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Gold is the most malleable and ductile of all metals; a
single gram can be beaten into a sheet of 1 square meter, or an ounce into
300 square feet. Gold leaf can be beaten thin enough to become
translucent. The transmitted light appears greenish blue, because gold
strongly reflects yellow and red. Such semi-transparent sheets
also strongly reflect infrared light, making them useful as infrared
(radiant heat) shields in visors of heat-resistant suits, and in
sun-visors for spacesuits.
Gold readily creates alloys with many other metals. These
alloys can be produced to modify the hardness and other metallurgical
properties, to control melting point or to create exotic colours (see
below). Gold is a good conductor of heat and electricity and reflects
infrared radiation strongly. Chemically, it is unaffected by air, moisture
and most corrosive reagents, and is therefore well suited for use in coins
and jewellery and as a protective coating on other, more reactive, metals.
However, it is not chemically inert.
Colour
Different colours of Ag-Au-Cu alloys
Whereas most other pure metals are grey or silvery white, gold is yellow.
This colour is determined by the density of loosely bound (valence)
electrons; those electrons oscillate as a collective "plasma"
medium described in terms of a quasiparticle called plasmon. The frequency
of these oscillations lies in the ultraviolet range for most metals, but
it falls into the visible range for gold due to subtle relativistic
effects that affect the orbital's around gold atoms.
Common coloured gold alloys such as rose gold can be created by the
addition of various amounts of copper and silver, as indicated in the
triangular diagram above. Alloys containing palladium or nickel are also
important in commercial jewellery as these produce white gold alloys. Less
commonly, addition of manganese, aluminium, iron, indium and other
elements can produce more unusual colours of gold for various
applications.
Applications:
Pure gold is too soft for day-to-day monetary use and is typically
hardened by alloying with copper, silver or other base metals. The gold
content of alloys is measured in carats (k). Pure gold is designated as
24k. Gold coins intended for circulation from 1526 into the 1930s were
typically a standard 22k alloy called crown gold, for hardness.
Because of the softness of pure (24k) gold, it is usually alloyed with
base metals for use in jewellery, altering its hardness and ductility,
melting point, colour and other properties. Alloys with lower caratage,
typically 22k, 18k, 14k or 10k, contain higher percentages of copper, or
other base metals or silver or palladium in the alloy. Copper is the most
commonly used base metal, yielding a redder colour. Eighteen-carat gold
containing 25% copper is found in antique and Russian jewellery and has a
distinct, though not dominant, copper cast, creating rose gold.
14k gold-copper alloy is nearly identical in colour to certain
bronze alloys, and both may be used to produce police, as well as other,
badges. Blue gold can be made by alloying with iron and purple gold can be
made by alloying with aluminium, although rarely done except in
specialised jewellery. Blue gold is more brittle and therefore more
difficult to work with when making jewellery. 14k and 18k carat
gold alloys with silver alone appear greenish-yellow and are referred to
as green gold. White gold alloys can be made with palladium or nickel.
White 18-carat gold containing 17.3% nickel, 5.5% zinc and 2.2% copper is
silvery in appearance. Nickel is toxic, however, and its release from
nickel white gold is controlled by legislation in
Europe
. Alternative white gold alloys are available based on palladium, silver
and other white metals, but the palladium alloys are more expensive than
those using nickel. High-carat white gold alloys are far more resistant to
corrosion than are either pure silver or sterling silver. The Japanese
craft of Mokume-gane exploits the colour contrasts between laminated
coloured gold alloys to produce decorative wood-grain effects.
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