Material of the Month: Gold

Materials World magazine
,
1 May 2016

This month, Anna Ploszajski explores gold production, its value throughout history and where it all began.

Gold is the most malleable metal, so even the ancients could manipulate it using simple tools to make ornate jewellery. One gramme can be beaten into a sheet of 1m2 and rolled so thin as to become transparent. This ductility is partially attributable to the fact that gold, being a noble metal, does not develop an oxide film on the surface, and this enables dislocations within the crystal structure to readily escape from the surface of the foil.

Pure gold’s unusual yellow colour comes from relativistic effects within the atoms themselves. Gold’s heavy nuclei mean that the electrons have to orbit at speeds nearing the speed of light to prevent them from falling into the nucleus. Special relativity states that anything moving near the speed of light must gain mass, with the consequence that the s and p orbitals of the gold atoms contract, such that the electrons are more tightly bound to the nucleus. This causes a screening effect on the d and f orbitals, which consequently expand. Our eyes register a yellowy-gold colour when incident white light hits a gold surface and wavelengths from the blue part of the spectrum are absorbed, such that the reflected light only contains wavelengths of the complementary colour. Blue light is absorbed by gold atoms because the energy of the blue light matches the energy difference between the unusually raised 5d orbital and the lowered 6s orbital. This effect can be altered by alloying gold with other metals, such as silver and copper, making a pink-coloured alloy called rose gold, or with palladium and nickel to make silver-coloured white gold. Alloying with the iron makes blue gold, aluminium makes purple gold, and silver makes green gold. 

The unusual optical response of gold makes it an excellent reflector of infrared light, lending it to shielding applications in satellites, and in the face-plates of astronaut helmets, as I found out during my time at the NASA Kennedy Space Center last year. McLaren uses gold foil to line the engine bay of its F1 supercar to protect the carbon fibre body parts from the heat generated by the engine. 

These relativistic effects are also the reason why gold is so reluctant to interact with other elements. The lone electron in the 6s orbital is closely drawn into the nucleus, making it extremely difficult for atoms to bond to others. Thus, gold is a very unreactive metal, so it is usually found naturally on its own in the form of nuggets or grains within rocks, in veins or in placer deposits, fine particles found in riverbeds, and floodplains formed during sedimentary processes. That said, it does naturally alloy with similar metals, such as silver, copper and palladium, or, less commonly, forms compounds such as gold tellurides. 

Alchemists believed that base metals like lead could be turned into noble metals like gold, and in their attempts to do so, unwittingly laid the foundation for modern chemistry. Their symbol for gold was a circle with a point in the centre, the same as the astrological symbol for the sun. We now know that this association between gold and the sun is actually more fitting than the alchemists realised, because gold existed on Earth after formation around 4.5 billion years ago, during the collapse of a supernova, in the dust from which our solar system formed. Most of this gold likely sank to the Earth’s core due to its high density. Today, much of the gold that we find in the Earth’s crust was delivered by asteroid impacts around four billion years ago. 

Exploration and extraction

Most gold is mined out of the ground using one of two methods. The first, originally developed by the ancient Egyptians, involves either panning, sluicing or dredging the granular deposits near rivers – a simple process, which takes advantage of gold’s greater density and lower melting temperature compared to sand.

The second method is open pit mining, used to access deposits encased in hard rock. During the 10th Century, it was discovered that gold dissolves in mercury to form an alloy called amalgam. This was the first method used to extract gold from hard rock deposits and is still in use today, although the associated hazards surrounding the use of mercury are an obvious hindrance. 

The alternative is not much safer. Gold is insoluble in most acids with the exception of aqua regia – nitro-hydrochloric acid – and so, nitric acid, which dissolves silver and other base metals, has long been used to verify the presence of gold in items, and is responsible for the term ‘acid test’. In 1783, Carl Wilhelm Scheele discovered that gold dissolves in aqueous cyanide solutions. This is exploited in the cyanidation process, in which metallic gold trapped in rocky deposits is converted to a water-soluble coordination complex, and can be subsequently extracted using an activated carbon sponge, electrowinning or the Merrill-Crowe process. 

Gold production is notoriously bad for the environment, since extraction is highly energy intensive, and produces huge amounts of waste ore – about one tonne per gramme of gold product. This waste is a source of heavy elements such as cadmium, lead, zinc, copper, arsenic, selenium and mercury, as well as sulphide-bearing minerals, which dissolve in rainwater to acidify the surrounding area. All of these pollutants can wreak havoc with drinking water supplies, plant and animal life, and nearby infrastructure. On top of this, the sodium cyanide used in the cyanidation process is a highly toxic chemical, and spills are not uncommon.

According to the World Gold Council, at the end of 2014 there were 183,600 tonnes of mined gold stocks above ground, the equivalent volume of just 21 cubic metres. Around 50% of this came from South Africa, although China has overtaken to become the world’s largest producer. Historically, the value and allure of gold has resulted in many gold rushes in North America, Canada, New Zealand, Australia, and South Africa, as well as many wars. 

The world’s oceans contain vast quantities of gold – 13,600 tonnes of it, but this boils down to only around 10–30 parts per quadrillion. Although it has not ever been economically viable to extract gold from seawater, many have tried, including Fritz Haber - of the ammonia-producing Haber process notoriety - in the 1920s, but he found that the initial concentration calculations were wrong by a factor of 1,000, rendering his venture a flop. The average human body contains about 0.2mg of gold, mostly in the blood, but this resource has not been recommended for mining just yet. 

Gold’s value throughout history can be put down to its unusual colour, rarity, easy handling, easy smelting, corrosion resistance and non-reactiveness. Around 2,600 years ago, electrum, a gold-silver alloy, is thought to have first been used as coins in Lydia, now part of modern Turkey. The Lydian people claimed that the alluvial deposits where they found their electrum were there because King Midas, with his golden touch, had turned the riverbed to gold when he tried to cleanse himself. The Lydians had also learnt how to separate gold from silver, and as the technology spread, different regions favoured the different metals – the Greeks preferred silver, as did the Romans, with bronze or copper for small change, until the death of Julius Caesar, when gold coins grew to prominence.

The value of gold

In Britain, the Anglo-Saxons introduced the pound as being equal to one-pound in weight of silver. This evolved into the modern pound sterling, introduced in 1489 by Henry VII, the unite in 1604, the laurel in 1619 and the guinea in 1663. All of these were originally worth one pound, although the fluctuating price of gold in the late 1600s caused the guinea’s value to change significantly. This led to the fixing of the value ratio of gold compared to silver by Sir Isaac Newton in his capacity as Master of the Royal Mint in 1717, and later the adoption of the ‘gold standard’ in 1816.

The Bank of England was established in 1694 to raise funds for King William III’s war against France. They issued notes that could be redeemed at the bank for gold or coinage by anyone presenting it for payment. ‘I promise to pay the bearer on demand the sum of X pounds’ remains on English banknotes today. Gold bullion, coins or bars are used as a hedge against inflation or other economic disruptions. 

Gold’s high malleability, ductility, electrical conductivity and corrosion resistance makes it an excellent, if expensive, choice for corrosion-resistant electrical connectors. Nevertheless, most of us walk around with 50mg of gold in our pockets in the form of smartphone components. Gold is also used for connectors for high-end audio, video and USB cables, and particularly for contacts with a high failure cost, such as communications equipment, spacecraft and
jet aircraft engines.

Gold is completely inert in the body, which is why gold leaf can be used in food and drink decoration. Historically, gold was used to treat all sorts of disorders, although today only gold salts and radioisotopes are of pharmacological value for treatments of maladies such as rheumatoid arthritis. Gold nanoparticles are used in aqueous solution for rapid diagnostic tests to detect diseases such as malaria, HIV/AIDS and cancer. There is also research ongoing into the use of gold nanoparticles for drug delivery to cancerous tumours.

Like many before us, we remain spellbound by gold’s allure. Our most aspirational achievements - Olympic medals, Nobel prizes, and Academy Awards are all rewarded with gold. But with advances in electronics and medical engineering, gold is a metal with value beyond pure aesthetics.