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Silver
Matters
For thousands of years silver has been used in coins and for ornamental purposes. Silver mines were probably worked before 2500 BC, mainly in Asia Minor. The alchemists called the metal Luna or Diana (the moon goddess), which is why silver is sometimes represented by the symbol of a crescent moon.
Silver
is a white, lustrous metal, that is able to take a high polish. Chemically, silver metal is not very active. It is insoluble in dilute acids and alkalies, but it does dissolve in concentrated nitric or sulphuric acid. It does not react to any great extent with oxygen or water at ordinary temperatures. However, sulphur and sulphides readily attack silver, to cause tarnishing through the formation of a surface layer of water insoluble black silver sulphide (Ag2S) . This explains why eggs, which contain sulphur (in the proteins), are efficient tarnishers of silver. Tarnishing is also slowly produced by the small amounts of hydrogen sulphide (H2S) present in the atmosphere. Silver is about 66th in natural abundance. It occurs in the pure state to a small extent. The richest deposits of native silver are in Peru and Norway. Pure silver may also be found alloyed with pure gold (electrum) and large amounts of silver are obtained in the production of gold. Silver is usually found combined with other elements (especially sulphur) in minerals and ores. Some of the important silver minerals are argentite, cerargyrite (horn silver), pyrargyrite, and sylvanite. Silver also occurs in lead, copper, and zinc ores; in fact about half of the world's production of silver is obtained from these ores. Nearly all of the silver produced in Europe is obtained from the ore galena (lead sulphide). In the U.S. the silver is usually mined along with lead, copper, and zinc. In 1988, the U.S. produced approximately 53 x 106 troy oz. of the estimated 444 million troy oz. of silver produced worldwide. Most of the mined silver is from Australia, Canada, Mexico, Peru and the U.S. Silver is usually recovered from silver ores by roasting the ore to convert the sulphides to sulphates, which are then precipitated. There are several different metallurgical processes used to extract silver from ores. (1) Algamation process:- Liquid mercury, which forms an amalgam with the silver, is added to the crushed ore. The amalgam is washed out of the ore and the mercury is recovered by distillation, leaving metallic silver. (2) Lixiviation process:- The silver is dissolved, usually in an aqueous solution of sodium cyanide. and metallic silver is precipitated by adding metallic zinc or aluminum. (3) Parkes process:- This is really more of a refinement process, in which impure silver, obtained in the metallurgical processes, is refined either by electrolytic methods or by cupellation. The latter process involves removing impurities by vaporisation or absorption.
The
use of silver in jewellery, tableware, and coinage is, of course,
well known.
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Silver
is one of the most malleable and ductile of all metals and is
generally superior to most other metals with respect to its ability
to conduct heat and electricity. Only gold is superior. The hardness
of silver lies between 2.5 and 2.7, which is harder than gold but
softer than copper, and it has a specific gravity of 10.5. Silver
melts at ~962 °C (ca. 1764 °F), boils ~2212 °C (ca.
4014 °F).
Usually.
the silver is alloyed with small amounts of other metals to make it
harder and more durable. For example, sterling silver, used in
tableware, is 92.5 % silver and 7.5 % copper. Silver is used to coat
clean glass surfaces to produce mirrors. This is achieved either by
vaporisation of the metal and its subsequent condensation on the
glass or (more commonly) by precipitation from a solution (largely
replaced by aluminium). Silver is also very widely used in electrical
circuitry (e.g., switch contacts) and electronic components. The
colloidal form of silver, dilute solutions of silver nitrate (AgNO3),
and other silver-containing compounds, have been used as antiseptics
and bactericides. Thus, Argyrol, a silver-containing protein, has
been used as a local antiseptic for the ears, eyes, nose and throat.
Silver-halide salts, such as silver bromide (AgBr), silver chloride
(AgCl), and silver iodide (AgI), which darken on exposure to light,
are incorporated in gels for photographic uses. The unexposed salts
are soluble in aqueous solutions of sodium thiosulphate, which is
used to 'fix' photographic film. The recent advent of the digital
camera may well replace the use of silver in the photographic process.
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