection of ships' bottoms, but more modern methods of cathodic protection have rendered its use less common. Its appearance resembles that of gold.[35]
  • Alpha brasses with less than 35% zinc, are malleable, can be worked cold, and are used in pressing, forging, or similar applications. They contain only one phase, with face-centered cubic crystal structure.
  • Prince's metal or Prince Rupert's metal is a type of alpha brass containing 75% copper and 25% zinc. Due to its beautiful yellow color, it is used as an imitation of gold.[36] The alloy was named after Prince Rupert of the Rhine.
  • Alpha-beta brass (Muntz metal), also called duplex brass, is 35–45% zinc and is suited for hot working. It contains both α and β' phase; the β'-phase is body-centered cubic and is harder and stronger than α. Alpha-beta brasses are usually worked hot.
  • Aluminium brass contains aluminium, which improves its corrosion resistance. It is used for seawater service[37] and also in Euro coins (Nordic gold).
  • Arsenical brass contains an addition of arsenic and frequently aluminium and is used for boiler fireboxes.
  • Beta brasses, with 45–50% zinc content, can only be worked hot, and are harder, stronger, and suitable for casting.
  • Cartridge brass is a 30% zinc brass with good cold working properties. Used for ammunition cases.
  • Common brass, or rivet brass, is a 37% zinc brass, cheap and standard for cold working.
  • DZR brass is dezincification resistant brass with a small percentage of arsenic.
  • Gilding metal is the softest type of brass commonly available. An alloy of 95% copper and 5% zinc, gilding metal is typically used for ammunition "jackets", e.g. full metal jacket bullets.
  • High brass contains 65% copper and 35% zinc, has a high tensile strength and is used for springs, screws, and rivets.
  • Leaded brass is an alpha-beta brass with an addition of lead. It has excellent machinability.
  • Lead-free brass as defined by California Assembly Bill AB 1953 contains "not more than 0.25 percent lead content".[10]
  • Low brass is a copper-zinc alloy containing 20% zinc with a light golden color and excellent ductility; it is used for flexible metal hoses and metal bellows.
  • Manganese brass is a brass most notably used in making golden dollar coins in the United States. It contains roughly 70% copper, 29% zinc, and 1.3% manganese.[38]
  • Muntz metal is about 60% copper, 40% zinc and a trace of iron, used as a lining on boats.
  • Nickel brass is composed of 70% copper, 24.5% zinc and 5.5% nickel used to make pound coins in the pound sterling currency.
  • Naval brass, similar to admiralty brass, is 40% zinc and 1% tin.
  • Nordic gold, used in 10, 20 and 50 cts euro coins, contains 89% copper, 5% aluminium, 5% zinc, and 1% tin.
  • Red brass is both an American term for the copper-zinc-tin alloy known as gunmetal, and an alloy which is considered both a brass and a bronze. It typically contains 85% copper, 5% tin, 5% lead, and 5% zinc.[39][40] Red brass is also an alternative name for copper alloy C23000, which is composed of 14–16% zinc, 0.05% iron and lead, and the remainder copper.[41] It may also refer to ounce metal, another copper-zinc-tin alloy.
  • Rich low brass (Tombac) is 15% zinc. It is often used in jewelry applications.
  • Tonval brass (also called CW617N or CZ122 or OT58) is a copper-lead-zinc alloy. It is not recommended for seawater use, being susceptible to dezincification.[42]
  • White brass contains more than 50% zinc and is too brittle for general use. The term may also refer to certain types of nickel silver alloys as well as Cu-Zn-Sn alloys with high proportions (typically 40%+) of tin and/or zinc, as well as predominantly zinc casting alloys with copper additive.
  • Yellow brass is an American term for 33% zinc brass.

History

Although forms of brass have been in use since prehistory,[43] its true nature as a copper-zinc alloy was not understood until the post medieval period because the zinc vapor which reacted with copper to make brass was not recognised as a metal.[44] The King James Bible makes many references to "brass".[45] The Shakespearean English form of the word 'brass' can mean any bronze alloy, or copper, rather than the strict modern definition of brass. [citation needed] The earliest brasses may have been natural alloys made by smelting zinc-rich copper ores.[46] By the Roman period brass was being deliberately produced from metallic copper and zinc minerals using the cementation process and variations on this method continued until the mid 19th century.[47] It was eventually replaced by speltering, the direct alloying of copper and zinc metal which was introduced to Europe in the 16th century.[46]

Early copper zinc alloys

In West Asia and the Eastern Mediterranean early copper zinc alloys are now known in small numbers from a number of third Millennium BC sites in the Aegean, Iraq, the United Arab Emirates, Kalmikia, Turkmenistan and Georgia and from 2nd Millennium BC sites in West India, Uzbekistan, Iran, Syria, Iraq and Palestine.[48] However, isolated examples of copper-zinc alloys are known in China from as early as the 5th Millennium BC.[49]

The compositions of these early "brass" objects are very variable and most have zinc contents of between 5% and 15% wt which is lower than in brass produced by cementation.[50] These may be "natural alloys" manufactured by smelting zinc rich copper ores in reducing conditions. Many have similar tin contents to contemporary bronze artefacts and it is possible that some copper-zinc alloys were accidental and perhaps not even distinguished from copper.[50] However the large number of copper-zinc alloys now known suggests that at least some were deliberately manufactured and many have zinc contents of more than 12% wt which would have resulted in a distinctive golden color.[51]

By the 8th–7th century BC Assyrian cuneiform tablets mention the exploitation of the "copper of the mountains" and this may refer to "natural" brass.[52] Oreichalkos, the Ancient Greek translation of this term, was later adapted to the Latin aurichalcum meaning "golden copper" which became the standard term for brass.[53] In the 4th century BC Plato knew oreichalkos as rare and nearly as valuable as gold[54] and Pliny describes how aurichalcum had come from Cypriot ore deposits which had been exhausted by the 1st century AD.[55]

Brass making in the Roman World

During the later part of first Millennium BC the use of brass spread across a wide geographical area from Britain[56] and Spain[57] in the west to Iran, and India in the east.[58] This seems to have been encouraged by exports and influence from the Middle-East and eastern Mediterranean where deliberate production of brass from metallic copper and zinc ores had been introduced.[59] The 4th century BC writer Theopompus, quoted by Strabo, describes how heating earth from Andeira in Turkey produced "droplets of false silver", probably metallic zinc, which could be used to turn copper into oreichalkos.[60] In the 1st century BC the Greek Dioscorides seems to have recognised a link between zinc minerals and brass describing how Cadmia (zinc oxide) was found on the walls of furnaces used to heat either zinc ore or brass and explaining that it can then be used to make brass.[61]

By the first century BC brass was available in sufficient supply to use as coinage in Phrygia and Bithynia,[62] and after the Augustan currency reform of 23 BC it was also used to make Roman dupondii and sestertii.[63] The uniform use of brass for coinage and military equipment across the Roman world may indicate a degree of state involvement in the industry,[64][65] and brass even seems to have been deliberately boycotted by Jewish communities in Palestine because of its association with Roman authority.[66]

Brass was produced by the cementation process where copper and zinc ore are heated together until zinc vapor is produced which reacts with the copper. There is good archaeological evidence for this process and crucibles used to produce brass by cementation have been found on Roman period sites including Xanten[67] and Nidda[68] in Germany, Lyon in France[69] and at a number of sites in Britain.[70] They vary in size from tiny acorn sized to large amphorae like vessels but all have elevated levels of zinc on the interior and are lidded.[69] They show no signs of slag or metal prills suggesting that zinc minerals were heated to produce zinc vapor which reacted with metallic copper in a solid state reaction. The fabric of these crucibles is porous, probably designed to prevent a build up of pressure, and many have small holes in the lids which may be designed to release pressure[69] or to add additional zinc minerals near the end of the process. Dioscorides mentioned that zinc minerals were used for both the working and finishing of brass, perhaps suggesting secondary additions.[71]

Brass made during the early Roman period seems to have varied between 20% to 28% wt zinc.[72] The high content of zinc in coinage and brass objects declined after the first century AD and it has been suggested that this reflects zinc loss during recycling and thus an interruption in the production of new brass.[73] However it is now thought this was probably a deliberate change in composition[74] and overall the use of brass increases over this period making up around 40% of all copper alloys used in the Roman world by the 4th century AD.[75]

Brass making in the medieval period

Baptism of Christ on the 12th century Baptismal font at St Bartholomew's Church, Liège.

Little is known about the production of brass during the centuries immediately after the collapse of the Roman Empire. Disruption in the trade of tin for bronze from Western Europe may have contributed to the increasing popularity of brass in the east and by the 6th–7th centuries AD over 90% of copper alloy artefacts from Egypt were made of brass.[76] However other alloys such as low tin bronze were also used and they vary depending on local cultural attitudes, the purpose of the metal and access to zinc, especially between the Islamic and Byzantine world.[77] Conversely the use of true brass seems to have declined in Western Europe during this period in favour of gunmetals and other mixed alloys[78] but by the end of the first Millennium AD brass artefacts are found in Scandinavian graves in Scotland,[79] brass was being used in the manufacture of coins in Northumbria[80] and there is archaeological and historical evidence for the production of brass in Germany[81] and The Low Countries[82] areas rich in calamine ore which would remain important centres of brass making throughout the medieval period,[83] especially Dinant – brass objects are still collectively known as dinanterie in French. The Baptismal font at St Bartholomew's Church, Liège in modern Belgium (before 1117) is an outstanding masterpiece of Romanesque brass casting.

The cementation process continued to be used but literary sources from both Europe and the Islamic world seem to describe variants of a higher temperature liquid process which took places in open topped crucibles.[84] Islamic cementation seems to have used zinc oxide known as tutiya or tutty rather than zinc ores for brass making resulting in a metal with lower iron impurities.[85] A number of Islamic writers and the 13th century Italian Marco Polo describe how this was obtained by sublimation from zinc ores and condensed onto clay or iron bars, archaeological examples of which have been identified at Kush in Iran.[86] It could then be used for brass making or medicinal purposes. In 10th century Yemen al-Hamdani described how spreading al-iglimiya, probably zinc oxide, onto the surface of molten copper produced tutiya vapor which then reacted with the metal.[87] The 13th century Iranian writer al-Kashani describes a more complex process whereby tutiya was mixed with raisins and gently roasted before being added to the surface of the molten metal. A temporary lid was added at this point presumably to minimise the escape of zinc vapor.[88]

In Europe a similar liquid process in open topped crucibles took place which was probably less efficient than the Roman process and the use of the term tutty by Albertus Magnus in the 13th century suggests influence from Islamic technology.[89] The 12th century German monk Theophilus described how preheated crucibles were one sixth filled with powdered calamine and charcoal then topped up with copper and charcoal before being melted, stirred then filled again. The final product was cast, then again melted with calamine. It has been suggested that this second melting may have taken place at a lower temperature to allow more zinc to be absorbed.[90] Albertus Magnus noted that the "power" of both calamine and tutty could evaporate and described how the addition of powdered glass could create a film to bind it to the metal.[91] German brass making crucibles are known from Dortmund dating to the 10th century AD and from Soest and Schwerte in Westphalia dating to around the 13th century confirm Theophilus' account as they are open topped, although ceramic discs from Soest may have served as loose lids which may have been used to reduce zinc evaporation, and have slag on the interior resulting from a liquid process.[92]

Brass making in Renaissance and post medieval Europe

The Renaissance saw important changes to both the theory and practice of brassmaking in Europe. By the 15th century there is evidence for the renewed use of lidded cementation crucibles at Zwickau in Germany.[93] These large crucibles were capable of producing c.20 kg of brass.[94] There are traces of slag and pieces of metal on the interior. Their irregular composition suggesting that this was a lower temperature not entirely liquid process.[95] The crucible lids had small holes which were blocked with clay plugs near the end of the process presumably to maximise zinc absorption in the final stages.[96] Triangular crucibles were then used to melt the brass for casting.[97]

16th century technical writers such as Biringuccio, Ercker and Agricola described a variety of cementation brass making techniques and came closer to understanding the true nature of the process noting that copper became heavier as it changed to brass and that it became more golden as additional calamine was added.[98] Zinc metal was also becoming more commonplace By 1513 metallic zinc ingots from India and China were arriving in London and pellets of zinc condensed in furnace flues at the Rammelsberg in Germany were exploited for cementation brass making from around 1550.[99]

Eventually it was discovered that metallic zinc could be alloyed with copper to make brass; a process known as speltering[100] and by 1657 the German chemist Johann Glauber had recognised that calamine was "nothing else but unmeltable zinc" and that zinc was a "half ripe metal."[101] However some earlier high zinc, low iron brasses such as the 1530 Wightman brass memorial plaque from England may have been made by alloying copper with zinc and include traces of cadmium similar those found in some zinc ingots from China.[100]

However the cementation process was not abandoned and as late as the early 19th century there are descriptions of solid state cementation in a domed furnace at around 900–950 degrees Celsius and lasting up to 10 hours.[102] The European brass industry continued to flourish into the post medieval period buoyed by innovations such as the 16th century introduction of water powered hammers for the production of battery wares.[103] By 1559 the Germany city of Aachen alone was capable of producing 300,000 cwt of brass per year.[103] After several false starts during the 16th and 17th centuries the brass industry was also established in England taking advantage of abundant supplies of cheap copper smelted in the new coal fired reverberatory furnace.[104] In 1723 Bristol brass maker Nehemiah Champion patented the use of granulated copper, produced by pouring molten metal into cold water.[105] This increased the surface area of the copper helping it react and zinc contents of up to 33% wt were reported using this new technique.[106]

In 1738 Nehemiah's son William Champion patented a technique for the first industrial scale distillation of metallic zinc known as distillation per descencum or "the English process."[107][108] This local zinc was used in speltering and allowed greater control over the zinc content of brass and the production of high zinc copper alloys which would have been difficult or impossible to produce using cementation, for use in expensive objects such as scientific instruments, clocks, brass buttons and costume jewellery.[109] However Champion continued to use the cheaper calamine cementation method to produce lower zinc brass [109] and the archaeological remains of bee-hive shaped cementation furnaces have been identified at his works at Warmley.[110] By the mid late 18th century developments in cheaper zinc distillation such as John-Jaques Dony's horizontal furnaces in Belgium and the reduction of tariffs on zinc[111] as well as demand for corrosion resistant high zinc alloys increased the popularity of speltering and as a result cementation was largely abandoned by the mid 19th century.[112]

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Bibliography

  • Bayley, J. (1990) "The Production of Brass in Antiquity with Particular Reference to Roman Britain" in Craddock, P.T. (ed.) 2000 Years of Zinc and Brass London: British Museum
  • Craddock, P.T. and Eckstein, K (2003) "Production of Brass in Antiquity by Direct Reduction" in Craddock, P.T. and Lang, J. (eds) Mining and Metal Production Through the Ages London: British Museum
  • Day, J. (1990) "Brass and Zinc in Europe from the Middle Ages until the 19th Century" in Craddock, P.T. (ed.) 2000 Years of Zinc and Brass London: British Museum
  • Day, J (1991) "Copper, Zinc and Brass Production" in Day, J and Tylecote, R.F (eds) The Industrial Revolution in Metals London: The Institute of Metals
  • Martinon Torres, M. and Rehren, T. (2002) Agricola and Zwickau: Theory and Practice of Renaissance Brass Production in SE Germany in Historical Metallurgy 36(2) pp. 95–111
  • Rehren, T. and Martinon Torres, M. (2008) "Naturam ars imitate: European brassmaking between craft and science" in Martinon-Torres, M and Rehren, T. (eds) Archaeology, History and Science Integrating Approaches to Ancient Material: Left Coast Press
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