Silver

Silver,  ₄₇Ag

Electrolytically refined silver
General properties
Name, symbol	silver, Ag
Appearance	lustrous white metal
Pronunciation	/ˈsɪlvər/ SIL-vər
Silver in the periodic table
Cu ↑ Ag ↓ Au palladium ← silver → cadmium
Atomic number (Z)	47
Group, block	group 11, d-block
Period	period 5
Element category	transition metal
Standard atomic weight (±) (Ar)	107.8682(2)[1]
Electron configuration	[Kr] 4d^10 5s^1
per shell	2, 8, 18, 18, 1
Physical properties
Phase	solid
Melting point	1234.93 K ​(961.78 °C, ​1763.2 °F)
Boiling point	2435 K ​(2162 °C, ​3924 °F)
Density near r.t.	10.49 g/cm^3
when liquid, at m.p.	9.320 g/cm^3
Heat of fusion	11.28 kJ/mol
Heat of vaporization	254 kJ/mol
Molar heat capacity	25.350 J/(mol·K)
vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1283 1413 1575 1782 2055 2433
Atomic properties
Oxidation states	−2, −1, 1, 2, 3, 4 ​(an amphoteric oxide)
Electronegativity	Pauling scale: 1.93
Ionization energies	1st: 731.0 kJ/mol 2nd: 2070 kJ/mol 3rd: 3361 kJ/mol
Atomic radius	empirical: 144 pm
Covalent radius	145±5 pm
Van der Waals radius	172 pm
Miscellanea
Crystal structure	​face-centered cubic (fcc)
Speed of sound thin rod	2680 m/s (at r.t.)
Thermal expansion	18.9 µm/(m·K) (at 25 °C)
Thermal conductivity	429 W/(m·K)
Thermal diffusivity	174 mm^2/s (at 300 K)
Electrical resistivity	15.87 nΩ·m (at 20 °C)
Magnetic ordering	diamagnetic[2]
Young's modulus	83 GPa
Shear modulus	30 GPa
Bulk modulus	100 GPa
Poisson ratio	0.37
Mohs hardness	2.5
Vickers hardness	251 MPa
Brinell hardness	206–250 MPa
CAS Number	7440-22-4
History
Discovery	before 5000 BC
Most stable isotopes of silver
iso NA half-life DM DE (MeV) DP ^105Ag syn 41.2 d ε – ^105Pd γ 0.344, 0.280, 0.644, 0.443 – ^106mAg syn 8.28 d ε – ^106Pd γ 0.511, 0.717, 1.045, 0.450 – ^107Ag 51.839% – (SF) <20.512 ^108mAg syn 418 y ε – ^108Pd IT 0.109 ^108Ag γ 0.433, 0.614, 0.722 – ^109Ag 48.161% – (SF) <19.241 ^111Ag syn 7.45 d β^− 1.036, 0.694 ^111Cd γ 0.342 –
Decay modes in parentheses are predicted, but have not yet been observed

Silver is the metallic element with the atomic number 47. Its symbol is Ag, from the Latin argentum, from a PIE root reconstructed as *h₂erǵ-, "grey" or "shining". A soft, white, lustrous transition metal, it possesses the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. The metal occurs naturally in its pure, free form (native silver), as an alloy with gold and other metals, and in minerals such as argentite and chlorargyrite. Most silver is produced as a byproduct of copper, gold, lead, and zinc refining.

Silver has long been valued as a precious metal. More abundant than gold, silver metal has functioned in many premodern monetary systems as coinable specie, sometimes even alongside gold. Its purity is typically measured on a per-mille basis; a 94%-pure alloy is described as "0.940 fine". In addition, silver has numerous applications beyond currency, such as in solar panels, water filtration, jewelry and ornaments, high-value tableware and utensils (hence the term silverware), and also as an investment in the forms of coins and bullion. Silver is used industrially in electrical contacts and conductors, in specialized mirrors, window coatings and in catalysis of chemical reactions. Its compounds are used in photographic film and X-rays. Dilute silver nitrate solutions and other silver compounds are used as disinfectants and microbiocides (oligodynamic effect), added to bandages and wound-dressings, catheters and other medical instruments.

Characteristics

Silver 1000 oz t (~31 kg) bullion bar

Silver is produced during certain types of supernova explosions by nucleosynthesis from lighter elements through the r-process, a form of nuclear fusion that produces many elements heavier than iron, of which silver is one.^[3]

Silver is a very ductile, malleable (slightly less so than gold), univalent coinage metal, with a brilliant white metallic luster that can take a high degree of polish.^[4] Protected silver has higher optical reflectivity than aluminium at all wavelengths longer than ~450 nm.^[5] At wavelengths shorter than 450 nm, silver's reflectivity is inferior to that of aluminium and drops to zero near 310 nm.^[6]

The electrical conductivity of silver is the highest of all metals, even higher than copper, but it is not widely used for electrical purposes due to its much higher cost. An exception to this is in radio-frequency engineering, particularly at VHF and higher frequencies, where silver plating is employed to improve electrical conductivity of parts and wires (at high frequencies current tends to flow on the surface of conductors, not their interior, hence silver plating greatly improves overall conductivity). Silver also has the lowest contact resistance of any metal.^[7] During World War II in the US, 13,540 tons were used in the electromagnets used for enriching uranium, mainly because of the wartime shortage of copper.^[8][9][10]

Pure silver has the highest thermal conductivity of any metal, although those of the nonmetal carbon in the form of diamond and Superfluid helium-4 are higher.^[7]

Silver halides are photosensitive and are remarkable for their ability to record a latent image that can later be developed chemically. Silver is stable in pure air and water, but tarnishes when it is exposed to air or water containing ozone or hydrogen sulfide, the latter forming a black layer of silver sulfide, which can be cleaned off with dilute hydrochloric acid.^[7] The most common oxidation state of silver is +1 (for example, silver nitrate, AgNO₃); the less common +2 compounds (for example, silver(II) fluoride, AgF₂), and the even less common +3 (for example, potassium tetrafluoroargentate(III), KAgF₄) and even +4 compounds (for example, potassium hexafluoroargentate(IV), K₂AgF₆)^[11] are also known.

Isotopes

Naturally occurring silver is composed of two stable isotopes, ¹⁰⁷Ag and ¹⁰⁹Ag, with ¹⁰⁷Ag being slightly more abundant (51.839% natural abundance). Their almost equal abundance is rare in the periodic table. Silver's atomic weight is 107.8682(2) g/mol.^[12][13]

Twenty-eight radioisotopes have been characterized, the most stable being ¹⁰⁵Ag with a half-life of 41.29 days, ¹¹¹Ag with a half-life of 7.45 days, and ¹¹²Ag with a half-life of 3.13 hours. This element has numerous meta states, the most stable being ^108mAg (t_1/2 = 418 years), ^110mAg (t_1/2 = 249.79 days) and ^106mAg (t_1/2 = 8.28 days). All of the remaining radioactive isotopes have half-lives of less than an hour, and the majority of these have half-lives of less than three minutes.

Isotopes of silver range in relative atomic mass from 93.943 (⁹⁴Ag) to 126.936 (¹²⁷Ag);^[14] the primary decay mode before the most abundant stable isotope, ¹⁰⁷Ag, is electron capture and the primary mode after is beta decay. The primary decay products before ¹⁰⁷Ag are palladium (element 46) isotopes, and the primary products after are cadmium (element 48) isotopes.

The palladium isotope ¹⁰⁷Pd decays by beta emission to ¹⁰⁷Ag with a half-life of 6.5 million years. Iron meteorites are the only objects with a high-enough palladium-to-silver ratio to yield measurable variations in ¹⁰⁷Ag abundance. Radiogenic ¹⁰⁷Ag was first discovered in the Santa Clara meteorite in 1978.^[15] The discoverers suggest the coalescence and differentiation of iron-cored small planets may have occurred 10 million years after a nucleosynthetic event. ¹⁰⁷Pd–¹⁰⁷Ag correlations observed in bodies that have clearly been melted since the accretion of the solar system must reflect the presence of unstable nuclides in the early solar system.^[16]

Compounds

Silver metal dissolves readily in nitric acid (HNO
3) to produce silver nitrate (AgNO
3), called 'Lunar Caustic', a transparent crystalline solid that is photosensitive and readily soluble in water. Silver nitrate is used as the starting point for the synthesis of many other silver compounds, as an antiseptic, and as a yellow stain for glass in stained glass. Silver metal does not react with sulfuric acid, which is used in jewelry-making to clean and remove copper oxide firescale from silver articles after silver soldering or annealing. Silver reacts readily with sulfur or hydrogen sulfide H
2S to produce silver sulfide, a dark-colored compound familiar as the tarnish on silver coins and other objects. Silver sulfide Ag
2S also forms silver whiskers when silver electrical contacts are used in an atmosphere rich in hydrogen sulfide.

4 Ag + O₂ + 2 H₂S → 2 Ag₂S + 2 H₂O

Cessna 210 equipped with a silver iodide generator for cloud seeding

Silver chloride (AgCl) is precipitated from solutions of silver nitrate in the presence of chloride ions, and the other silver halides used in the manufacture of photographic emulsions are made in the same way, using bromide or iodide salts. Silver chloride is used in glass electrodes for pH testing and potentiometric measurement, and as a transparent cement for glass. Silver iodide has been used in attempts to seed clouds to produce rain.^[7] Silver halides are highly insoluble in aqueous solutions and are used in gravimetric analytical methods.

Silver oxide (Ag
2O), produced when silver nitrate solutions are treated with a base, is used as a positive electrode (anode) in watch batteries. Silver carbonate (Ag
2CO
3) is precipitated when silver nitrate is treated with sodium carbonate (Na
2CO
3).^[17]

2 AgNO₃ + 2 OH⁻ → Ag₂O + H₂O + 2 NO₃⁻

2 AgNO₃ + Na₂CO₃ → Ag₂CO₃ + 2 NaNO₃

Silver fulminate (AgONC), a powerful, touch-sensitive explosive used in percussion caps, is made by reaction of silver metal with nitric acid in the presence of ethanol (C
2H
5OH). Other dangerously explosive silver compounds are silver azide (AgN
3), formed by reaction of silver nitrate with sodium azide (NaN
3),^[18] and silver acetylide, formed when silver reacts with acetylene gas.

Latent images formed in silver halide crystals are developed by treatment with alkaline solutions of reducing agents such as hydroquinone, metol (4-(methylamino)phenol sulfate) or ascorbate, which reduce the exposed halide to silver metal. Alkaline solutions of silver nitrate can be reduced to silver metal by reducing sugars such as glucose, and this reaction is used to silver glass mirrors and the interior of glass Christmas ornaments. Silver halides are soluble in solutions of sodium thiosulfate (Na
2S
2O
3), which is used as a photographic fixer, to remove excess silver halide from photographic emulsions after image development.^[17]

Silver metal is attacked by strong oxidizers such as potassium permanganate (KMnO
4) and potassium dichromate (K
2Cr
2O
7), and in the presence of potassium bromide (KBr); these compounds are used in photography to bleach silver images, converting them to silver halides that can either be fixed with thiosulfate or redeveloped to intensify the original image. Silver forms cyanide complexes (silver cyanide) that are soluble in water in the presence of an excess of cyanide ions. Silver cyanide solutions are used in electroplating of silver.^[17]

Although silver normally has oxidation state +1 in compounds, other oxidation states are known, such as +3 in AgF
3, produced by the reaction of elemental silver or silver fluoride with krypton difluoride.^[19]

Silver artifacts undergo three forms of deterioration, the most common of which is the formation of a black film of silver sulfide tarnish. Fresh silver chloride, formed when silver objects are immersed for long periods in salt water, is pale yellow colored, becoming purplish on exposure to light and projects slightly from the surface of the artifact or coin. The precipitation of copper in ancient silver can be used to date artifacts.^[20]

Applications

Many well-known uses of silver involve its precious metal properties, including currency, decorative items, and mirrors. The contrast between its bright white color and other media makes it very useful to the visual arts. By contrast, fine silver particles form the dense black in photographs and in silverpoint drawings. It has also long been used to confer high monetary value to objects (such as silver coins and investment bars) or make objects symbolic of high social or political rank. Silver salts have been used since the Middle Ages to produce a yellow or orange color in stained glass, and more complex decorative color reactions can be produced by incorporating silver metal in blown, kilnformed or torchworked glass.^[21]

Currency

Silver, in the form of electrum (a gold–silver alloy), was coined to produce money around 700 BC by the Lydians. Later, silver was refined and coined in its pure form. Many nations used silver as the basic unit of monetary value. In the modern world, silver bullion has the ISO currency code XAG. The name of the pound sterling (£) reflects the fact it originally represented the value of one pound Tower weight of sterling silver; other historical currencies, such as the French livre, have similar etymologies. During the 19th century, the bimetallism that prevailed in most countries was undermined by the discovery of large deposits of silver in the Americas; fearing a sharp decrease in the value of silver and thus the currency, most states had switched to a gold standard by 1900. In some languages, including Sanskrit, Spanish, French, and Hebrew, the word for silver may be used to mean money.

A Canadian 50 cent piece from 1951, with King George the 6th on the obverse and Canada's (now former) coat of arms on the reverse. It is made of 80% silver and 20% copper.

The 20th century saw a gradual movement to fiat currency, with most of the world monetary system losing its link to precious metals after the United States dollar came off the gold standard in 1971; the last currency backed by gold was the Swiss franc, which became a pure fiat currency on 1 May 2000. During this same period, silver gradually ceased to be used in circulating coins. In the UK the silver standard was reduced from .925 to .500 in 1920. Coins that had been made of silver started to be made of cupro-nickel in 1947; existing coins were not withdrawn, but ceased circulating as the silver content came to exceed the face value. In 1964 the United States stopped minting the silver dime and quarter; the last circulating silver coin was the 1970 40% half-dollar.^[22] In 1968 Canada minted their last circulating silver coins, the 50% dime and quarter.

For most of the century after the Civil War in the United States, the price of silver was considerably less than the face value of circulating silver coins, reaching its nadir of about $.25 per ounce in 1932.^[23] By this measure, the silver coins of the United States were effectively fiat coins for much of their history. Not until 1963 did the price of silver rise above the threshold of $1.29 per ounce, at which point the silver content of pre-1965 United States coins was equal to in value to the face values of the coins themselves.^[24]

Silver coins are still minted by several countries as commemorative or collectible items, but they are not in general circulation.

Silver is used as a currency by many individuals, and is legal tender in the US state of Utah.^[25] Silver coins and bullion are also used as an investment to guard against inflation and devaluation.

Jewelry and silverware

Shallow silver bowl, Persian, 6th century BC (Achaemenid). The deeper depressions represent lotus buds, an Egyptian motif. Walters Art Museum collections.

Chain, Worn by a Woman. Silver, made in Syria. Brooklyn Museum.

Jewelry and silverware are traditionally made from sterling silver (standard silver), an alloy of 92.5% silver with 7.5% copper. In the US, only an alloy consisting of at least 0.900-fine silver can be sold as "silver" (thus frequently stamped 900). Sterling silver (stamped 925) is harder than pure silver, and has a lower melting point (893 °C) than either pure silver or pure copper.^[7] Britannia silver is an alternative, hallmark-quality standard containing 95.8% silver, often used to make silver tableware and wrought plate. With the addition of germanium, the patented modified alloy Argentium sterling silver is formed, with improved properties, including resistance to firescale.

Sterling silver jewelry is often plated with a thin coat of .999-fine silver to give the item a shiny finish. This process is called "flashing". Silver jewelry can also be plated with rhodium (for a bright, shiny look) or gold (to produce silver gilt).

Silver is a constituent of almost all colored carat gold alloys and carat gold solders, giving the alloys paler color and greater hardness.^[26] White 9-carat gold contains 62.5% silver and 37.5% gold, while 22-carat gold contains a minimum of 91.7% gold and 8.3% silver or copper or other metals.^[26]

Historically, the training and guild organization of goldsmiths included silversmiths, and the two crafts remain largely overlapping. Unlike blacksmiths, silversmiths do not shape the metal while it is red-hot, but work it at room temperature with gentle and carefully placed hammer blows. The essence of silversmithing is to take a flat piece of metal and to transform it into a useful object using different hammers, stakes and other simple tools.^[27]

While silversmiths specialize in, and principally work silver, they also work with other metals, such as gold, copper, steel, and brass. They make jewelry, silverware, armor, vases, and other artistic items. Because silver is such a malleable metal, silversmiths have a large range of choices with how they prefer to work the metal. Historically, silversmiths are mostly referred to as goldsmiths, which was usually the same guild. In the western Canadian silversmith tradition, guilds do not exist; however, mentoring through colleagues becomes a method of professional learning within a community of craftspeople.^[28]

Traditionally, silversmiths mostly made "silverware" (cutlery, tableware, bowls, candlesticks and such). Much less solid silver tableware is handmade now than in the past.

Solar energy

Solar modules mounted on solar trackers

Silver is used in the manufacture of crystalline solar photovoltaic panels.^[29] Silver is also used in plasmonic solar cells. 100 million ounces of silver are projected for use by solar energy in 2015.^[30]

Silver is the reflective coating of choice for concentrated solar power reflectors.^[31] In 2009, scientists at the National Renewable Energy Laboratory (NREL) and SkyFuel teamed to develop large curved sheets of metal that have the potential to be 30% less expensive than today's best collectors of concentrated solar power by replacing glass-based models with a silver polymer sheet that has the same performance as the heavy glass mirrors, but at much lower cost and weight. It also is much easier to deploy and install. The glossy film uses several layers of polymers, with an inner layer of pure silver.

Air conditioning

In 2014 researchers invented a mirror-like panel that, when mounted on a building, acts like an air conditioner.^[32] The mirror is built from several layers of wafer-thin materials. The first layer is silver, the most reflective substance on Earth. On top of this are alternating layers of silicon dioxide and hafnium oxide. These layers improve the reflectivity, but also turn the mirror into a thermal radiator.

Water purification

Silver is used in water purifiers. It prevents bacteria and algae from building up in filters. The catalytic action of silver, in concert with oxygen, sanitizes water and eliminates the need for chlorine. Silver ions are also added to water purification systems in hospitals, community water systems, pools and spas, displacing chlorine.^[30]

Dentistry

Silver can be alloyed with mercury at room temperature to make amalgams that are widely used for dental fillings. To make dental amalgam, a mixture of powdered silver and other metals such as tin and gold is mixed with mercury to make a stiff paste that can be adapted to the shape of a cavity. The dental amalgam achieves initial hardness within minutes, and sets hard in a few hours.

Photography and electronics

The use of silver in photography, in the form of silver nitrate and silver halides, has rapidly declined due to the lower demand for consumer color film from the advent of digital technology. From the peak global demand for photographic silver in 1999 (267,000,000 troy ounces or 8304.6 metric tonnes) the market had contracted almost 70% by 2013.^[33]

Some electrical and electronic products use silver for its superior conductivity, even when tarnished. The primary example of this is in high quality RF connectors. The increase in conductivity is also taken advantage of in RF engineering at VHF and higher frequencies, where conductors often cannot be scaled by 6%, due to tuning requirements, e.g. cavity filters. As an additional example, printed circuits and RFID antennas can be made using silver paints,^[7][34] and computer keyboards use silver electrical contacts. Silver cadmium oxide is used in high-voltage contacts because it can withstand arcing.

Some manufacturers produce audio connector cables, speaker wires, and power cables using silver conductors, which have a 6% higher conductivity than ordinary copper ones of identical dimensions, but cost much more. Though debatable, many hi-fi enthusiasts believe silver wires improve sound quality.

Small devices, such as hearing aids and watches, commonly use silver oxide batteries due to their long life and high energy-to-weight ratio. Another usage is high-capacity silver-zinc and silver-cadmium batteries.

In World War II, there was a shortage of copper and silver was borrowed from the United States Treasury for electrical windings for several production facilities including those of the Manhattan Project; see below under History, WWII.

Glass coatings

Telescopic mirrors

Mirrors in almost all reflective telescopes use vacuum aluminium coatings.^[35] However thermal or infrared telescopes use silver coated mirrors because of silver's ability to reflect some types of infrared radiation more effectively than aluminium, as well as silver's ability to reduce the amount of radiation actually emitted from the mirror (its thermal emissivity).^[36]

Silver, in protected or enhanced coatings, is seen as the next generation metal coating for reflective telescope mirrors.^[37]

Windows

Using a process called sputtering, silver, along with other optically transparent layers, is applied to glass, creating low emissivity coatings used in high-performance insulated glazing. The amount of silver used per window is small because the silver layer is only 10–15 nanometers thick.^[38] However, the amount of silver-coated glass worldwide is hundreds of millions of square meters per year, leading to silver consumption on the order of 10 cubic meters or 100 metric tons/year. Silver color seen in architectural glass and tinted windows on vehicles is produced by sputtered chrome, stainless steel or other alloys.

Silver-coated polyester sheets, used to retrofit windows, are another popular method for reducing light transmission.^[30]

Other industrial and commercial applications

This Yanagisawa A9932J alto saxophone has a solid silver bell and neck with a solid phosphor bronze body. The bell, neck, and key-cups are extensively engraved. It was manufactured in 2008.

Silver and silver alloys are used in the construction of high-quality musical wind instruments of many types.^[39] Flutes, in particular, are commonly constructed of silver alloy or silver plated, both for appearance and for the frictional surface properties of silver. Brass instruments, such as Trumpets and Baritones, are also commonly plated in silver.^[40]

Silver's catalytic properties make it ideal for use as a catalyst in oxidation reactions, for example, the production of formaldehyde from methanol and air by means of silver screens or crystallites containing a minimum 99.95 weight-percent silver. Silver (upon some suitable support) is probably the only catalyst available today to convert ethylene to ethylene oxide (CH2-O-CH2) in the synthesis of ethylene glycol, used for making polyesters) and polyethylene terephthalate. It is also used in the Oddy test to detect reduced sulfur compounds and carbonyl sulfides.

Because silver readily absorbs free neutrons, it is commonly used to make control rods to regulate the fission chain reaction in pressurized water nuclear reactors, generally in the form of an alloy containing 80% silver, 15% indium, and 5% cadmium.

Silver is used to make solder and brazing alloys, and as a thin layer on bearing surfaces can provide a significant increase in galling resistance and reduce wear under heavy load, particularly against steel.

Biology

Silver stains are used in biology to increase the contrast and visibility of cells and organelles in microscopy. Camillo Golgi used silver stains to study cells of the nervous system and the Golgi apparatus.^[41] Silver stains are used to stain proteins in gel electrophoresis and polyacrylamide gels, either as primary stains or to enhance the visibility and contrast of colloidal gold stain.^[42] Different yeasts from Brazilian gold mines, bioaccumulate free and complexed silver ions. A sample of the fungus Aspergillus niger was found growing from gold mining solution; and was found to contain cyano metal complexes; such as gold, silver, copper iron and zinc. The fungus also plays a role in the solubilization of heavy metal sulfides.^[43]

Medicine

The medical uses of silver include its incorporation into wound dressings, and its use as an antibiotic coating in medical devices. Wound dressings containing silver sulfadiazine or silver nanomaterials may be used to treat external infections. Silver is also used in some medical applications, such as urinary catheters and endotracheal breathing tubes, where there is tentative evidence that it is effective in reducing catheter-related urinary tract infections and ventilator-associated pneumonia respectively.^[44][45] The silver ion (Ag+
) is bioactive and in sufficient concentration readily kills bacteria in vitro. Silver and silver nanoparticles are used as an antimicrobial in a variety of industrial, healthcare and domestic applications.^[46]

Investing

Silver coins and bullion are used for investing. Various types of silver investments can be made on the stock markets, including mining or silver streaming stocks, or silver-backed exchange-traded funds.^[47]

Canada's Maple leaf 1 troy ounce silver bullion coin.

Clothing

Silver inhibits the growth of bacteria and fungi on clothing, such as socks, so is sometimes added to reduce odors and the risk of bacterial and fungal infections. It is incorporated into clothing or shoes either by integrating silver nanoparticles into the polymer from which yarns are made or by coating yarns with silver.^[48][49] The loss of silver during washing varies between textile technologies, and the resultant effect on the environment is not yet fully known.^[50][51]

History

The crescent moon has been used since ancient times to represent silver.

Silver has been used for thousands of years for ornaments and utensils, trade, and as the basis for many monetary systems. Its value as a precious metal was long considered second only to gold. The word "silver" appears in Anglo-Saxon in various spellings, such as seolfor and siolfor. A similar form is seen throughout the Germanic languages (compare Old High German silabar and silbir). The chemical symbol Ag is from the Latin word for "silver", argentum (compare Ancient Greek ἄργυρος, árgyros), from the Proto-Indo-European root *h₂erǵ- (formerly reconstructed as*arǵ-), meaning "white" or "shining". Silver has been known since ancient times; it is mentioned in the Book of Genesis. Slag heaps found in Asia Minor and on the islands of the Aegean Sea indicate silver was being separated from lead as early as the 4th millennium BC using surface mining.^[7] One of the earliest silver extraction centres in Europe was Sardinia in early Chalcolithic.^[52]

The stability of the Roman currency relied to a high degree on the supply of silver bullion, which Roman miners produced on a scale unparalleled before the discovery of the New World. Reaching a peak production of 200 t per year, an estimated silver stock of 10,000 t circulated in the Roman economy in the middle of the second century AD, five to ten times larger than the combined amount of silver available to medieval Europe and the Caliphate around 800 AD.^[53][54] Financial officials of the Roman Empire worried about the loss of silver to pay for highly demanded silk from Sinica (China).

Mines were made in Laureion during 483 BC.^[55]

In the Gospels, Jesus' disciple Judas Iscariot is infamous for having taken a bribe of 30 coins of silver from religious leaders in Jerusalem to turn Jesus of Nazareth over to soldiers of the High Priest Caiaphas.^[56]

The Chinese Empire during most of its history primarily used silver as a means of exchange. In the 19th century, the threat to the balance of payments of the United Kingdom from Chinese merchants demanding payment in silver in exchange for tea, silk, and porcelain led to the Opium War because Britain had to find a way to address the imbalance in payments, and they decided to do so by selling opium produced in their colony of British India to China.^[57]

Silver mining and processing in Kutná Hora, Central Europe, 1490s

Islam permits Muslim men to wear silver rings on the little finger of either hand.^[58] Muhammad himself wore a silver signet ring.^[59]

In the Americas, high temperature silver-lead cupellation technology was developed by pre-Inca civilizations as early as AD 60–120.^[60]

World War II

During World War II, the shortage of copper led to the substitution of silver in many industrial applications. The United States government loaned out silver from its massive reserve located in the West Point vaults to a wide range of industrial users. One very important use was for bus bars for new aluminium plants needed to make aircraft. During the war, many electrical connectors and switches were silver plated. Another use was aircraft master rod bearings and other types of bearings. Since silver can replace tin in solder at a lower volume, a large amount of tin was freed up for other uses by substituting government silver. Silver was also used as the reflector in searchlights and other types of lights. Silver was used in nickels during the war to save that metal for use in steel alloys.^[61]

The Manhattan Project to develop the atomic bomb used about 14,700 tons of silver borrowed from the United States Treasury for calutron windings for the electromagnetic separation process in the Y-12 National Security Complex at the Oak Ridge National Laboratory. The oval "racetracks" had silver bus bars with a cross-section of one square foot. ^[62] After the war ended, the silver was returned to the vaults.^[63]

Occurrence and extraction

Time trend of silver production

Native silver

Silver is found in native form, as an alloy with gold (electrum), and in ores containing sulfur, arsenic, antimony or chlorine. Ores include argentite (Ag₂S), chlorargyrite (AgCl), which includes horn silver, and pyrargyrite (Ag₃SbS₃). The principal sources of silver are the ores of copper, copper-nickel, lead, and lead-zinc obtained from Peru, Bolivia, Mexico, China, Australia, Chile, Poland and Serbia.^[7] Peru, Bolivia and Mexico have been mining silver since 1546, and are still major world producers. Top silver-producing mines are Cannington (Australia), Fresnillo (Mexico), San Cristóbal (Bolivia), Antamina (Peru), Rudna (Poland), and Penasquito (Mexico).^[64] Top near-term mine development projects through 2015 are Pascua Lama (Chile), Navidad (Argentina), Jaunicipio (Mexico), Malku Khota (Bolivia),^[65] and Hackett River (Canada).^[64] In Central Asia, Tajikistan is known to have some of the largest silver deposits in the world.^[66]

The metal is primarily produced as a byproduct of electrolytic copper refining, gold, nickel, and zinc refining, and by application of the Parkes process on lead metal obtained from lead ores that contain small amounts of silver. Commercial-grade fine silver is at least 99.9% pure, and purities greater than 99.999% are available. In 2014, Mexico was the top producer of silver (5,000 tonnes or 18.7% of the world's total of 26,800 t), followed by China (4,060 t) and Peru (3,780 t).^[67]

Price

Silver price history in 1960–2011

As of 4 April 2016, the price of silver is US$482.42 per kilogram (US$15.01 per troy ounce). This equates to approximately ¹⁄₈₁ the price of gold. The ratio has varied from ¹⁄₁₅ to ¹⁄₁₀₀ in the past 100 years. Physical silver bullion prices are higher than the paper prices, with premiums increasing when demand is high and local shortages occur.^[68]

In 1980, the silver price rose to a peak for modern times of US$49.45 per troy ounce (ozt) due to market manipulation of Nelson Bunker Hunt and Herbert Hunt (equivalent to $142 in 2015). Some time after Silver Thursday, the price was back to $10/oz troy.^[69] From 2001 to 2010, the price moved from $4.37 to $20.19 (average London US$/oz).^[70] According to the Silver Institute, silver's recent gains have greatly stemmed from a rise in investor interest and an increase in fabrication demand.^[70] In late April 2011, silver reached an all-time high of $49.76/ozt.

In earlier times, silver has commanded much higher prices. In the early 15th century, the price of silver is estimated to have surpassed $1,200 per ounce, based on 2011 dollars.^[71] The discovery of massive silver deposits in the New World during the succeeding centuries has been stated as a cause for its price to have diminished greatly.

The price of silver is important in Judaic law. The lowest fiscal amount a Jewish court, or Beth Din, can convene to adjudicate a case over is a shova pruta (value of a Babylonian pruta coin). This is fixed at .025 grams (0.00088 oz) of pure, unrefined silver, at market price. In a Jewish tradition, still continuing today, on the first birthday of a first-born son, the parents pay the price of five pure-silver coins to a Kohen (priest). Today, the Israel mint fixes the coins at 117 grams (4.1 oz) of silver. The Kohen will often give those silver coins back as a gift for the child to inherit.^[72]

Human exposure and consumption

Silver plays no known natural biological role in humans, and possible health effects of silver are a disputed subject.^[73] Silver itself is not toxic to humans, but most silver salts are. In large doses, silver and compounds containing it can be absorbed into the circulatory system and become deposited in various body tissues, leading to argyria, which results in a blue-grayish pigmentation of the skin, eyes, and mucous membranes. Argyria is rare, and although, so far as known, this condition does not otherwise harm a person's health, it is disfiguring and usually permanent. Mild forms of argyria are sometimes mistaken for cyanosis.^[7]

Monitoring exposure

Overexposure to silver can occur in workers in the metallurgical industry, persons taking silver-containing dietary supplements, patients who have received silver sulfadiazine treatment, and individuals who accidentally or intentionally ingest silver salts. Silver concentrations in whole blood, plasma, serum, or urine may be measured to monitor for safety in exposed workers, to confirm the diagnosis in potential poisoning victims, or to assist in the forensic investigation in a case of fatal overdosage.^[74]

Use in food

Silver is used in food coloring; it has the E174 designation and is approved in the European Union.^[75]

Traditional Indian dishes sometimes include the use of decorative silver foil known as vark,^[76] and in various cultures, silver dragée are used to decorate cakes, cookies, and other dessert items.^[73]

Occupational safety and health

People can be exposed to silver in the workplace by breathing it in, swallowing it, skin contact, and eye contact. The Occupational Safety and Health Administration (OSHA) has set the legal limit (Permissible exposure limit) for silver exposure in the workplace as 0.01 mg/m³ over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a Recommended exposure limit (REL) of 0.01 mg/m³ over an 8-hour workday. At levels of 10 mg/m³, silver is immediately dangerous to life and health.^[77]

See also

• Free silver
• List of countries by silver production
• List of elements
• Hardnesses of the elements (data page)
• List of silver compounds
• Silverpoint drawing

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