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Gold as a means of payment
Gold is one of the precious metals and is a dense but soft metal, slightly harder than zinc. Before it can be used, it has to be purified and for most purposes it is alloyed with other materials to make it harder. The purity of gold for jewelry is measured in carats; pure gold is 24 carats. Common purity levels in the Netherlands are 14 carats (58.3% gold), 18 carats (75%) and 22 carats (91.7%).
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Gold has long been seen as a valuable metal. Egyptian hieroglyphs from the 26th century BC already mention gold as a means of payment. Until about the middle of the 20th century A.D., gold was still widely used for coins.
In antiquity gold was not only known as valuable, but it also emanated magic and was the symbol of purity. Alchemists have long been looking for the philosopher’s stone to transform other materials into gold. The fact that they never succeeded in this can be explained by the current insights about the construction of atoms.
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In the 19th century many rich gold sources were discovered, especially in the United States and Australia, which led to gold rushes. Well-known examples are the Californian gold rush and the Klondike Goldrush.
The symbol of gold Au is the abbreviation of the Latin aurum. The name gold is derived from the Indo-European root ĝhel- from which also yellow comes.
Remarkable properties
The gold jewellery from the Grave Field of Varna is one of the oldest known gold jewellery in the world.
Golden death mask of Agamemnon
The Bush Barrow discharge is known as the finest example of a worked gold object from the bronze age from England.
Metallic gold has a yellow shiny color. Very finely divided, it can also take on other colours such as black or dark purple. Of all known metals that are solid at room temperature, gold is the easiest to bend and deform after lead. A cube of gold of 1 gram (a cube with sides of 3.73 mm) can be crushed and rolled into a sheet of gold leaf with an area of 1 square meter. Gold leaf can be glued to objects that make them gilded. It is also possible to apply gold to objects by means of electrolysis.
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Gold is a very good electrical and thermal conductor and almost inert. The density of gold (19,320 kg/m3) is almost twice that of lead (11,300 kg/m3).
Gold chemistry
Gold is known as an inert precious metal with little reactivity; for example, it does not react with oxygen. However, there are quite a few known gold compounds such as gold halogenides and gold calcogenides. The metal is soluble in royal water and forms an AuCl4 ion. With heavier elements, such as tellurium, the reactivity is even quite high.
As a rule, gold forms compounds with an oxidation number of +1 or +3. However, there are also a small number of compounds in which it itself acts as an oxidizer and assumes the oxidation number -1, the aurides.
Calaverite (gold telluride) is a mineral and gold ore.
Appearance
Due to its relative inertness, gold is often found in nature in an unbound form. It usually occurs as a trace element in minerals, but sometimes also in the form of nuggets (nuggets), and sometimes in the form of a large nugget. The largest nugget ever found is the Welcome Stranger Nugget with a weight of over 71 kilos. It was found in 1869 in Moliagul, Victoria, Australia at a depth of almost a metre and has been melted down. The Serra Pelada (Canaã) Nugget was found in 1983 near Serra Pelada, Pará, Brazil. With a weight of over 61 kilos, this is the largest nugget still existing in its original form.
In almost the entire earth’s crust, gold is found in very low concentrations in the minerals Petzite, Calaverite and Sylvanite. However, the quantities are completely unprofitable for commercial extraction.
The highest concentrations have been found in South Africa since 1880. Other large gold mines are located in Nevada and South Dakota in the United States. Australia, Indonesia and the People’s Republic of China are among the most important gold producing countries (in 2001). Approximately two thirds of the extraction takes place in open mines (open-cast mining) and produces large quantities of waste. Annual production from the mines in the years 2010 to 2017 was between 2745 and 3270 tonnes or at least two thirds of the total supply. The extraction of the gold from a 10-gram ring with an 18-carat content produces 18,000 kg of waste. Recycling old gold accounts for a quarter to a third of the total supply.
Two well-known gold mining methods use the toxic sodium cyanide and also toxic mercury by first dissolving the gold according to:
4 Au + 8 NaCN + O2 + 2 H2O → 4 NaAu(CN)2 + 4 NaOH.
Among other things, the toxic sodium cyanide has caused a major environmental disaster at the Summitville mine in Rio Grande County, Colorado in the United States.
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