Introduction to Silver
Silver, a metallic chemical element. Like gold and platinum, silver is a precious metal because of its beauty and scarcity. Silver is harder than gold but not as hard as copper. Of all the metals, pure silver is the best conductor of heat and electricity. It is ductile (can be drawn into wire), malleable (can be hammered or rolled into a thin sheet), and an excellent reflector of light.
Silver is one of the most corrosion-resistant metals. It resists attacks by alkalies and all acids except nitric acid and hot concentrated sulfuric acid. Silver is not easily oxidized by air, and is insoluble in water. Because of its softness, silver must be alloyed with other metals for uses that subject it to wear.
Silver for centuries has been used in the arts and for coinage. Silver smithing, the art of producing decorative silver objects, was highly developed in Egypt by the 16th century B.C. Jewelry, tableware, vases, religious articles, and many other items are made of pure silver or alloys containing a high percentage of silver. The most important use of silver compounds is for the light-sensitive materials used in photographic films and papers.
The use of silver for coinage has virtually disappeared because of its high price. In the United States, silver was eliminated from dimes and quarters in 1965 and from half dollars in 1971. Silver certificates, last issued in 1963, were paper bills redeemable in silver until 1968. In 1986, a silver dollar weighing one troy ounce (31.1 g) was issued for collectors and investors. The coin's metal value is much higher than its face value, so it does not circulate.
Silver tarnishes on contact with sulfur or most sulfides. Unprotected silver will tarnish in air because of hydrogen sulfide gas, which comes mainly from the combustion of coal or oil in furnaces. To help prevent tarnish, silver objects may be stored in specially treated paper or felt, or in airtight containers. Display pieces are often coated with clear lacquer or silicone. Tarnish may be removed either by a fine abrasive polish or by various chemical processes.
Pure silver is called fine silver. The proportion of silver in jewelry, coins, and bullion is usually expressed in terms of fineness (the number of parts of silver in 1,000 parts of alloy). Example: an alloy that is 95 per cent silver is said to be 950 fine.
Uses of Silver and Its Alloys
Silver is widely used in alloys. Sterling silver, an alloy of 92.5 per cent silver and 7.5 per cent copper, is used for tableware, jewelry, and decorative items. Silver coins usually consist of silver-copper alloys and silver-copper-nickel alloys. Silver solders, which consist primarily of silver, copper, and zinc, are strong alloys that resist heat and corrosion. They are used for joining metals in car radiators, air conditioners, rocket nozzles, and other products. Silver-cadmium alloys are used for bearings. Silver amalgams are silver-mercury alloys used for dental fillings. Silver is alloyed with palladium, cadmium oxide, gold, or nickel to make electrical contacts. Some silver-gold alloys are used for jewelry and dental fillings.
Silver metal is used as a catalyst in the manufacture of chemicals, and to make electrical contacts, electrical wire, and silver compounds. Mirrors are often coated with silver. Because of its ability to resist corrosion, silver is used to line pipes, vats, and nozzles for chemical and food processing equipment. Silver powder is used in coatings for watch and clock dials and to decorate glass. Silver paints are used to make printed circuits for portable radios and television sets. Storage batteries made with silver-zinc or silver-cadmium cells are used in guided missiles, telemetry equipment, jet aircraft, and portable television cameras.
Silver plate is a base metal coated with pure silver or a silver alloy. The coating is used to improve the appearance or electrical conductivity of metal objects or to protect them from corrosion. Most silver plate is made by an electrolytic process known as electroplating. Jewelry, tableware, trays, musical instruments, and novelties are plated with silver to make them more attractive. Silver is used as a protective coating for bearings, surgical instruments, and many electrical and electronic devices.
Silver Compounds
Silver compounds have many commercial uses. Silver nitrate is used for silvering mirrors, for coloring porcelain, and in the manufacture of photographic chemicals, hair dyes, indelible inks, glass, and other silver compounds. A solution of silver nitrate is used as an astringent and as an antiseptic. It is put on the eyes of children at birth to prevent eye infection.
Silver bromide, silver chloride, and silver iodide are used as light-sensitive substances in photographic emulsions. In artificial rain-making, silver iodide crystals are dispersed into clouds to produce rainfall. Silver cyanide is used in silver plating. Silver fulminate, a high explosive, is sometimes used as a detonator for dynamite and other explosives. Several silver compounds, including silver sulfate and silver ckromate, are used as reagents in chemical analysis. Silver oxide is used in storage batteries, in water purification, as a catalyst in the manufacture of chemicals, and to color and polish glass.
Occurrence and Production
Silver sometimes occurs free (chemically uncombined) in nature, but is usually found combined with other elements. The principal ores of silver are argentite, consisting of silver and sulfur; polybasite, stephanite, and pyrargyrite, consisting of silver, antimony, and sulfur; proustite, consisting of silver, arsenic, and sulfur; and cerargyrite, consisting of silver and chlorine. Small amounts of silver occur in most gold ores and in various base metal ores, chiefly those of copper, lead, and zinc.
Silver deposits that lie far underground are mined by using deep shafts. Sometimes uncombined silver or silver ores occur as small nuggets or flakes that lie on or near the surface of the earth. Such deposits are worked by placer mining methods.
Most of the world's silver is produced as a by-product in the refining of base metals, chiefly copper, lead, and zinc. Substantial amounts of silver are obtained from silver ores and in refining gold. The leading silver-mining countries are Mexico, the United States, and Peru. Silver is mined in about 20 states; Nevada, Idaho, Montana, and Arizona usually account for two-thirds to three-fourths of the United States output.
The largest percentage of silver is obtained from the electrolytic refining of copper. The silver collects in the anode slimes and is recovered by smelting. Silver in lead ores is recovered by the Parkes process. In this process molten lead that has been partially purified is treated with zinc. The zinc combines with the silver and any gold that may be present, forming an alloy that rises to the surface. The alloy is skimmed off, and then distilled to remove the zinc. Silver and gold are separated either by chemical processes or by electrolysis. Silver in zinc or gold ores may be recovered by various metallurgical processes.
Silver is usually extracted from its ores by the flotation process, and then refined by smelting. Silver is sometimes obtained from its ores by the cyanidation process and the amalgamation process.
Cyanidation ProcessCrushed ore is treated with a dilute cyanide solution to dissolve the silver. Lime is usually added to the mixture to control acidity and to reduce chemical reactions between the cyanide and the impurities. After the silver dissolves, the mixture is filtered to remove the solid impurities. Zinc powder is added to precipitate the silver. The silver is then refined by electrolysis.
Amalgamation ProcessCrushed ore is passed over copper plates coated with mercury. The mercury combines with the silver to form silver amalgam. The amalgam is washed, and then distilled to drive off the mercury. The silver is refined by electrolysis.
The recovery of silver from scrap is increasingly important to the world silver supply. Much silver is recovered from the wastes of manufacturing processes and the arts. Silver is also recovered from various used products, including batteries, mirrors, silverware, and jewelry. Also, considerable amounts of silver have been recovered from silver coins that have been withdrawn from circulation.
History
Silver was one of the first metals known to humans. It was used for jewelry and ornaments in the Near East as early as 3500 B.C. The metal was first used in coins about 600 B.C. The silver mines at Laurium, Greece, were worked for several centuries after 1000 B.C., and their output provided much of the wealth of Athens. Silver was mined extensively by the Romans. Production was relatively small during the Middle Ages.
World silver production increased greatly after the discovery of the New World. During the 16th century rich deposits were mined in Bolivia, Peru, and Mexico. Bolivia was the largest producer until the beginning of the 18th century. Silver production in the United States was small until the discovery of the Comstock Lode in Nevada in 1859.
Symbol: Ag (from argentum, Latin for silver). Atomic number: 47, Atomic weight: 107.8682. Specific gravity: 10.5 Melting point: 1,7634 F. (961.9 C.). Boiling point: 4,013.6 F. (2,212 C.) Silver has two stable isotopes: Ag-107 and Ag-109. It belongs to Group IB of the Periodic Table and may have a valence of +1, +2, +3.