Gold and it
Gold and its applications:


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.  



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.  


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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