Brass

• 1. Introduction

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  Brass, the alloy of copper and zinc, turns up in a 2,600-year-old shipwreck off Sicily. From that wreck came 39 ingots. X-ray fluorescence analysis found the metal to be an alloy of 75-80% copper and 15-20% zinc, with small percentages of nickel, lead and iron.

  Making brass did not require understanding it. Zinc vapor did the work in the furnace, and its nature as a metal went unrecognised until the post-medieval period. Brass has been in use since prehistoric times, yet its true recipe stayed hidden for thousands of years.

  How do people make an alloy from a metal they cannot identify? Why did the Roman world stamp brass onto its coins, and why did a German monk write down instructions for melting it? And how can the same two ingredients produce a golden doorknob in one workshop and a nearly silver casting in another? The answers run from ancient furnaces to modern plumbing and the bell of a trumpet.

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• 2. Golden Copper

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  In the 3rd millennium BC, small numbers of copper-zinc objects appear at sites in the Aegean, Iraq, the United Arab Emirates, Kalmykia, Turkmenistan and Georgia. Sites of the 2nd millennium BC in western India, Uzbekistan, Iran, Syria, Iraq and Canaan add more. Their recipes vary widely. Most hold between 5% and 15% zinc, which is less than later brass, and many carry tin in amounts similar to the bronzes of the same period.

  Some of these objects were probably natural alloys, made by smelting copper ores that were already rich in zinc. Others may have been accidents that nobody distinguished from plain copper. Yet the large number of finds suggests that at least some were deliberate, and many contain more than 12% zinc, enough to give a distinctly golden colour. In China, only isolated examples are known, dating from the 1st century AD, long after bronze was widely used.

  Assyrian cuneiform tablets of the 8th-7th century BC mention the exploitation of the copper of the mountains, which may refer to natural brass. The ancient Greeks rendered the phrase as oreikhalkon. The Latin aurichalcum, meaning golden copper, later became the standard word for brass. In the 4th century BC, Plato knew orichalkos as rare and nearly as valuable as gold. Pliny wrote that the Cypriot ore deposits behind aurichalcum had been exhausted by the 1st century AD.

  The King James Bible shows how muddled the vocabulary stayed. It uses the word brass in many places to translate the Hebrew nechosheth, which means bronze or copper. Brass was also sometimes called yellow copper.

  The writer Theopompus, quoted by Strabo in the 4th century BC, described heating earth from Andeira in Turkey until it gave off droplets of false silver. That substance was probably metallic zinc, and it could turn copper into oreichalkos.

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• 3. Roman Crucibles

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  In the 1st century BC, the Greek writer Dioscorides seems to have recognised a link between zinc minerals and brass. He described Cadmia, which is zinc oxide, collecting on the walls of furnaces that heated either zinc ore or copper. He explained that it could then be used to make brass.

  By then the metal was travelling. During the later first millennium BC, its use spread from Britain and Spain in the west to Iran and India in the east. Exports and influence from the Middle East and the eastern Mediterranean seem to have encouraged this, since deliberate production from metallic copper and zinc minerals had been introduced there.

  By the first century BC, brass was plentiful enough for coinage in Phrygia and Bithynia. After the Augustan currency reform of 23 BC, it was also used for Roman dupondii and sestertii. The same alloy appears in military equipment across the empire, which may point to some degree of state involvement in the industry. Jewish communities in Palestine even seem to have boycotted brass deliberately, because of its association with Roman authority.

  The method was cementation. Copper and zinc ore were heated together until zinc vapor formed and reacted with the copper. Crucibles from this process have been found at Xanten and Nidda in Germany, at Lyon in France and at a number of sites in Britain. They range from tiny vessels the size of an acorn to large ones like amphorae. All are lidded and show raised zinc levels inside, but no slag or metal prills. That pattern suggests zinc vapor reacting with solid copper. The porous fabric may have prevented pressure building up, and small holes in the lids may have let pressure out or let extra zinc minerals in near the end.

  Early Roman brass held between 20% and 28% zinc. The zinc content of coins and objects then fell after the 1st century AD. Some scholars read this as zinc lost in recycling, which would mean a break in new production. The current view is that the change was probably deliberate. Brass use actually grew, and by the 4th century AD it made up around 40% of all copper alloys used in the Roman world.

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• 4. Medieval Recipes

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  By the 6th-7th centuries AD, over 90% of copper alloy artefacts from Egypt were made of brass. Disruption in the tin trade from Western Europe may have helped. Low tin bronze still circulated, and the mix of alloys depended on local attitudes, the purpose of the object and access to zinc, especially between the Islamic and Byzantine worlds.

  Western Europe went the other way for a time, favouring gunmetals and other mixed alloys. Yet by about 1000, brass objects appear in Scandinavian graves in Scotland, and Northumbria used brass for coins. Germany and the Low Countries, rich in calamine ore, were producing calamine brass. They stayed important centres through the Middle Ages, especially the town of Dinant. French still calls brass objects collectively dinanderie.

  The baptismal font at St Bartholomew's Church in Liège, made before 1117, is an outstanding piece of Romanesque casting, though it is often described as bronze. The early 12th-century Gloucester Candlestick is stranger. Its metal combines copper, zinc, tin, lead, nickel, iron, antimony and arsenic, with silver ranging from 22.5% in the base to 5.76% in the pan below the candle. That recipe may mean it was cast from a hoard of old coins, probably Late Roman. Medieval workers also used latten, a loose term for alloys of uncertain composition cut into sheets for decorative borders. Aquamaniles were typically made in brass in both the European and Islamic worlds.

  Islamic brassmakers often used zinc oxide, known as tutiya or tutty, in place of zinc ores. This gave a metal with fewer iron impurities. Marco Polo in the 13th century and several Islamic writers describe how tutiya was gathered by sublimation from zinc ores and condensed onto clay or iron bars, and archaeological examples have been identified at Kush in Iran. In 10th century Yemen, al-Hamdani described spreading al-iglimiya, probably zinc oxide, over molten copper so that tutiya vapor reacted with the metal. The 13th century Iranian writer al-Kashani added raisins to the mixture, gently roasted it, and fitted a temporary lid to keep the zinc vapor in.

  The 12th century German monk Theophilus gave a European recipe. Preheated crucibles were one sixth filled with powdered calamine and charcoal, then topped up with copper and charcoal. The charge was melted, stirred, filled again, cast, and then melted once more with calamine. The 13th century writer Albertus Magnus used the word tutty, which hints at Islamic influence. He also suggested that powdered glass could form a film to bind the power of calamine to the metal. Open-topped crucibles from Dortmund in the 10th century, and from Soest and Schwerte in Westphalia around the 13th century, match Theophilus. Ceramic discs from Soest may have served as loose lids to slow the loss of zinc.

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• 5. Brass in West Africa

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  The Bronze Head of Queen Idia, now in the British Museum, belongs to a tradition of lost wax casting from what is now Nigeria. The Kingdom of Ife produced such work first, and the Benin Empire followed. The famous Benin Bronzes, mostly held today in the British Museum and other Western collections, are usually called bronzes. They are better described as brass, though their compositions vary.

  The large portrait head from Ife in the British Museum, another celebrated example, is made of what the source calls "heavily leaded zinc-brass". Work in brass or bronze stayed central to Benin art. West African communities valued the metal more highly than Europeans did, and Akan goldweights are among the traditions that used it.

  Brass also served as money. Manilla bracelets of brass were used as a means of exchange across West Africa, and they were still circulating into the 20th century.

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• 6. From Calamine to Zinc

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  At Zwickau in Germany, 15th century workers were again using lidded cementation crucibles. Each large crucible could produce c.20 kg of brass. The lids had small holes that were blocked with clay plugs near the end of the process, probably to maximise zinc absorption in the final stages. Traces of slag and metal inside, together with an irregular composition, suggest a lower temperature process that was not entirely liquid. Triangular crucibles then melted the brass for casting.

  The 16th century technical writers Biringuccio, Ercker and Agricola came closer to the truth. They noted that copper grew heavier as it became brass, and more golden as more calamine was added. Zinc metal was becoming more common too. By 1513, ingots of metallic zinc from India and China were arriving in London. From around 1550, pellets of zinc that condensed in furnace flues at the Rammelsberg in Germany were used for cementation brass. India itself used brass for the hilts of Sirohi swords between the 16th and 19th centuries.

  Metallic zinc could be alloyed directly with copper, a process called speltering. The 1530 Wightman brass memorial plaque in England may have been made this way, since it is high in zinc and low in iron and contains traces of cadmium like those in some Chinese zinc ingots. In 1657 the German chemist Johann Glauber recognised that calamine was "nothing else but unmeltable zinc" and that zinc was a "half ripe metal".

  By 1559, the German city of Aachen alone could produce 300,000 cwt of brass a year. Water powered hammers, introduced in the 16th century, helped turn out wares such as pots. England established its own brass industry after several false starts in the 16th and 17th centuries, using cheap copper smelted in the new coal fired reverberatory furnace. In 1723 the Bristol brass maker Nehemiah Champion patented granulated copper, made by pouring molten metal into cold water. The larger surface area helped the copper react, and zinc contents of up to 33% were reported.

  His son William Champion patented the first industrial scale distillation of metallic zinc in 1738, called distillation per descencum or the English process. It gave makers tighter control over zinc content and allowed high-zinc alloys for expensive objects such as scientific instruments, clocks, brass buttons and costume jewelry. William still used cheaper calamine cementation for lower-zinc brass, and bee-hive shaped cementation furnaces have been identified at his works at Warmley.

  Cheaper distillation, such as John-Jaques Dony's horizontal furnaces in Belgium, and lower tariffs on zinc made speltering more popular by the mid-to-late 18th century. Demand for corrosion-resistant high-zinc alloys pushed the same way. Cementation was largely abandoned by the mid-19th century, though descriptions from the early 19th century still record solid-state cementation in domed furnaces at around 900-950 degrees Celsius, lasting up to 10 hours.

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• 7. Tuning the Alloy

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  Brass melts at 900 to 940 degrees Celsius, depending on composition, and its flow characteristics make it relatively easy to cast. Its density is 8.4 to 8.73 g/cm3. It is more malleable than bronze or zinc. Changing the balance of copper and zinc changes the metal. More copper gives a softer, more golden brass, while more zinc gives a harder and more silvery one.

  Metallurgists sort brass by its crystal phases. Alpha brasses have more than 65% copper and less than 35% zinc. They have a single face-centred cubic structure and are worked cold, with best formability at 32% zinc. Alpha-beta brasses, with 55-65% copper, are usually worked hot, and strength peaks at 45% zinc. Beta brasses have 50-55% copper, can only be worked hot, and suit casting. White brass, with under 50% copper, is too brittle for general use. A gamma phase also exists, a cubic intermetallic compound written Cu5Zn8.

  Named recipes fill the catalogue. Gilding metal, 95% copper and 5% zinc, is the softest common brass, almost red, and is used for bullet jackets. Cartridge brass, 70% copper and 30% zinc, makes ammunition cases and token coins. Muntz metal, 60% copper and 40% zinc with traces of iron, lined boats. Manganese brass clads United States golden dollar coins, and nickel brass forms the outer ring of the one pound coin and the one euro coin. Nordic gold appears in the 10, 20 and 50 cent euro coins.

  Some alloys are bound up with people. Pinchbeck, invented in the early 18th century by Christopher Pinchbeck, passes for gold in budget jewelry. Prince's metal, a gold-coloured alpha brass, takes its name from Prince Rupert of the Rhine. The firm Doehler Die Casting Co. of Toledo, Ohio, produced Brastil, a copper, zinc and silicon alloy that was tested in 1932 on an M1911 pistol because it was cheaper than steel at the time.

  Small additions change behaviour. Aluminium strengthens brass and forms a thin, transparent, self-healing layer of aluminium oxide on the surface. Tin has a similar effect. As little as 1% iron gives a noticeable magnetic attraction. Because ordinary brass is not ferromagnetic, a powerful magnet separates ferrous scrap from it, and almost 90% of all brass alloys are recycled. The scrap is melted into billets and extruded. Brass is also used in non-sparking fittings and tools near flammable or explosive materials.

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• 8. Where Brass Fails

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  In the 1920s, rifle cartridges in the British Indian Army began to crack. The brass cases carried high residual stress from cold forming during manufacture. The cartridges were stored in stables, where ammonia levels rose in the hot summer months and attacked the metal. The problem became known as season cracking. Annealing the cases and storing the cartridges elsewhere solved it.

  Brass corrodes in the presence of moisture, chlorides, acetates, ammonia and certain acids. The copper often reacts with sulfur to form a brown and eventually black layer of copper sulfide. Urban rainwater can then turn that into a green-blue patina of copper carbonate, which in some cases protects the metal underneath. Contact with a more noble metal such as silver or gold makes brass corrode. Contact with a less noble metal such as zinc or iron protects the brass instead.

  Saltwater attacks the zinc and leaves porous copper behind, so ordinary brass is unsuitable for marine use. Naval brass answers this with 40% zinc and 1% tin. Aich's alloy, with 1.02% tin and 1.74% iron, was designed to protect ships' bottoms. Dezincification-resistant brasses suit boiler systems. One example, C352, has about 30% zinc, 61-63% copper, 1.7-2.8% lead and 0.02-0.15% arsenic. NSF International requires brasses with more than 15% zinc in piping and plumbing fittings to resist dezincification.

  Lead adds a different problem. About 2% is often added to make brass easier to machine. As the metal cools, the lead gathers in globules at the grain boundaries, and cutting can smear it across the surface. Lead can then leach out even from brasses with comparatively little of it.

  In October 1999, the California attorney general sued 13 key manufacturers and distributors over lead content. State researchers found that the average brass key, new or old, exceeded the Proposition 65 limit by a factor of 19, assuming handling twice a day. In April 2001 the makers agreed to cut lead to 1.5% or warn consumers. Plated keys were exempt. On the 1st of January 2010, California cut the lead limit for lead-free brass from 4.0% to 0.25%.

  Saxophones are classed as woodwinds, yet they are normally made of brass. Their wide, conical bores and thin-walled bodies are easier to form from sheet metal than to cut from wood. The same logic puts brass at the heart of the instruments collectively called the brass. Trombone, tuba, trumpet, cornet, flugelhorn, baritone horn, euphonium, tenor horn, French horn and the saxhorns all use long, narrow tubing, often folded or coiled. Silver and even gold have worked for such tubing, but brass is the most economical choice.

  Other wind instruments borrow it too. Most modern student flutes and piccolos are made of a brass variety, usually a cupronickel alloy similar to nickel silver. Low clarinets such as the contrabass and subcontrabass are sometimes metal because dense tropical hardwoods are in limited supply. Some low clarinets, bassoons and contrabassoons are hybrids, with straight wooden sections and curved metal joints, necks or bells. Metal also avoids the sudden cracking that temperature or humidity changes can cause in wood.

  Cymbals, gongs and orchestral tubular bells are the most notable percussion uses. Large church bells are normally bronze, while small handbells and jingle bells are often brass. Harmonicas, brass reed tongues in reed organ pipes, and some snare drums use it as well. On electric guitars, brass inertia blocks on tremolo systems, string nuts and saddles are valued for their tone, and the nuts and saddles for low friction too.

  Centuries of observation, especially in marine settings where brass prevents biofouling, point to a bactericidal effect. Depending on the pathogens and the medium, brass kills microorganisms within a few minutes to hours of contact. Many independent studies confirm this, including against antibiotic-resistant bacteria such as MRSA and VRSA. How copper and its alloys achieve it is still under intense investigation.

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