Gold: the essentials

Most metals are metallic grey or silvery white whereas gold is characteristically a metallic yellow colour, in other words gold-coloured. Caesium is also gold coloured. The gold colour seems related to relativistic effects of the outermost gold orbitals.

Small amounts of other metals alloyed with gold change the colour as well as mechanical properties such as hardness. White gold for jewellery is formed by mixing palladium, silver, or nickel with gold, although the result is green gold with certain proportions of silver. White gold is commonly used for wedding rings in the USA. Addition of some copper gives "rose gold", a soft pink colour. Remarkably other colours such as purple (a gold:aluminium alloy), blue (a gold:indium alloy) and even black (a gold:cobalt alloy) may be formed.

Gold is usually alloyed in jewellery to give it more strength, and the term carat describes the amount of gold present (24 carats is pure gold). It is estimated that all the gold in the world, so far refined, could be placed in a single cube 60 ft. on a side. It is metallic, with a yellow colour when in a mass, but when finely divided it may be black, ruby, or purple.

It is the most malleable and ductile metal; 1 ounce (28 g) of gold can be beaten out to 300 square feet. It is a soft metal and is usually alloyed to give it more strength. It is a good conductor of heat and electricity, and is unaffected by air and most reagents.

Gold is readily available commercially and its price (see the UtiliseGold directory) changes day by day and is one of the most widely tracked commercial prices.

The most common gold compounds are auric chloride (AuCl3) and chlorauric acid (HAuCl4). A mixture of one part nitric acid with three of hydrochloric acid is called aqua regia (because it dissolved gold, the King of Metals). It is unaffected by air and most reagents. It is found free in nature and associated with quartz, pyrite and other minerals. Two thirds of the world's supply comes from South Africa, and 2/3 of USA production is from South Dakota and Nevada. Gold is found in sea water, but no effective economic process has been designed (yet) to extract it from this source.

Table: basic information about and classifications of gold.

Gold: historical information

Gold was discovered by Known since ancient times. at no data in not known. Origin of name: from the Anglo-Saxon word "gold" (the origin of the symbol Au is the Latin word "aurum" meaning "gold").

Gold has always fascinated people and gold is certainly one of the very first metals known. Nobody knows who picked up a gold nugget first but it would have been because it was shiny. Gold was highly valued from from the earliest recorded times in history. It seems that the Egyptians developed gold smelting some 5600 years ago (about 3600 BC) using clay blowpipes to heat the smelter contents. Egyption inscriptions dating back to 2600 BC describe gold. Gold workers from Mesopotamia (known now as Iraq) made one of the earliest known pieces of gold jewellery in about 2600 BC. Gold is mentioned several times in the Old Testament. Tutankhamun's funeral mask is one of the most iconic gold pieces known. It was made around 1223 BC and is a stunning piece of ancient gold craftsmanship.

The first use of gold in dentistry as the Etruscans (an ancient Italian civilisation from an area corresponding now to Tuscany) begin securing substitute teeth with gold wire, and with astonishing skill. This was as early as 700-600 BC. Apparently Etruscan craftsmen made gold dental bridges for women so that they could show display their wealth and status. Some women had their incisors removed so that they could be fitted with the gold prosthetics. The Etruscans probably learned dentistry from the Egyptians, Phoenicians, and Greeks but took the art much further. Many specimens of Etruscan dental fittings are to be found in Italian and other museums.

Etruscan gold dental bridge.png
Adapted from M. Don Clawson: Phoenician Dental Art and History of Dentistry Research Page, Newsletter, 8 May 2006.

The dental work illustrated above from the 7th century BC is a fine Etruscan specimen. Gold loops were fitted over the remaining natural teeth, set in place, and maintained by solder. The patient's missing teeth were replaced by either the teeth of human beings and on occasion the tooth of an ox.

Bio-compatibility, malleability and corrosion resistance make gold valuable even today in dental applications.

Colloidal gold has been used since Roman times to colour glass shades of yellow, red, or violet. Melting gold powder into glass diffuses gold nanoparticles into the glass, these refract light, giving the glass a luminous red glow. THe precise colour depends upon the form of the colloidal gold. Faraday (1857) recognised that the colour is due to the minute size of the gold particles and called the sample he made 'activated gold'. In 1898 Richard Adolf Zsigmondy prepared the first colloidal gold in dilute solution. Today, colloidal gold is made by the addition of reducing agents to dilute solutions of Au(III) (the auric ion). One famous form is that made by the use of tin dichloride (SnCl2) as reducing agent. This is a very stable form of colloidal gold and is known as the "Purple of Cassius". It is suitable for colouring ceramics and glasses, and in addition is a good test for Au(III). The form that Faraday studied was made using phosphorus to reduce gold chloride.

Colloidal gold has been used since Ancient Roman times to colour glass intense shades of yellow, red, or mauve, depending on the concentration of gold,

Gold is one of the elements which has an alchemical symbol, shown below (alchemy is an ancient pursuit concerned with, for instance, the transformation of other metals into gold).

alchemical symbol of gold

Hallmarking practice established The world's first "hallmarking" system was established at Goldsmith's Hall in London in 1300 AD. London's Assay Office is still there. THe purpose of the hallmarking system is to determine and guarantee the quality of precious metals such as gold.

Sometime prior to the autumn of 1803, the Englishman John Dalton was able to explain the results of some of his studies by assuming that matter is composed of atoms and that all samples of any given compound consist of the same combination of these atoms. Dalton also noted that in series of compounds, the ratios of the masses of the second element that combine with a given weight of the first element can be reduced to small whole numbers (the law of multiple proportions). This was further evidence for atoms. Dalton's theory of atoms was published by Thomas Thomson in the 3rd edition of his System of Chemistry in 1807 and in a paper about strontium oxalates published in the Philosophical Transactions. Dalton published these ideas himself in the following year in the New System of Chemical Philosophy. The symbol used by Dalton for gold is shown below. [See History of Chemistry, Sir Edward Thorpe, volume 1, Watts & Co, London, 1914.]

Dalton's symbol for gold

Gold has been the basis of many currencies over the centuries and so for economic reasons, among others, the possession of gold was or is restricted in some countries. Notably, private ownership of gold (apart from as jewellery and coins) was banned between 1933 and 1975. In 1422 the Venice Mint made ("struck") 1.2 million gold ducats using 4.26 metric tonnes of gold. These small coins (each about 3.5 g) were popular perhaps because they were easy to make and are highly transportable. In 1717 Great Britain moved to a pure gold standard. The British government linked the currency to gold at a fixed rate (77 shillings, ten and a half pennies per ounce of gold).

gold flakes in Goldschläger.jpg
Image: Classical Numismatic Group, Inc.

The Californian Gold Rush of 1848+1855 started on 24 January 1848 when gold was found by James W. Marshall at Sutter's Mill, in Coloma, California. Although attempts were made to keep the discovery quiet, news leaked out and the result was the migration of 300,000 people to California from the United States and beyond. The New York Herald was the first East Coast newspaper to report the discovery of gold and on 5 December 1848, President James Polk confirmed the discovery in an address to Congress. The gold-seekers were called the "Forty-niners" and often had a very hard time. Prospectors retrieved the gold from streams and riverbeds using low-technology methods such as panning. More sophisticated methods of gold recovery were eventually developed. Gold worth tens of billions of US dollars (2010 equivalent price) was extracted. Some became wealthy but many finished with little profit and a lot of heartache. Today, the forty-niners name lives on in the name of the San Francisco 49ers, an American football team in California.

Some years later in 1885 the South African Gold Rush began. An Australian miner, George Harrison, found gold on Langlaagte farm near Johannesburg while digging to build a house. South Africa became the source of almost one-half of the world's gold.

Gold: physical properties

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Gold: orbital properties

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Isolation: it would not normally be necessary to make gold in the laboratory as it is readily available commercially. The most romantic way to extract gold is by panning it out from a stream in some pleasant valley but most such sources are now depleted. Panning relies upon the density of gold (which is very high) being much greater than that of the sand and other particulates. It therefore settles to the bottom of the pan. The amount of gold recoverable in this way is declining.

One suggestion regarding the golden fleece in the Jason and the Argonauts story (Greek mythology) is that the golden fleece is a consequence of gold mining. There are suggestions that, perhaps 1500 years ago sheep fleeces were stretched out over wooden frames and be submerged in streams. Gold particles swept down from from upstream deposits would then become embedded in them. The fleeces were then dried in trees before shaking or combing the gold out. Similarly, sheep fleeces may have been used on washing tables at alluvial gold mines with much the same effect. Perhaps such methods predated panning of gold from river sands.

Today, more often than not, gold is extracted from ores. These ores often contain relatively little gold. Some of these processes cause environmental concern. Much gold is recovered from ores that are low in gold concentration using a cyanide extraction process. Cyanide extraction was first used around 1887, when the MacArthur-Forrest Process was developed in Glasgow by John Stewart MacArthur. Many worry about the envoronmental effects of the cyanide extraction process and the risks of using cyanide on a large scale. There are three main steps.

The first step is leaching - the ore is crushed to a powder so as to expose the small gold particles. and mixed with water. The resulting mixture of powdered ore and water (the slury) is then reacted with cyanide in the presence of oxygen.

4Au(s) + 8CN-(aq) + 2H2O(l) + O2(g) → 4[Au(CN)2]-(aq) + 4OH-(aq)

The result is that electrons from oxygen are used to convert the gold metal into a Au(I) complex, [Au(CN)2]-. The acidity of the process must be slightly alkaline (pH 10.1 for instance) to minimise the release of highly toxic hydrogen cyanide while optimising the leaching rate.

The next stage is concentration. Once in solution the gold must be converted back to gold metal. One way to do this is by adsorption of the gold onto activated carbon. Most of the impurities are left behind in the solution. This would appear to cause some anion exchange of [Au(CN)2]- with anions associated with the carbon, the precipitation of insoluble AuCN, and the formation of some metallic gold within the carbon pore structure.

The final step is recovery and refining. Gold is stripped from the carbon by mixing it with NaCN and NaOH at 1108C forming a new solution of [Au(CN)2]-. This solution is now fairly pure as the activated carbon process removes many of the impurities. The gold is then converted back to elemental gold in the following electrolysis reactions known as "electrowinning":

At the anode: 4OH- → O2 + 2H2O +4e-
At the cathode: e- + [Au(CN)2]- → Au + 2CN-
Overall: 4OH- + 4[Au(CN)2]- → 4Au + 8CN- + O2 + 2H2O

The gold is "won" onto stainless steel electrodes or precipitated out as a fine black mud. The mud is then smelted and poured into moulds to make gold ingots.

As an laternative to this, zinc powder is added to the solution or reasonably pure of [Au(CN)2]-. This results in a metal displacement reaction:

2[Au(CN)2]-(aq) + Zn(s) -> [Zn(CN)4]2-(aq) + 2Au(s)

As before a fine black 'mud' of gold and residual zinc precipitates from the solution, which is then smelted.

Extraction of gold from seawater

There is some gold in seawater, but the concentration of dissolved gold is very low, perhaps 10 ng l-1. The most determined attempt to recover gold from sea water was undertaken by Fritz Haber, [F. Haber, Z. Angew. Chem. 1927, 40, 303.], who researched the matter extensively after the First World War. He wanted to find a way to pay Germany's war reparation debts. He developed a method involving gold reduction to the metal by sodium polysulfide and removal using sulphur-coated sand filters. Four expeditions were made on ships equipped with the extraction technology but with disappointing results. Following this Haber estimated the gold concentration in sea water to be 4 ng l-1, just one-thousandth of the amount which he had expected.

With the current estimate for the concentration of gold in sea water as 10 ng-1 and the total volume of the oceans at 1.37 x 109 km3, then the total quantity of gold dissolved in sea water is calculated to be 13.7 million tons. A lot of gold, but not extractable on a commercial basis as yet.

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