From Fort Knox to Wagner's Ring, from Eldorado to the treasure of King Tut - mankind has been obsessed with gold since the dawn of civilisation. Read on to find out what it is, where it came from and why we're so fond of it.

Gold is a metal which is a pure element. It has an atomic number of 79, making it one of the heaviest naturally-occurring metals. It is soft, heavy and yellow in colour. Gold has a number of properties which make it ideal for making jewellery: it looks nice, it is soft, ductile, malleable, and it doesn't tarnish. Gold has been used for jewellery since metals were first discovered about 6,500 years ago.

Properties

Inertness

Gold's most important property is, perhaps more accurately, a lack of a property. It is chemically inert; that is, it doesn't react with other chemicals. This means that it is permanent. Whereas iron rusts away and copper gets covered in a green coating over time (verdigris), gold stays exactly the way it is.

There are a few chemical substances that can attack gold: the most notable is called aqua regia (literally, 'royal water') and is made from a mixture of hydrochloric acid and nitric acid. This is sometimes used for etching a gold surface. Another is sodium cyanide (NaCN) which is used in the extraction of gold from ore. These are unusual, though, and in general, gold is safe from attack by most chemicals.

Beauty

Gold and copper are the only pure elemental metals that have a colour. All other metals have a silvery-grey look. Copper is arguably as beautiful as gold, but it doesn't keep that look for long, because copper very quickly gets coated in a dirty tarnish and over time turns green. Because gold is chemically inert, it retains its colour more-or-less forever. Golden jewellery from thousands of years ago is as beautiful now as it was when it was made.

Ductility

Ductility is the ability to be stretched into a wire. Gold is the most ductile metal and can be made into very thin wire which can be used for the most intricate of decoration in the style known as filigree. A single gram of gold can be stretched into a wire 3.2km long.

Malleability

Malleability is the ability to be hammered out into a flat sheet. Gold is the most malleable metal by far. It can be flattened into sheets only four-millionths of an inch thick (100 nanometres) without breaking. A gram of gold can be flattened into a sheet with an area of 0.6 square metres (6.7 sq ft). Such sheets of gold, known as gold leaf are so thin that they are slightly transparent - the light shining through is green in colour, giving us the bizarre fact that gold is 'green inside'.

Gold leaf is used in decoration: it can be used in frescoes, ceiling ornaments or on furniture to give an opulent look. It is also sometimes used in India as decoration in cookery - slivers of gold leaf adorn certain desserts. And yes, you are supposed to eat it!

Industrial properties

Gold also has properties which make it useful in industry. It is a good conductor of electricity. Although not as good a conductor as copper, gold does not tarnish so it is very good for exposed contacts such as in slot-in boards in computer equipment. It is also used for the tiny connections to silicon chips in integrated circuits.

An ultra-thin coating of gold on glass can reflect 98% of infrared light while letting through visible light. This is used on the windows of some office buildings in hot countries to reduce the effect of heating by the sun. The gold coating also reflects ultraviolet light and other potentially dangerous high energy radiation, so it is also used on the visors of spacesuits and the windscreens of high-altitude aircraft to protect the people inside from radiation. A single gram of gold can be used to coat 10 m² of glass.

Gold is also used in its pure form for some tooth fillings. These are expensive but are still popular with some dentists.

One final important use of gold is as a catalyst in certain chemical reactions - for example, in the oxidation of carbon monoxide. This is important in air-conditioning systems to remove poisonous carbon monoxide from the air.

(In a parody of this, Doctor Who's enemies the Cybermen are killed by the addition of gold dust to their breathing apparatus.)

A Puzzle

Which weighs more, an ounce of feathers or an ounce of gold?

The correct answer, paradoxically, is an ounce of gold, because gold is traditionally weighed using special units called 'Troy ounces', probably named after the medieval trading city of Troyes. An ounce of gold always means a Troy ounce, which is 31.1g, about 10% greater than a normal ounce. This Entry will use grams throughout, but bear in mind when looking up information on gold, that if ounces are mentioned, that they are these 'special' ounces.

Gold and Money

Since gold was the most valuable metal available in antiquity, it was used from Greek and Roman times as money. The earliest use of gold in coins dates from about 700 BC in Greece, although the gold was alloyed with silver (see Electrum later in this Entry). Gold was used for coins by many different countries after this, even up to the 20th Century in Britain, where a 'sovereign' was a gold coin worth one pound.

In England in 1698, Sir Isaac Newton was put in charge of the Royal Mint. In 1717, he decided that the economy was best served by setting a standard price for gold of four pounds five shillings per troy ounce. In 1816, this 'gold standard' was adopted as the basis for the currency. Gold was considered to hold its value even in times of war or economic depression, so it became the fundamental unit of currency. Every banknote bore the legend 'I promise to pay the bearer one pound sterling'. This meant that the note could be exchanged for an equivalent value in gold at the Bank of England.

The gold standard was adopted by Germany in 1871 and by the USA in 1900. Countries began to stockpile gold. By the beginning of the 21st Century, the USA was the world's greatest hoarder of gold, with over 8,000 tonnes of the stuff. Much of this was held in the famous Fort Knox.

Gold can be traded on the stock market as a commodity, and in fact more than 930 tonnes of gold are cleared through London every day. However, this is virtual gold which is bought and sold. No actual metal changes hands. The stock market price is artificially much higher than the actual value of the metal. This is not a good basis for any currency. This was brought home to the governments of the world in 1968 when gold was allowed to float freely rather than being fixed in price. At first the value rose, so that by 1980 it had reached a record high of US$850 per troy ounce. Then the price started to fall and by 1999 was down to less than a third of that. Although the price has risen again since then, standing at approximately $1,350 per troy ounce as of 2010, governments no longer use gold as the standard and are selling off their gold reserves.

Alloys

Pure gold is soft, which means that jewellery made from it can get damaged easily. To strengthen the gold, other metals are normally added - such a mixture of metals is called an alloy. The amount of gold in the alloy is indicated by a number of carats: 24-carat is pure gold, 12-carat is 50% gold, 50% other metals (by weight). Alloys can often have very different characteristics from the original gold, depending on what metals are added and in what quantities.

Yellow Gold Alloys

Yellow gold alloys are ones which have been designed to retain the golden colour of the metal. There are three main yellow gold alloys used in jewellery making:

• 9-carat gold is typically 9 parts of gold, 2 of silver, 11 of copper and 2 of zinc.
• 14-carat gold is typically 14 parts of gold, 1 of silver, 7 of copper and 2 of zinc.
• 18-carat gold is typically 18 parts of gold, 4 parts of silver and 2 parts of copper.

So: copper is the main ingredient of 9-carat gold!

White Gold Alloys

White gold alloys change the colour so that they look silvery or gray in colour. These aim to look as close to platinum as possible, without the expense of using platinum.

There are two main types of white gold alloy:

Nickel white gold uses nickel as the main ingredient other than gold. This is cheap, it strengthens the metal and it bleaches the gold to a colour very closely approximating platinum. But many people are allergic to nickel - it can cause dermatitis. A typical nickel white gold alloy has 18 parts of gold, 4 parts of nickel, 1 of copper and 1 of zinc.

Palladium white gold is much more expensive because palladium is expensive. It does not cause any allergic reaction. A typical palladium white gold alloy contains 18 parts gold, 4 parts palladium, 1 part silver and 1 part copper.

Electrum

One alloy which is rarely found now but was important in antiquity was 'electrum' - it was a mixture of one part of silver to four parts of gold. This alloy occurs naturally in many mines, so no refining was needed to produce it.

Electrum looks very much like gold. Legend has it that the Ancient Greek philosopher Archimedes was given the task of determining whether a crown offered to the king was made from electrum or pure gold. One way would be to estimate the density - electrum is less dense than gold, but the crown was too complex a shape to calculate its volume. While Archimedes was taking a bath, he noticed that as he got into tub, the water level rose. He realised that he could measure the volume of the crown by immersing it in water. He jumped out of the bath and ran naked down the street shouting 'Eureka, eureka!' (which means 'I have found it'). Whether this is true or not, Archimedes went on to discover the principle of flotation, which shows how the weight of a solid changes when it is immersed in a liquid.

Where Gold Comes From

Creation

Current thinking is that gold is created when stars explode. Stars are initially composed of the gaseous element hydrogen. This 'burns' in a nuclear reaction (fusion) to produce helium. When all the hydrogen is used up, the helium burns to produce carbon, which in turn burns to produce heavier elements. This process continues until the core of the star becomes iron, compressed to an incredible density and at an enormously high temperature. Since iron is unable to burn in the same way, the nuclear process stops and the star collapses inwards. This causes massive shockwaves which literally rip the star apart. As the shockwave travels through the upper layers of the star, elements are flung together in all sorts of haphazard arrangements and some combine to produce gold and other heavy elements. Then the star explodes, shooting off most of its mass into space. A small remnant star is left behind. This explosion is known as a supernova - it is the ultimate fate of all large stars. Supernovas are so bright that they can briefly outshine all the other stars in the galaxy put together.

Gradually over the aeons, all the debris from one such supernova explosion gathered together to form our solar system. The heavier elements clumped together to form the inner planets while the lighter elements formed the sun at the centre and the gas giant planets further out. Gold is distributed in microscopic quantities through the Earth, both on land and in the sea, although in such tiny concentrations that it is not worth extracting.

Concentration

For gold to be present in sufficient quantities for mining, some sort of mechanism is needed to concentrate the gold. This is achieved by flows of hot water, which often occur deep underground, near volcanic vents and along tectonic plate boundaries. If the water is hot enough and has just the right chemical constituents, it will dissolve the gold out of the surrounding bedrock. As the water flows it carries the gold along. Eventually at some point along the water flow, the conditions change: perhaps the water cools below some critical temperature. The cooler water is now not able to hold the gold in solution, so it stops being dissolved and reverts to solid form: it 'precipitates'. Since the point at which this occurs is fixed, the gold concentration builds up at this point. This process might take 50 - 100,000 years. Eventually a vein of gold is formed. This may be pure or it may be mixed with other precious metals - silver is very common.

Extraction

Gold-bearing veins are often discovered because a river may erode the rock and carry particles of gold downstream. These can be lumps of gold known as nuggets, or may be as small as grains of sand. Prospectors in times gone by found this gold in the rivers and extracted it from the river sand and gravel by 'panning'. A flat pan is filled with a mixture of river sand and water. This is swirled around so that some of the sand spills over the edge. The much heavier gold particles will tend to stay in the pan and can eventually be picked out of the sand by hand.

Once the gold vein or 'mother lode' has been found, it can be mined directly. There are three main ways of extracting the gold from the rock:

1. Crushing and mechanical separation - the gold-bearing rock is crushed into dust and then centrifuged in a mechanical process similar to panning. The heavier gold can be extracted from the lighter rock.

2. Chemical separation using mercury - mercury dissolves gold into a mixture called an amalgam. The gold can then be recovered by boiling off the mercury. This method was much favoured in the past, but leads to mercury vapour, which is extremely toxic, being introduced into the environment. This method is no longer used by big mining corporations, but is probably still used by small mines in underdeveloped countries.

3. Chemical separation using cyanide compounds - cyanide will also dissolve gold. Other chemicals can then be used to recover the gold from the solution. The cyanide is poisonous, but only for a short time; it breaks down rapidly and is not a major threat to the environment.

The biggest goldmine in the world is the Witwatersrand (also known as 'the Rand') in South Africa. It currently produces about 40% of all gold in the world and since it opened has produced more than half of all the gold ever mined in the world. Other major producers are the USA and Australia.

Gold in History and Legend

Because of its beauty and rarity, gold has always been highly valued in human society.

The Ancient Egyptian Pharaohs worked on the principle of 'You can take it with you'. They crammed their tombs with possessions, much of them made of pure gold or plated with gold; so much so that the world's gold reserves would have been very quickly depleted were it not for the tomb-robbers, who quickly plundered the pharaohs' tombs and put the gold back into circulation. An indication of how much gold the Egyptians had is the contents of the tomb of Pharaoh Tutankhamen. He was a minor pharaoh who died suddenly in a time of crisis in the Kingdom, and was given a quick burial in a tomb that had been built for someone else. Nevertheless, he was put in a solid gold coffin and his death mask was also solid gold. These were placed inside two more coffins, each one plated in gold. This gold was probably obtained by panning of river silt deposits. Despite this, it has proved to be remarkably pure, at 22 carats (about 92% gold).

Other civilisations held gold in equal regard. The Ancient Irish made golden bracelets and collars (torcs, gorgets and lunulas). These were often buried deep in bogs, apparently as offerings to the gods. The ancient Central American civilisations regularly also gave gold to the gods, often in the form of gold goods thrown into a sacred lake or well.

Greece

Ancient Greek legends often mention gold and are still well known. King Midas was given the gift that anything he touched would turn to gold, but it all went horribly wrong when his young daughter ran up to greet him and he turned her to gold too. One of Aesop's fables tells of a man who had a goose that laid golden eggs. He killed the goose to get the gold more quickly, but found nothing inside. Jason sailed his ship Argo in search of a 'Golden Fleece'. This story was probably based on an early method of separating gold particles from river sand, which involved straining the grit through a sheep fleece, which collected the gold.

Northern Europe

Ancient German legend became the basis for Wagner's series of Operas known as 'The Ring of the Nibelung.' Three water spirits called the Lorelei guarded some magic gold on a rock in the river Rhine. A dwarf called Alberich stole the gold from the Lorelei and made a magic ring and a magic helmet out of it. The gods themselves got involved, using the magic gold as a payment to atone for a broken promise. A complex plot evolves, resulting in the destruction of the world and the gods themselves.

Gold in Christianity

Gold has always been considered a sacred metal by Christians. The ceremony of marriage is sealed by the gift of a gold ring. The cup used to hold the wine in the ceremony of Eucharist was traditionally lined with gold, as only gold was considered pure enough to touch the wine that represented or became the blood of Christ.

The Golden Stool of the Ashanti

In the Middle Ages, West Africa came to the attention of various European nations who had heard of the legends of gold-swathed kings. Present-day Ghana became known as the Gold Coast when it was colonised by the British Empire in the 18th Century.

The Golden Stool is the seat of the soul of the Ashanti tribe, one of the tribes of Ghana. It is said to have come down from the heavens in around 1700 to unite the nation. The Golden Stool is a symbol of the political, religious and cultural authority and unity of the people of Ashanti. All Ashanti kings (asantehenes) are referred to as 'occupying the Golden Stool' but this is not a throne and has never been sat upon, nor is it permitted to touch the ground. For this reason, the Golden Stool always rests on its side.

Eldorado - a Legend Drives History

When the first Spanish explorers arrived in South America, they heard stories of a ruler who covered himself in gold and dived into a lake as part of a religious ceremony. His subjects would throw gold into the lake as offerings. The Spaniards named him 'Eldorado' (the Golden One). Later, the name Eldorado came to mean the mythical country where the gilded man lived, and it was supposed to have cities called Omagua and Manoa.

Fueled by the desire for gold, the explorers conquered much of South America. One such explorer was Gonzalo Pizarro, who in his quest for gold discovered the Inca Empire. He pulled a few dirty tricks on the Inca emperor Atahualpa, holding him to ransom for a room full of gold. When the ransom was duly paid, Pizarro slew the Inca leader and took the gold.