The gold proportion fineness of alloys is measured by karat k.
Pure gold commercially termed fine gold is designated as 24 karat, abbreviated 24k. English gold coins intended for circulation from into the s were typically a standard 22k alloy called crown gold ,  for hardness American gold coins for circulation after contain an alloy of 0. Although the prices of some platinum group metals can be much higher, gold has long been considered the most desirable of precious metals , and its value has been used as the standard for many currencies.
Gold has been used as a symbol for purity, value, royalty, and particularly roles that combine these properties.
Gold as a sign of wealth and prestige was ridiculed by Thomas More in his treatise Utopia. On that imaginary island, gold is so abundant that it is used to make chains for slaves, tableware, and lavatory seats. When ambassadors from other countries arrive, dressed in ostentatious gold jewels and badges, the Utopians mistake them for menial servants, paying homage instead to the most modestly dressed of their party.
Modern bullion coins for investment or collector purposes do not require good mechanical wear properties; they are typically fine gold at 24k, although the American Gold Eagle and the British gold sovereign continue to be minted in 22k 0. The special issue Canadian Gold Maple Leaf coin contains the highest purity gold of any bullion coin , at In , the United States Mint began producing the American Buffalo gold bullion coin with a purity of The Australian Gold Kangaroos were first coined in as the Australian Gold Nugget but changed the reverse design in Like other precious metals, gold is measured by troy weight and by grams.
The proportion of gold in the alloy is measured by karat k , with 24 karat 24k being pure gold, and lower karat numbers proportionally less. The purity of a gold bar or coin can also be expressed as a decimal figure ranging from 0 to 1, known as the millesimal fineness , such as 0. The price of gold is determined through trading in the gold and derivatives markets, but a procedure known as the Gold Fixing in London , originating in September , provides a daily benchmark price to the industry.
The afternoon fixing was introduced in to provide a price when US markets are open. Historically gold coinage was widely used as currency; when paper money was introduced, it typically was a receipt redeemable for gold coin or bullion. In a monetary system known as the gold standard , a certain weight of gold was given the name of a unit of currency. By , it was becoming hard to maintain this price, and a pool of US and European banks agreed to manipulate the market to prevent further currency devaluation against increased gold demand. Bullion Depository at Fort Knox.
In the World Gold Council estimated total global gold supply to be 3, tonnes and demand to be 3, tonnes, giving a surplus of tonnes. In late , gold markets experienced renewed momentum upwards due to increased demand and a weakening US dollar.
Because of the softness of pure 24k gold, it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties. Alloys with lower karat rating , typically 22k, 18k, 14k or 10k, contain higher percentages of copper or other base metals or silver or palladium in the alloy. The Japanese craft of Mokume-gane exploits the color contrasts between laminated colored gold alloys to produce decorative wood-grain effects. By , the gold jewelry industry was escalating despite a dip in gold prices.
Gold solder is used for joining the components of gold jewelry by high-temperature hard soldering or brazing. If the work is to be of hallmarking quality, the gold solder alloy must match the fineness purity of the work, and alloy formulas are manufactured to color-match yellow and white gold. Gold solder is usually made in at least three melting-point ranges referred to as Easy, Medium and Hard. By using the hard, high-melting point solder first, followed by solders with progressively lower melting points, goldsmiths can assemble complex items with several separate soldered joints.
Gold can also be made into thread and used in embroidery.
Though gold is attacked by free chlorine, its good conductivity and general resistance to oxidation and corrosion in other environments including resistance to non-chlorinated acids has led to its widespread industrial use in the electronic era as a thin-layer coating on electrical connectors , thereby ensuring good connection. For example, gold is used in the connectors of the more expensive electronics cables, such as audio, video and USB cables. The benefit of using gold over other connector metals such as tin in these applications has been debated; gold connectors are often criticized by audio-visual experts as unnecessary for most consumers and seen as simply a marketing ploy.
However, the use of gold in other applications in electronic sliding contacts in highly humid or corrosive atmospheres, and in use for contacts with a very high failure cost certain computers , communications equipment, spacecraft , jet aircraft engines remains very common. Besides sliding electrical contacts, gold is also used in electrical contacts because of its resistance to corrosion , electrical conductivity , ductility and lack of toxicity.
Fine gold wires are used to connect semiconductor devices to their packages through a process known as wire bonding. The concentration of free electrons in gold metal is 5. For example, gold electrical wires were used during some of the Manhattan Project 's atomic experiments, but large high-current silver wires were used in the calutron isotope separator magnets in the project. Metallic and gold compounds have long been used for medicinal purposes.
Gold, usually as the metal, is perhaps the most anciently administered medicine apparently by shamanic practitioners  and known to Dioscorides. Even some modern esotericists and forms of alternative medicine assign metallic gold a healing power. In the 19th century gold had a reputation as a "nervine", a therapy for nervous disorders. Depression , epilepsy , migraine , and glandular problems such as amenorrhea and impotence were treated, and most notably alcoholism Keeley, The apparent paradox of the actual toxicology of the substance suggests the possibility of serious gaps in the understanding of the action of gold in physiology.
Some gold salts do have anti-inflammatory properties and at present two are still used as pharmaceuticals in the treatment of arthritis and other similar conditions in the US sodium aurothiomalate and auranofin.
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