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Copper, Silver and Gold The Royal Family Minerals. They are also called noble metals, and all others base metals. They are the royal family of the periodic table, family IB. Their alchemical symbols are the signs of the three brightest planets, the sun, the moon and Venus, their colors yellow, white and red.
Copper, silver and gold are found in elemental or "native" form at the earth's surface. The best nuggets have been picked up already, since the land has been scoured for them wherever there are people who appreciate them. They are called the currency metals, since they have been used in coins since the beginning of money. The three largest U.S. coins in each of the metals are shown at the right. The large cent was issued 1793-1857, and weighs 11g. The Peace Dollar was minted 1921-1935, and weighs 27g. The St. Gaudens Double Eagle, $20, was minted 1907-1933, and weighs 34 g. At today's prices, the metal in these coins is worth $0.02, $4.27 and $405.61. Now, however, copper alloys predominate in the miserable debased coins of no artistic merit we use every day.
All three are not only delightful in appearance and resistant to corrosion, but can be hammered into ornaments of artistic distinction. The three are ductile, capable of being drawn into fine wires, and malleable, capable of being hammered into thin sheets. Gold, indeed, is outstandingly ductile and malleable, able to be drawn into wires thinner than a hair and beaten into sheets thin enough to transmit light. All three metals are important in technology, especially in electronics.
Copper and silver, and perhaps even gold, harden when they have been worked, and it becomes impossible to work them further, even with great effort, without danger of cracking and crumbling. This work hardening is seen even when you bend the lid of a tin can back and forth to break it off. However, if the metal is heated sufficiently to allow the metal to recrystallize, on cooling it will be restored to its original soft, workable state. This process of annealing could have been the first metallurgical practice discovered. Copper anneals at a temperature of 200°C - 750°C, the pure native copper at lower rather than higher temperatures. These temperatures are less than a red heat, and are obtained in any common fire.
However, the artisans were still limited to the size of the nugget that was picked up. Large objects required large nuggets, and were correspondingly more valuable than a handful of small objects of the same weight. To make larger objects, the metal has to be melted and poured into a mould. The temperature required to melt the noble metals is close to 1000°C, practically a "white heat." This requires a fire blown by bellows or some other means, though in favorable circumstances a good natural draft might suffice. The fusion of metals could have been the second metallurgical process discovered.
It should be mentioned that most stories about the history of metallurgy are just that: stories. There is no detailed evidence of methods, only the rare object found in excavations, often misidentified, mislabeled and misinterpreted by those unversed in metallurgy or chemistry. Nobody knows the order of discovery, or who the discoverers were. From the scraps of information, a story is constructed on the basis of our current understanding of metals, just as I did in making annealing the first discovery, and fusion the second. This is logical, but not proved. We have much better evidence from classical times, but still do not know much in detail.
Copper knives and weapons were made in pre-dynastic Egypt, perhaps as long ago as 6000 BC by some authorities. Pipes and tubes were found from 2750 BC, and a bronze mirror dates from 1800 BC. However, metals were rare in Egypt, and a non-metallic culture predominated. The smelting of metals from ores is a significant and much later discovery than the use of native metals, annealing and fusion. This is discussed at more length in Tin. This metallurgy may have originated in Mesopotamia around 2000 BC, and perhaps also in India. The Phoenicians made smelted copper and tin available to the Mediterranean world, and with it the alloy bronze, around 1800-1500 BC. I would like to have better estimates of these dates, but the authorities seem to be in disagreement, and not all that knowledgeable about metals anyway.
Although copper was probably originally encountered in nature as the native metal, it was also probably the first to be extracted by smelting from ores. Zippe proposes that this was discovered when heat was used to break rocks for the native copper they contained, since stone tools would not suffice, and some of the ore, so different in appearance from metallic copper, was reduced. Copper was the first metal whose ores were mined, and some of the earliest mines have left traces, as in the Sinai peninsula.
Copper was the predominant early metal, especially after the technique of smelting it from its ores and trade made it widely available. It is still the third most important industrial metal, after iron and aluminium. Our word "copper" comes from the Plattdeutsch coper or koper, still used in Dutch. In Latin, it was known as cyprium aes, "brass from Cyprus" or "brass of Venus." Aes was the Latin for all coppery-bronzy-brassy alloys, which I have translated as "brass," though what we call brass was a later development, but still Classical. Copper in particular was also called adhenus, as was a copper pot. However, the Venusian name stuck in the West, and gave us cobre in Spanish, cuivre in French and Kupfer in German. In Welsh it is copr, which suggests that this word comes from old Celtic, and is unassociated with Venus. In Russian, copper is med', a quite different word, whose etymology might give us valuable information.
Greek, also, had a different word. Here, the equivalent of aes was calkos, "chalcos," named after the copper mines at Chalcis in Euboea. This stem is often seen as referring to copper, notably in mineralogy. "Chalcopyrite" is copper pyrites. The word was applied to iron as well after its introduction, long before the coining of sideros, "sideros." The word for steel is caluy, clearly related to chalcos. The Phoenicians, and others, worked copper mines and smelters in Cyprus, Kupris. Venus, or Aphrodite, the Kupris, was born there, always loved the island, and was its patron. She is very often referred to as Cypris. The word "cuprium" could have come either from the island or the goddess, and it is impossible to make a distinction. The word cuprum was probably later Latin, and gave us the chemical symbol Cu. The alchemical symbol was the sign of Venus, shown in the title of this article. The Phoenicians were surely the ones who spread the knowledge and use of copper around the Mediterranean, and their metallurgical procedures were probably kept secret.
The fact that pure copper and its alloys were not distinguished in ancient words shows that a fundamental difference was not appreciated. Our recognition of chemical elements is recent, and quite foreign to earlier thought. Most ancient "copper" is indeed bronze, containing tin but also lead and zinc. The alloys have a somewhat lower melting point than pure copper, and so would be easier to work, besides being much harder and more usable as tools and weapons. Zippe concludes from the various names of copper and its alloys used by different peoples the probable multiple discovery of its smelting. He also remarks, quite to the point, that it is just as possible that a source of a metal was named after the metal as that the metal was named after a place.
Copper is often described as a "red" metal, though its actual color is an orange-red of lower intensity, not a bright signal red. It is not a spectral color by any means, but a particular impure one requiring its own name, such as "copper-red." The red color is produced by the density of electrons being insufficient to cause a high plasma frequency, so the shorter wavelengths are not reflected as efficiently as the longer, redder ones. The red color is unique to copper and its alloys.
Copper has atomic number 29, atomic weight 63.57, and density 8.94 g/cc. Its naturally-occurring isotopes have mass numbers 63 (69%) and 65 (31%). The electron configuration has one 4s electron outside a filled 3d shell in the ground state. However, the energies of the 3d and 4s orbitals are about equal, so a 3d94s2 configuration is as favored as a 3d104s. Copper exhibits valences +1 (cuprous) and +2 (cupric), with the +2 predominating. Copper metal has a face-centered cubic structure, with a = 0.361 nm. Each ion has donated one electron to the Fermi sphere. The work function of copper is about 4.7 eV (tabulated values vary from 3.85 to 4.86). The Fermi energy is 7.0 eV. The electrical resistivity of annealed copper is 1.7241 μΩ-cm, of hard-drawn, 1.771 μΩ-cm, and the temperature coefficients are 0.00393 and 0.00382 per °C, respectively. For pure copper, the resistivity is 1.683 μΩ-cm. The thermal conductivity is 0.923 cal/cm-s-K, and the linear coefficient of expansion is 16.42 x 10-6 per °C. The specific heat is 0.0918 cal/g-K. The melting point of copper is 1083°C, boiling point 2325°C, and the heat of fusion is 50.6 cal/g. Its hardness is 3.0 on the Mohs scale. The tensile strength of annealed copper is about 30 ksi, of hard-drawn copper, 60 ksi. The Young's modulus is 16 x 106 psi.
For most uses, copper is alloyed with other metals, though copper usually predominates. Bronze is discussed in Tin and brass in Zinc. Copper-nickel alloys are used in coinage. Aluminium makes the excellent aluminium bronze. Pure copper is used for electrical conductors, since any alloy usually greatly increases the resistivity. Silver, cadmium and zinc have the least effect. 1% Cd decreases the conductivity to 94%, while hardening and strengthening the copper. Cadmium copper is used for contact wires. Arsenic, a common hardening addition to copper, has a large effect. 0.1% As lowers the conductivity to 75% of the value for pure copper, 0.5% As to 40%. Arsenic raises the softening (annealing) temperature to around 550°C from 190°C for pure copper. Arsenical copper with 0.5% As is used in boiler fireboxes, tubes and rivets to give strength at elevated temperatures. More arsenic embrittles the copper, and is very undesirable. Speculum metal, 89 Cu 33 Sn, is white and can be polished to make good mirrors. It was known in antiquity and used for mirrors.
Copper, like the other members of the royal family, is unreactive, with an electrode potential of 0.47V. It is not attacked by nonoxidizing acids or alkalies. In moist air, however, it may be attacked even by dilute acids. The CO2 in moist air creates a layer of CuCO3·Cu(OH)2, a basic copper carbonate called verdigris of a greenish color, called a patina, considered to be attractive. Otherwise, as in household copper, oxygen and sulphur make a film of black CuO or CuS tarnish. This happens very slowly, and the copper is not corroded. If regularly polished, it remains shiny and attractive.
The process of "flashing" a deep red surface layer of glass containing colloidal copper oxide was discovered in Bohemia around 1842, and became an alternative to the ruby glass that was a colloidal suspension of gold. This process was widely used to make signal lenses and similar products in place of the more expensive ruby glass. Note that the colour here is superficial, while in ruby glass it is a body colour. Copper also makes green, blue and brown pigments.
Cuprous oxide, Cu2O is red, and cuprous sulphide, Cu2S is black. Both are very insoluble. In any soluble cuprous compound, auto-oxidation generally occurs, 2Cu+ → Cu + Cu++, producing the cupric salt. Cupric oxide, CuO is produced by heating copper in air, or by strongly heating any oxygen-containing cupric salt. CuS is produced in an analogous way. In solution, the copper ion forms complex ions, such as Cu(H2O)4++, or Cu(NH3)4++. These are flat, square ions, of blue color. The water ion is medium blue, the ammonia ion dark blue.
Soluble copper compounds are all very poisonous. Indeed, they are often used as insecticides and algacides. This is not unexpected, since most heavy metal ions are also poisonous. Copper, however, seems to have escaped the notice of chemophobes who have persecuted lead and mercury.
Copper tarnish is composed of CuO, and the basic carbonate if moisture and air are present. It can be removed by solutions containing the ammonium ion, which forms the soluble ammonia complex. The commercial preparation Brasso is an example. Other copper polishes use different means not revealed on the labels, but boast that they do not contain ammonia. Copper polishes should not be used on silver, since they will attack it. Copper polishes seem to attack the copper as well, but not seriously. Kitchen ammonia does not seem to remove copper tarnish. Lemon juice, salt and vinegar, however, seem to remove tarnish well. None of the three works on its own, but all three together do. Mechanical scouring is also recommended for copper.
Cupric sulphate, CuSO4·5H2O, called bluestone or blue vitriol, is soluble in 0°C water to 24.3%, in 100°C water to 205%. The blue color requires the water. The anhydrous salt, prepared by heating, is white or pale green. Its solutions are slightly acidic, and are strong germicides and fungicides. Bordeaux mixture is a garden insecticide that contains copper sulphate. The triclinic crystals are easily grown. This is the substance that supplies the copper ions in a Daniell cell or gravity cell. The sulphate can be recovered by dissolving the copper cathodes of the cells in sulphuric acid, so the copper is continuously regenerated as the zinc is burned. The Daniell and gravity cells are discussed in Electrochemistry.
Copper is easily electrolytically plated from an acid solution of Cu(SO)4, since it is below hydrogen on the electrochemical scale, or from an alkaline solution of potassium cuprocyanide, KCu(CN)2. It will not give cathodic protection to iron, so care must be taken that the coating is continuous to avoid corrosion at pinholes. The copper layer is usually excellent, and can be a base for other plating for decorative effect. Chromium plating, which is always porous and non-protective, is conveniently done on a thin copper plate, or on nickel on top of the copper. Many automotive parts are zinc die-castings, plated with copper, nickel and finally chromium. Chromium is protected by a thin, invisible adherent layer of Cr2O3 like aluminium, but this does not rob its lustre after polishing of the plated layer.
Cuprous oxide was used in one kind of metallic rectifier, a predecessor of the semiconductor diodes of today. A plate of copper was strongly oxidized on one side to produce a thick layer of CuO. This was heat-treated so that a thin layer of Cu2O would grow between the Cu and the CuO. Then the CuO was stripped off with acid, and a contact of lead or similar material applied. The copper base plate acted as an n-type material, while the Cu2O was p-type, and a pn-junction was formed. This junction had a forward bias voltage of a few tenths of a volt, a reverse breakdown voltage of 5V or 6V, and a rather low maximum operating temperature. A stack of such elements could be assembled for higher voltages. The reverse current was larger than would be tolerated now, but was satisfactory for battery chargers and other similar devices. Similar rectifiers were made from Cu2S on magnesium, and selenium on iron or aluminium. These were quite satisfactory for low voltages and moderate currents, but have been completely replaced by the cheaper silicon diodes with far superior characteristics.
The United States was once the leading producer of copper, but the mines of Michigan, Montana, Utah, Nevada and Arizona are now exhausted, and most copper has to be imported. The copper nugget shown at the left is like those that were found in Upper Michigan, and provided the purest copper available, "Lake copper," for many years (image ©Amethyst Galleries). Rich copper ores are very rare, and much effort has been expended to make the more widely available lean ores economic. Most copper now comes from enriched zones that are mined in open pits as earth containing less than 1% copper, not as rich vein minerals of 20% Cu and more. The primary ores are sulphides and oxides, usually mixed with iron. Chalcocite is Cu2S, chalcopyrite CuFeS2, covellite CuS, and bornite Cu3FeS3. The oxides are cuprite Cu2O, tenorite CuO, malachite CuCO3·Cu(OH)2 (mineral verdigris), azurite 2CuCO3·Cu(OH)2, and the silicate chrysocolla CuSiO3·2H2O. Many of these minerals make attractive specimens, because of their green and blue colors, and can be carved into ornaments. They are not seen as crystals or massive forms in the ore mined today, but widely disseminated in an earthy matrix.
The sulphide ores are roasted to drive off the sulphur, arsenic and antimony, then calcined with sand or lime to slag off iron and more sulphur, producing a black matte consisting mainly of sulphides, like Cu2S. The matte is reduced in a blast furnace with air and the sulphur it contains to produce blister copper. Oxide ores are reduced with carbon. The blister copper is then dissolved in H2SO4, which can be made from the gases evolved in roasting and reduction, and the solution electrolyzed to produce cathodes of pure electrolytic copper. Gold and silver are usually recovered from the electrolysis sludge, and help the economics of the process. Modern smelting is much different from ancient smelting, since the ores treated are much leaner. Electrolytic copper was once traded principally in "wire bar," which was ready for the wire-drawing process. It is now supplied as "refined cathodes," which are continuous-cast at the beginning of wire drawing. The price is often quoted for 100 lb cathodes. The current price of copper is about $0.77 per pound.
Early copper smelting in the United States was established in the vicinity of New York, and the copper was used in the brass works in the Housatonic Valley of Connecticut. Electrolytic reduction favored locations on the coal fields of western Pennsylvania where power could be produced cheaply, and even later in the far west where hydroelectric power was used. This expanded greatly with the creation of government-subsidized electricity from large hydroelectric projects, since there was otherwise no need for the electricity produced, and it was all but given away to create a little benefit to the cost-benefit ratio. Free enterprise thrives with government's aid to channel the public's wealth into private pockets, then as now.
Silver is the same as Dutch zilver, German Silber, and the Anglo-Saxon seolfor, except for the orthography. The sound is the same, except for the well-known s/z and b/v alternatives. The Greek argyros and Latin argentum are clearly cognate, and Latin would even have used argyrum, with its normal transliteration of a Greek word. These give argento in Italian, and argent in French. Spanish is different, using plata instead. In Welsh, silver is arian, a Celtic word probably also related to argyros. In Russian, we have cerebro, with still another root. This suggests that silver was earlier than copper in daily life, and had a name in Indo-European. The alchemical symbol for silver is the sign of the Moon, a white crescent.
Silver is a white metal, like tin and cadmium, which resemble it, but are softer and lighter. When polished and untarnished, it has excellent, uniform reflectivity and so was preferred for mirrors. The white color is different from this perfect reflectivity, and is seen on matte surfaces. It was by far the earliest and most used coinage metal, and its name in many languages also is the simple word for money of any kind: j'ai de l'argent--I have money. In America, plata is money, though in Spain it is dinero, from the Arabic dinars that were once familiar there.
Silver has atomic number 47, atomic weight 107.880, and density 10.5 g/cc. Its naturally-occurring isotopes have mass numbers 107 (52%) and 109 (48%). The electron configuration has one 5s electron outside a filled 4d shell in the ground state, just like copper. However, the energies of the 4d and 5s orbitals are about equal, so a 4d95s2 configuration is as favored as a 4d105s. Silver exhibits valence +1 in most of its compounds, and forms complex ions. Silver metal has a face-centered cubic structure, with a = 0.408 nm. Each ion has donated one electron to the Fermi sphere. The work function of silver is about 3.7 eV (tabulated values range from 3.0 to 4.75). The Fermi energy is 5.5 eV. The electrical resistivity of silver is 1.62 μΩ-cm and the temperature coefficient is 0.0038 per °C. The thermal conductivity is 0.974 cal/cm-s-K, and the linear coefficient of expansion is 18.6 x 10-6 per °C. The electrical and thermal conductivities are slightly greater than those of copper, and the largest of any metal's. The specific heat is 0.0558 cal/g-K. The melting point of silver is 961°C, boiling point 1955°C, and the heat of fusion is 24.3 cal/g. Its hardness is 2.7 on the Mohs scale. The tensile strength of cast silver is about 40 ksi, of hard-drawn, 51 ksi. The Young's modulus is 10.3 x 106 psi.
Silver has a relatively simple chemistry. It dissolves in concentrated nitric acid to form colorless silver nitrate, AgNO3, called lunar caustic. This is a powerful antiseptic. Adding sodium hydroxide to a solution of silver nitrate precipitates brown Ag2O, which is only slightly soluble. Adding silver oxide to halogen acids makes the silver halides, such as AgCl, which, like most silver compounds, are insoluble. Silver chloride is white, and the bromide is pale yellow. On exposure to light, they gradually turn black by the photochemical reduction of the silver to metal. Silver forms soluble ammonia Ag(NH3)2+, thiosulphate Ag(S2O3)2--- and cyanide Ag(CN)2- complex ions. The thiosulphate ion is used to dissolve undeveloped AgBr and "fix" a photographic image. The cyanide ion is used in silver plating and in the cyanidation process for the recovery of silver. Silver fulminate, Ag2(CNO)2, is a powerful explosive. It has been formed in small amounts when mirrors are chemically silvered.
The salt KAg(CN)2, potassium argenticyanide, is soluble, and is made the electrolyte in a silver plating bath. The object to be plated forms the cathode (the negative electrode) and the anode can be silver or an inert substance. For every electron entering the cathode from the external circuit, one atom of silver is reduced. The concentration of uncomplexed silver ions is very small in the argenticyanide solution, which aids the deposition of a dense and uniform coat of silver. Many precautions must be observed to get a good result, among them thorough cleaning of the surface to be plated, and plating on a suitable metal, such as copper or nickel. Silver is plated at a current density of 5-15 A/sqft, which requires 1-2V. Each Faraday of charge, 96 480 C, deposits 108 grams of silver.
Silver is used for tableware, a practice that used to be much more common than it is now. The best silverware is made from Sterling, 0.925 fine silver, and has an excellent appearance, though it was expensive. To bring silver within the reach of more of the public, Sheffield Plate was introduced in 1743. Silver was melted onto brass or copper ingots, and the ingots were then rolled, giving a thin cladding of silver on the base metal, greatly reducing the cost of items made from it, which were as beautiful as solid Sterling. This was the earliest industrial use of cladding, which has become a very useful process. In Alclad, the pure aluminium cladding gives corrosion resistance to the underlying Duralumin allow. A later method of making inexpensive silverware was also introduced in Sheffield, the EPNS or electro-plated nickel silver process that uses electroplating.
Silver is quite resistant to corrosion, but is tarnished by materials containing sulphur, such as eggs, mustard and rubber. Silver tarnish contains only black Ag2S, since silver resists oxidation and does not form a carbonate. The tarnish can be removed by rubbing with a sodium bicarbonate, NaHCO3, paste. In the alkaline environment, soluble Na2S is formed and can be washed away. There are proprietary silver polishes that do the same thing, possibly more rapidly than the bicarbonate. Household hints suggest the use of ammonia, or salt and ammonia. One household hint suggested the use of electrolysis. In an enameled kettle, bring a solution of one tablespoon each of bicarbonate of soda and salt to one quart water to a boil. Put an aluminium pie pan in the bottom of the kettle, and in it the silver to be cleaned. Boil for 2-5 minutes. There is a warning against using this for hollow ware or plate. If you have valuable silver coins, it is best not to clean them, since the "toning" is often desirable, especially when blue, violet or orange.
Only limited amounts of silver are found as the native metal. Most comes from argentite, Ag2S, a soft black mineral that can be cut by a knife. The fresh surface is metallic and shiny, but tarnishes rapidly. Argentite is often associated with native silver. Mexico's wealth in silver was revealed through the Patio process, developed at Pachuca in 1557 by Bartoloméo de Medina. Crushed ore was roasted with salt and cupric sulphate to reduce the silver to metal, after which the matte was amalgamated with mercury on an amalgamating table. The mercury coated the surfaces of the small particles of silver and made them coalesce into a mass that could be scraped from table and distilled to recover the silver. A similar amalgamation process was used for gold. Amalgamation does not give as great a recovery as the cyanidation process that has succeeded it. In this process, the finely divided silver or gold is dissolved by a dilute cyanide solution, which is filtered, and silver and gold are precipitated from the filtrate by zinc. Infinitely more wealth was acquired by mining than from filching the metal the natives had picked up from the ground. Mexico is probably still the largest producer of silver, as it has been since the 16th century.
Silver is often associated with lead, from which it is separated by the process known as cupellation, in which the lead is oxidized and leaves the silver behind. For more information on cupellation, see Lead.
Silver is also found as AgCl, cerargyrite, or horn silver. It is soft and translucent, like animal horn, not silver used for horns. It rapidly darkens on exposure to light as silver is photochemically reduced, becoming violet-brown. It is a supergene ore, created in zones enriched by percolating water containing chlorine. Cerargyrite was common at Leadville, Colorado, and also in the Comstock Lode in Nevada, as well as other places.
The silver photographic process uses an emulsion of AgBr in gelatin coated on a cellulose acetate film or glass plate. The emulsion may be sensitized by treatment with a solution that deposits active impurities in the emulsion. On exposure to light, these centers are photochemically altered to become centers that catalyze the reduction of metallic silver. The reduction is carried out by means of a reducing agent called a developer. When a sufficient density of silver has been reached, the film is passed through a stop bath of acetic acid to stop the developing action, and then soaked in a bath of Na2S2O3, sodium thiosulphate or "hypo" to remove the remaining AgBr before it can darken. The film is then thoroughly washed and dried. Positive prints are made from the transparent negative in the same way. This is not the only photographic process, but it is by far the fastest and most sensitive, and has been extensively developed.
Silver, or analog, photography is now being replaced by digital photography, where the image is created and scanned on a matrix of sensors, and stored digitally. Silver photography is to digital photography as a manuscript is to a disk file.
Silver was also used in photosensitive cathodes. Although silver does not have a particularly low work function, Ag2O and Cs on Ag do, and can be sensitive into the infrared. Such cathodes were widely used in vacuum phototubes and photomultipliers. Ag2O is also used in alkaline battery cathodes.
Silver and gold were traditionally weighed in troy measure. A troy pound contained 12 troy ounces or 240 pennyweights (pwt), and was 373.2417 g. The prices of silver and gold are quoted in dollars per troy ounce for 0.900 fine metal. On 22 November 2002, the New York spot price of silver was $4.475, and gold $317.65. Sterling silver is 92.5 Ag, 7.5 Cu. Purity is also indicated by the carat system. Pure metal is 24 carat. 0.900 fine is 21.6 carat. Sterling silver is 22.2 carat. 18 carat gold is 0.750 fine. In Britain, silver and gold were "hallmarked" by stamps to show the fineness and the source. U.S. silver coins, minted up to the 1960's, were .900 fine were beautiful, very unlike the unattractive base tokens that replaced them. British silver coins had been debased much earlier, before finally being replaced by base metal.
Gold is Gold in German, goud in Dutch. In Latin, it was aurum, from which come French or, and Italian and Spanish oro. In the country, "au" sounded more like "o" than the city pronunciation "ow," and this has been reflected in the later spelling. The Welsh is aur, cognate to aurum. The Greek is chrysos, from a different stem, as is Russian zóloto. Like silver, gold is an ancient metal with different words for its name in the Teutonic, Celtic/Italic and Greek languages. The word aura in Latin and Greek is a light, cool breeze, while aurion is "tomorrow" in Greek. Neither of these has much to do with gold. "Aurum" could have given its name to the dawn as well as the dawn to aurum, and I don't believe they have anything to do with each other. The alchemical symbol for gold is the sign of the Sun. Gold is generally regarded as the first metal known and prized by mankind. This is likely, but there is absolutely no hard evidence. Copper and silver, as native metals, are also contenders, and less rare. To the alchemist, gold was associated with the sun, whose name and symbol were adopted for it.
The yellow of gold is an orangish-yellow, not a bright cadmium yellow, a complex color that has its distinctive name. It has long been used for the most precious ornaments, as a repository of wealth, and for transactions of great value. It can be drawn into fine wire or hammered to a foil with ease. It can be inlaid on heated base metals by hammering, or applied as an amalgam with mercury. For normal daily transactions, it has been much less used than silver. Anyone who has acquired gold does not generally like to give it away; the hoarding of gold is notorious. It is among the heaviest of metals. At 19.3 g/cc, it is nearly twice as heavy as silver, and even lead. Like Martha Stewart, gold is not only beautiful, but is quite useful.
Archimedes was faced with the problem of determining if the crown made for Hiero, tyrant of Syracuse, contained the full weight of gold furnished to the goldsmith who made it. The crown, of course, weighed the full weight, but there was the possibility that the goldsmith had taken a certain weight of the gold for himself, and replaced it with base metal. No base metal had half the density of gold, so although the crown was of the same weight, it would be larger in volume. Archimedes immersed the crown in a bath just full with water, and measured the volume of the water that overflowed when the crown was submerged in it. He found that the crown was of larger volume than it should have been, and that the goldsmith was indeed guilty of defalcation. The story is told in Vitruvius, de Architectura, IX 9-12.
Gold has atomic number 79, atomic weight 197.2, and density 19.32 g/cc. Its single naturally-occurring stable isotope has mass number 197. The electron configuration has one 6s electron outside a filled 3d shell in the ground state, just like copper. However, the energies of the 5d and 6s orbitals are about equal, so a 5d96s2 configuration is as favored as a 5d106s. Gold forms very few stable compounds, and exhibits valences of +1 and +3. Complex ions are important in its chemistry. Gold metal has a face-centered cubic structure, with a = 0.408 nm, almost exactly the same as silver. Each ion has donated one electron to the Fermi sphere. The work function of gold is about 4.7 eV (tabulated values range from 4.0 to 4.86). The Fermi energy is 5.5 eV. The electrical resistivity of gold is 2.44 μΩ-cm and the temperature coefficient is 0.0034 per °C. The thermal conductivity is 0.7003 cal/cm-s-K, and the linear coefficient of expansion is 14.43 x 10-6 per °C. The electrical and thermal conductivities are less than those of copper and silver, but gold is still a good electrical and thermal conductor. The specific heat is 0.0316 cal/g-K. The melting point of gold is 1063°C, boiling point 2530°C, and the heat of fusion is 16.3 cal/g. Its hardness is 2.5 on the Mohs scale. The tensile strength of cast gold is about 20 ksi, of hard-drawn wire, 37 ksi. The Young's modulus is 11.3 x 106 psi.
Gold is found almost entirely as the native metal, originally widely disseminated in quartz veins in hypothermal deposits. As the veins weather, the gold particles get into streams where they are concentrated by the natural action of the water into placer deposits. Many of the famous California placers were "elevated placers," covered with lava and then left high on the hillsides by erosion. The problems of winning gold are those of separating it from the gangue that accompanies it. No smelting is required. Some gold is found in the rare minerals calaverite, AuTe2 and sylvanite AuAgTe2. These tellurides have been found at Cripple Creek, Colorado, Kalgoorlie, Australia, and in Transylvania (Siebenbürgen), with smaller amounts elsewhere, such as Calaveras County, California. There does not appear to be very much, if any, at Telluride, Colorado. Heating the telluride ore is sufficient to liberate the gold. Gold is frequently associated with silver, and the two metals are alloyed in the recovered metal.
Unlike other metals, gold has not been acquired by mining until modern times. It was traditionally found in placer deposits in rivers, which form easily since the density of gold, 19.3, is much higher than the density of gangue, which is about 2.6. The gold is concentrated at certain locations where the flow is locally diminished so the gold can settle out. These deposits originate when gold-bearing quartz veins are weathered out, but often the source of placers was not known. The Tajo (in Spain), the Po, the Pactolus (in Asia Minor) and the Ganges were historically well-known sources of gold, mentioned in Pliny. In more recent times, the "mother lodes" in granitic rocks have been located and mined, sometimes by tracing placer gold back to its source. Although nuggets of gold weighing as much as 600 lbs have been found, most gold is very finely divided. Even in a rich placer, the gold may not be detectable by the eye, even when aided with a magnifier. Sometimes, with the passage of time, an exhausted placer may become productive again as its source continually weathers and more gold is deposited.
Gold can be separated from gangue by panning, using water in a shallow pan to sluice away the lighter gangue from the heavy gold. This method, also called washing, gives very poor recovery, especially of the smaller particles, even when mechanized. Waste heaps from this gravity recovery can be profitably exploited by using more efficient methods of recovery. The finer gold can be separated by amalgamation with mercury. At 100°C, mercury can dissolve 15.7% gold. An amalgamation table consists of a copper trough which is thoroughly amalgamated. The finely ground mixture ("slime") of gold and gangue is then treated with excess mercury and agitated, so that the amalgamated gold particles adhere to the table. At intervals, this amalgam is scraped off and distilled to recover the gold. Amalgamation gives much better recovery, but there is still a considerable loss of gold. Placer gold was discovered in California in 1848, and mercury in 1850, a convenient circumstance. Still more thorough recovery is possible with the cyanide process, introduced in 1893. Gold is recovered as sodium aurocyanide, NaAu(CN)2 on leaching of the concentrate with NaCN, sodium cyanide. The amount of gold recoverable by cyanidation of tailings is probably comparable that originally recovered by amalgamation, and amalgamation probably recovered more gold than the initial washing.
Zippe gives an excellent account of the exploitation of gold in Europe from ancient times. Once-rich sources have been exhausted, in locations like Spain and France. Spain was the most prolific gold producer in ancient times, enriching the Visigoths as well as the Moors before its exhaustion. The Rhine, Garonne and the Rhône carried gold from the Alps. Little gold was found in Scandinavia, but the Harz, Thüringer Wald, and the Fichtelgebirge all contributed. Ireland had placer gold near Wicklow, and Scotland near Leadhills. In the middle ages, Bohemia was a prolific gold producer, in the Sudeten mountains and Silesia; the gold rush there was like later California and Australia. Gold washing left large heaps of refuse, destroying most of the valuable farmland in mountain valleys, and sometimes leading to famine, always to conflict betweeen miners and farmers. It is not mentioned if these tailings have been later treated by amalgamation or cyanidation to recover the large amounts of gold they must still retain. By the 19th century, Austria-Hungary was the only considerable producer of gold remaining in Europe. The Siebenbürgen (57%) and Hungary (40%) were the major producers, with smaller contributions (3%) from Salzburg, Tirol, the Balkans, and what remained from Bohemia. The average annual production from 1823 to 1848 was 450,000 ducats, about the same as the southern U.S. states. California produced about 22,000,000 ducats a year in its early days.
Currently, the gold and gangue, sometimes of very lean concentration--often tailings of the amalgamation process--are leached in the open air with a dilute cyanide solution. The solution is drawn off and filtered at intervals, and gold is precipitated in the filtrate by metallic zinc. This can give 97% recovery, an excellent result. There is also a chlorine process, where the gold dissolves as AuCl2- ion when chlorine is bubbled through the ore. Open-heap cyanidation is much less hazardous than would be imagined. Attempts to outlaw it are ignorant and misguided; much better would be to ensure that it is done in a safe manner.
Most gold is accompanied by up to 10% silver, while other metals are exceptional. If there is 20%-40% silver, the alloy is called electrum. Incidentally, alloys in which gold predominates still have the properties of gold, including its resistance to corrosion, but are harder and of lighter color. Electrum may be white like silver, but will not tarnish. Separating gold from impurities is called parting, and was a very difficult thing in antiquity, when strong acids were unknown. Nevertheless, it was eventually possible, and Roman gold coins were quite pure, though later gold coins often had considerable silver content. The ancient process is probably no longer known. Modern methods of parting include the use of sulphuric or nitric acid, which dissolves the silver, or electrolysis. Silver can be parted from gold by bubbling chlorine through the molten metal; under these conditions it produces AgCl and leaves the pure gold behind.
Gold forms complex ions with cyanide and chlorine, as we have seen. Aside from this, gold has very little chemistry. It should be noted that although gold will not combine with other elements willingly, it combines very well with itself to form the metal. Any gold compound will decompose to the metal on heating. The metal is not attacked by the air, simple acids (except selenic), or alkalis. Gold oxides are difficult to form and unstable. It is attacked by selenic acid, but more notably by aqua regia, "royal water," composed of 1 part concentrated nitric acid to 3 parts concentrated hydrochloric acid. The Cl- forms the complex chloride ion with the gold, thereby dissolving it. If stannous chloride, SnCl2, solution is added, stannic ion is formed which immediately hydrolyzes to a hydrosol of stannic hydroxide. Simultaneously, gold is reduced from the complex ion and is adsorbed on the hydrosol, making a brilliant purple color called the Purple of Cassius. This is a very sensitive test for gold, detecting one part in a hundred million (10 ppb).
Aqua regia, in German Königswasser, appears to have been first prepared by Andreas Libau (Libavius) around 1600. Libau discovered concentrated hydrochloric acid, while strong nitric acid was discovered by the "False" Geber around 1300. The ascription of aqua regia or nitric acid to Geber (Jabir ibn-Hayyan, ca. 760-815) appears to be erroneous. Among other things, nitrates were yet unknown in the West in Geber's times. Alchemists long strove to purify lead so that the pure heavy substance, gold, would appear, or to make gold from mercury and sulphur, combining their attributes. Aqua regia made the purification of gold much easier.
In the AuCl4- ion, the gold is bonded covalently to the four chlorines at the corners of a square. In acid solution, this becomes chlorauric acid, which makes salts: for example, sodium chloraurate, NaAuCl4·2H2O. Auric chloride, AuCl3, is formed by oxidizing the ion. Chlorauric acid crystallizes out when an aqua regia solution of gold is evaporated.
Gold is used in microelectronics to plate connection pads, since it solders very well, and for electrical contacts, because it does not corrode. Fine gold wires connect chips to the pads leading to the package pins. Because gold is so inactive, it is not poisonous. It is excellent for dental work, making durable crowns. If gold were cheaper, it would find many additional uses in technology.
W. N. Jones, Inorganic Chemistry (Philadelphia: Blakiston, 1949), Chapter 34.
J. L. Bray, Non-Ferrous Production Metallurgy, 2nd ed. (New York: John Wiley & Sons, 1947), Chapter 26.
R. A. Higgins, Engineering Metallurgy, 3rd ed. (London: The English Universities Press, 1971). pp. 315-324, 363f.
R. B. Leighou, Chemistry of Engineering Materials (New York: McGraw-Hill, 1942). pp 164-174.
C. S. Hurlbut, Dana's Manual of Mineralogy, 16th ed. (New York: John Wiley & Sons, 1952). pp. 195, 198, 237, 273, 378, 440f, 454f.
F. X. M. Zippe, Geschichte der Metalle (Wien: W. Braumüller, 1857). History and mythology of metals. The geology, still accepting Noah's flood, is struggling to escape from supersitition in this work. It is interesting to see how Zippe reconciles biblical myths with his evident rationality.
Image of native copper kindly furnished by Amethyst Galleries, Inc.. This is an excellent website and specimens can be purchased online. This is probably the best mineral website, and the company should be supported for making it available.
Current prices of copper can be found at the London Metals Exchange website. The LME is very influential in stabilizing the markets in aluminium, copper, lead, nickel, tin and zinc.
Composed by J. B. Calvert
Created 24 November 2002
Last revised 7 March 2004
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