SILVER

(Ag), chemical element, a white, lustrous metal valued for its decorative beauty and electrical conductivity. Silver is located in Group Ib Period 5 of the periodic table, between copper (Period 4) and gold (Period 6), and its physical and chemical properties are intermediate between those two metals. Silver is treated in a number of Macropaedia articles. For treatment of the recovery and refining of silver, see Industries, Extraction and Processing: Silver. For treatment of silver's use in ornamental and household objects, see Decorative Arts and Furnishings: Metalwork. Properties, uses, and occurrence. Together with gold, and the platinum-group metals, silver is one of the so-called precious metals. Because of its comparative scarcity, brilliant white colour, malleability, and resistance to atmospheric oxidation, silver has long been used in the manufacture of coins, ornaments, and jewelry. Silver has the highest known electrical and thermal conductivity of all metals and is used in fabricating printed electrical circuits and as a vapour-deposited coating for electronic conductors; it is also alloyed with such elements as nickel or palladium for use in electrical contacts. Silver also finds use as a catalyst for its unique ability to convert ethylene to ethylene oxide, which is a precursor of many organic compounds. Silver ornaments and decorations have been found in royal tombs dating back as far as 4000 BC. It is probable that both gold and silver were used as money by 800 BC in all countries between the Indus and the Nile. Silver is widely distributed in nature, but the total amount is quite small when compared with other metals; the metal constitutes 0.05 parts per million of the Earth's crust. Practically all sulfides of lead, copper, and zinc contain some silver. Silver-bearing ores may contain amounts of silver from a trace to several thousand troy ounces per avoirdupois ton, or about 10 percent. Unlike gold, silver is present in many naturally occurring minerals. The most abundant include argentite (Ag2S) and tetrahedrite. Deposits of native (chemically free, or uncombined) silver are also commercially important. Because the majority of the ores that contain silver also contain the important metals lead, copper, or zinc or a combination of the three, the silver-bearing fraction of these ores is frequently recovered as a by-product of copper and lead production. Pure silver is then recovered from the crude fraction by a combination of smelting and fire- or electrorefining. Historically, a major use of silver has been monetary, in the form of reserves of silver bullion and in coins. By the 1960s, however, the demand for silver for industrial purposes, in particular the photographic industry, exceeded the total annual world production. Use of the metal for sterling and plated silverware, ornaments, jewelry, and similar products has continued to be important. Alloys of silver with copper are harder, tougher, and more fusible than pure silver and are used for jewelry and coinage. The proportion of silver in these alloys is stated in terms of fineness, which means parts of silver per thousand of the alloy. Sterling silver contains 92.5 percent of silver and 7.5 percent of another metal, usually copper; i.e., it has a fineness of 925. Jewelry silver is an alloy containing 80 percent silver and 20 percent copper (800 fine). Gold dental alloys contain about 75 percent gold and 10 percent silver. The yellow gold that is used in jewelry is composed of 53 percent gold, 25 percent silver, and 22 percent copper. Natural silver consists of a mixture of two stable isotopes: silver-107 and silver-109. The metal does not react with moist air or dry oxygen but is oxidized superficially by moist ozone. It is quickly tarnished at room temperature by sulfur or hydrogen sulfide. In the molten state, silver can dissolve up to 22 times its volume of oxygen; on solidification, most of the oxygen is expelled, a phenomenon known as the spitting of silver. This can be controlled by the addition of a deoxidant such as charcoal to the molten silver. Silver dissolves readily in nitric acid and in hot concentrated sulfuric acid.