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GOLD :
History
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Known and highly valued
since ancient times, gold is found in nature as the free metal and in
tellurides. It is widely distributed and is almost always associated
with quartz or pyrite. It occurs in veins and alluvial deposits, and is
often separated from rocks and other minerals by sluicing and panning
operations. The metal is recovered from its ores by cyaniding,
amalgamating and smelting processes, while refining is carried out by
electrolysis.
Gold in its pure state
is considered the most beautiful and hence most valuable of the pure
metals. It is metallic, having a yellow colour when in mass, though when
finely divided it may be black, ruby or purple.
Gold is soft, ductile
and the most malleable of metals, and because of this is usually alloyed
to give improved strength and durability. Gold’s reflectivity of
ultraviolet and visual light rays is low, however it has high
reflectivity of infrared and red wavelengths.
Gold is a good
conductor of heat and electricity, and is unaffected by air, nitric,
hydrochloric, or sulphuric acid and most other reagents. A mixture of
one part nitric acid with three parts hydrochloric acid, called aqua
regia, dissolves gold. It is also dissolved by solutions of azoimide.
Further, it is attacked by sodium, and potassium cyanide plus oxygen.
The most common gold
compounds are auric chloride (AuCl3) and chlorauric acid
(HAuCl4).
Gold has eighteen
isotopes.
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Applications |
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Gold is chiefly used
for:
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Coinage
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Ornaments
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Jewellery
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Gilding
Green gold is generally used for jewellery. Green
gold is an alloy of gold, silver and copper, and is graded from 14 to 18
karats. Coinage gold (USA) or standard gold (UK) contains gold and
copper in slightly varied amounts, while Australian gold (Australia)
contains silver instead of copper.
Other uses for gold
include:
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Textile industry
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Gold
flake is used for a radiation-control coating for spacecraft
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In
electronic tubes, as gold-plated grid wire, to give high conductivity
and suppressing secondary emissions
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Gold
powder and gold sheet is used for soldering semiconductors, with gold
having a good ability to wet
silicon at 371°C (725°F)
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Gold
is used as a plating material, where sodium gold cyanide [NaAu(CN)2]
is used as a gold plating solution. The plating has good chemical
resistance and electrical properties, however the plating lacks wear
resistance, in which case gold-indium plate is utilised.
Gold alloys
also have a number of applications such as:
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Gold-gallium and gold-antimony are used in electronic industry
(primarily as wire)
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Gold
is used for dental applications and is rightly termed dental gold,
where gold is alloyed with silver,
platinum and on occasion palladium.
It is sometimes alloyed with iridium for hardening.
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Source: AZoM.com |
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The noble metals, gold is found in the native state, and as
is well known, gold was used to make jewelry and sheet metal due
to the great ductility and lustre of the pure metals. Some of the early rich
finds of gold artifacts were from the cemeteries in Bulgaria in Europe (5th
millennium BC) with accouterments of hammered and sheet gold. Some of the
most elegant gold vessels made by the repousse technique come from the
Mesopotamia (ca 2500 BC). Spectacular gold castings are known from ancient
Pharaohnic Egypt, such as the enigmatic face of the young Pharaoh
Tutenkhamen (ca 1300 BC). Early gold ornaments from the Indian
subcontinent are found from Indus Valley sites such as Mohenjodaro (ca 3000
BC). These are on display in the National Museum, New Delhi.
Some of the most interesting artifacts of gold are the unique tumbaga
alloys from medieval South America which are gold-copper-silver alloys of
about 40% gold used for making castings and sheet metals. These were
chemically treated to give a surface of pure gold. Also from South America
comes unique evidence for use of the metal platinum in antiquity where the
sintering of platinum granules with gold was practiced.
In antiquity gold would usually have been collected by panning alluvial
sands from placer deposits. However India has the distinction that the
deepest ancient mines in the world for gold come from the Maski region of
Karnataka with carbon dates from the mid 1st millennium BC. A rather
delightful piece of conjecture is that tales of Herodotus, the Greek, about
‘gold-digging ants’ from India refers to marmot, a type of rodent found in
Afghanistan, who dig up the river sand which could then have been panned for
gold by the inhabitants. Prof. R.K. Dube has produced literary evidence that
the ants gold, refer to in the epic Mahabharath must have been actually
produced by ants, if the size of the gold powder is any indication. The
interesting technique of granulation of gold in making jewelry was developed
in Egypt and was used extensively by the ancient Greeks (ca 600 BC). Surface
tension was used to turn melted gold filings into spheres. The granulation
technique was also used to make gold jewelry in India in the late 1st
millennium BC to early Christian era.
Source:
http://en.wikipedia.org/wiki/Chemical_elements_data_references
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