Gold (Au)
Throughout history, gold has been unequalled in its value. Gold has
several qualities that have made it so attractive as a currency, adornment
and technological metal.
- Its yellow colour and brightness
- Its inertness to chemical reaction, it does not tarnish or corrode
- It is highly malleable and ductile
- It can be found in nature in pure form
- Its high density
- It is a good conductor of heat and electricity
Gold's
highly visual appeal and workability made it one of the first metals
to attract man's attention. Examples of gold workmanship and coins have
survived from ancient Egypt, Greece, Syria and Etruscia.
Gold is universally accepted in exchange for goods and services, with
approximately 45% of the world's gold held by governments and central
banks in the form of coins and bullion as a medium of international
exchange.
Natural gold
Gold is found in low concentrations in all igneous
rocks. It's abundance in the Earth's crust is estimated at about 0.005
parts per million. Gold often occurs in copper and lead deposits. Gold
can be found in underground veins associated with quartz and pyrite
(or fool's gold) and as alluvial deposits created by the weathering
of gold-bearing rocks and the deposit of gold in river beads and lakes.
Gold in igneous rock is usually found as near invisible grains, more
rarely as flakes large enough to be seen, and even more rarely as nuggets.
The largest nuggets, up to 90 kg, come from Australia.
Gold production
Because gold is found naturally as a metal, little is required to extract
it from its surrounding rock. Production usually starts with crushing
the rock to release the gold, this can then be washed to separate the
gold from the rock. With the gold being much denser than normal rock
it will sink to the bottom of a pan or slats of a sluice.
Gold can also be dissolved from the crushed rock ore using a mixture
of cyanide and acids and then recovered from solution formed. Another
method passes the crushed ore through a bath of liquid mercury. The
dense gold sinks in the mercury, forming an alloy, while the lighter
rock floats on the surface of the mercury.
Gold alloys
Because pure gold is very soft it is usually alloyed with other metals
to increase its hardness for use in jewellery and coinage. Gold is alloyed
with silver, copper, nickel and zinc to produce various shades of yellow
gold. Alloys with platinum and palladium are also used in jewellery
to form "white gold".
The gold content of gold alloys is expressed in karats. 1 karat = 1/24th
of the content being gold, 24-karat gold being pure gold. A 18 karat
gold alloy is 75% gold, 12 karat is 50% gold and 9 karat is 38% gold.
Modern
gold
Modern technology uses gold for its high electrical conductivity and
chemical inertness. The largest industrial user of gold is in the electrical
and electronics industry, where gold is used for wiring, printed circuits
and electronic electrical connections. Thin films of gold that reflect
up to 98% of infrared radiation are used on satellites, on space-suit
visors and windows of large office buildings to control temperature.
Impregnated into glass, gold creates a "rose coloured glass".
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