How Gold Works

When most people think of prehistoric humans transitioning from the Stone Age to the Bronze Age, they picture a movement from stone tools to tools made from copper or copper combined with tin. But in some regions of the world, early humans may have worked gold before other metals. For example, in Bulgaria, archaeologists have found decorative gold objects dating to about 4000 B.C. Most civilizations in North Africa, Asia and Europe transitioned out of the Stone Age between 6000 B.C. and 2500 B.C., so gold was certainly embraced by humans early in their development.

There's no doubt that ancient Egyptians had a voracious appetite for gold. Descriptions of the metal appeared in hieroglyphs as early as 2600 B.C. By 1500 B.C., gold had become the recognized medium of exchange for international trade. The source of this gold was Nubia, or Kush, a sub-Saharan empire located on the Nile south of Egypt. Pharaohs sent expeditions to Nubia to mine the quartz lodes for gold, which Egyptian goldsmiths transformed into vessels, furniture, funerary equipment and sophisticated jewelry.

­By 550 B.C., the Greeks had started mining the Mediterranean and Middle East for gold. The Romans continued the practice, introducing sophisticated techniques, such as hydraulic mining, or hushing, which involved using large volumes of water to dislodge rock and remove debris. They also minted coins on a scale never before seen, producing millions of gold aureus coins, each stamped with the emperor's head, between A.D. 200 and 400.

At about the same time, South American civilizations were making great headway with gold metalworking. People of the Middle Sicán era (A.D. 900 to 1100), living in present-day Peru, produced enormous quantities of precious metal artifacts. Their goldsmiths specialized in using sheet metal created by hammering gold ingots with stone hammers on stone anvils. The result was a startling array of gold ornaments, masks, headdresses and other objects.

These are the kinds of treasures Europeans were hungry to find when they began to explore the world to their west. Up next, we'll see how lust for gold helped to shape the fortune of empires and ordinary men.

­­Marco Polo, the Venetian explorer who traveled to China in 1271 and served on the court of Kublai Khan, did much to inspire Europe's notion that vast gold treasures could be found in distant lands. His book, "The Travels of Marco Polo," told of great palaces with walls covered in gold and silver. Christopher Columbus most certainly read Polo's book and hypothesized that one might reach the trade-rich Orient by sailing west across the Atlantic.

­Columbus' successful voyage to the Caribbean in 1492-93 touched off an unprecedented wave of exploration. Although spreading Christianity was a major objective of Spain, obtaining gold and silver were high priorities, too. In 1511, King Ferdinand, who financed Columbus on his fateful journey to the New World, spoke unequivocally when he said, "Get gold, humanely if you can, but at all hazards, get gold." Throughout the 16th century, the Spanish concentrated on conquering Central and South America, all the while searching for El Dorado, a city in which gold was supposed to be as common as sand.

Every time explorers discovered a major source of gold, they reported that they had found the mythical city and started a gold rush. One of the first rushes occurred in Brazil's Minas Gerais region in 1700. Gold mining in this area became the main economic activity, making Brazil the largest gold producer by 1720. Slaves, brought in from Africa, used primitive technologies, such as panning, to do the mining.

Panning became iconic of the California Gold Rush, as well, although the first U.S. gold rush occurred 3,000 miles (4,828 kilometers) away and 45 years earlier -- in North Carolina. Until the 1830s, North Carolina supplied all of the domestic gold coined for currency by the U.S. Mint in Philadelphia. Then, on Jan. 24, 1848, New Jersey-born contractor and builder John Marshall found flakes of gold while overseeing construction of a sawmill near Sacramento, Calif. By the end of the year, an estimated 5,000 people were mining in California. That number exploded to 40,000 by the end of 1849 [source: PBS].

­Similar rushes gripped other nations in the 19th century. A gold rush in Australia began in 1850 when Edward Hammond Hargraves found gold in New South Wales. And in 1868, George Harrison uncovered gold in South Africa while digging up stones to build a house.

More than 90 percent of the world's gold has been produced following the rushes of the mid- to late 1800s [source: World Gold Council]. The entire process of gold mining can be broken down roughly into four steps: prospecting, mining, extracting and refining.

Early discoveries of gold relied on the blind luck of someone spotting a yellow glint in a stream or in a crack between rocks. But the search today is more systematic and precise. First, geologists know more about how gold forms. They know, for example, that the metal is present in almost all rocks and soil, but the grains are so small that they're invisible. Only in a few areas is the gold concentrated enough to be mined profitably. Scientists, known as prospectors or explorationists, search for these deposits. This is known as prospecting. Sometimes, these deposits contain pure gold. In most deposits, however, gold is combined with silver or another metal. After finding indications of gold, scientists drill to obtain samples from below the surface, which they analyze for their gold content. If there's enough gold in the deposit, the mining company may set up a large-scale mining operation.

How gold is mined depends on the deposits. Lode deposits are concentrations of gold found in solid rock. If the gold-bearing rock is located at the earth's surface, the mining company will use open-pit techniques. First, miners drill a pattern of holes, which they then fill with explosives. Next, they detonate the explosives to break up the ground so it can be loaded into haul trucks.

If the lode deposit is located beneath the Earth's surface, underground mining is necessary. In this case, miners drill a shaft, or an adit, into the ground to access the lode. Then they dig long vertical tunnels, known as stopes, that extend from the top of the ore block to the bottom. After they drill and load explosives into the ore block, the miners detonate the explosives, causing broken ore to fall to the bottom of the stope. There, ore is loaded into trucks and taken to the surface.

Placer deposits -- accumulations of loose gold in the sediments of a streambed or a beach -- are mined differently. Miners scoop up sand, gravel and rock, and mix it with generous amounts of water. The gold, because of its greater density, sinks faster than the other materials and collects at the bottom. Many miners use a metal or plastic pan to separate the gold from sediments, a process known as panning.

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Removing the gold-bearing rock from the ground is just the first step. To isolate pure gold, mining companies use a complex extraction process. The first step in this process is breaking down large chunks of rock into smaller pieces. At a mill, large machines known as crushers reduce the ore to pieces no larger than road gravel. The gravel-like material then enters rotating drums filled with steel balls. In these drums, the ore is ground to a fine slurry or powder.

Next, mill operators thicken the slurry with water to form pulp and run the pulp through a series of leaching tanks. Leaching dissolves the gold out of the ore using a chemical solvent. The most common solvent is cyanide, which must be combined with oxygen in a process known as carbon-in-pulp. As the cyanide and oxygen react chemically, gold in the pulp dissolves. When workers introduce small carbon grains to the tank, the gold adheres to the carbon. Filtering the pulp through screens separates the gold-bearing carbon.

The carbon moves to a stripping vessel where a hot caustic solution separates the gold from the carbon. Another set of screens filters out the carbon grains, which can be recycled for future processing. Finally, the gold-bearing solution is ready for electrowinning, which recovers the gold from the leaching chemicals. In electrowinning, operators pour the gold-bearing solution into a special container known as a cell. Positive and negative terminals in the cell deliver a strong electric current to the solution. This causes gold to collect on the negative terminals.

­Smelting, which results in nearly pure gold, involves melting the negative terminals in a furnace at about 2,100 degrees F (1,149 degrees C). When workers add a chemical mixture known as flux to the molten material, the gold separates from the metal used to make the terminals. Workers pour off the flux and then the gold. Molds are used to transform the liquid gold into solid bars called doré bars. These low-purity bars are then sent to refineries all over the world for further processing.

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­The final stage of gold production -- refining -- involves removing impurities that remain after the smelting process. Refining companies receive doré bars, as well as scrap gold, and reliquefy the metal in a furnace. Workers add borax and soda ash to the molten metal, which separates the pure gold from other precious and less precious metals. A sample is then taken to a lab for tests, or assays, that measure the gold content. In most cases, the gold is 99.9 percent pure. Workers cast the gold produced during refining into bars.

What happens next depends on how the gold will be used. Pure gold is generally too soft for most practical applications, so other metals are nearly always added to it. When gold is combined in this way, it forms an alloy. Scientists and goldsmiths often use colors to designate the various gold alloys that are possible. For example, white gold is made by combining gold with nickel, silver or palladium. Red or pink gold is an alloy of gold and copper. And blue gold is the result of mixing gold with iron.

­Karatage refers to how much gold is present in an object versus another alloy. A higher karatage indicates a higher proportion of gold in the sample. So, 24-karat gold is 100 percent gold, while 12-karat gold has exactly half as much. The common karatages are shown in the accompanying sidebar.

Interestingly, different cultures prefer different karatages. For example, the people of India are partial to 22-karat gold, while Europeans prefer 18-karat gold. In the United States, 14-karat gold, which offers a balance between gold content, hardness and affordability, is by far the most popular [source: World Gold Council: Jewelry Inspirations].

­Most people are familiar with karatage as it applies to jewelry, and jewelry accounts for nearly two-thirds of the global demand for gold [source: Gerlach]. In the next section, we'll examine the other uses of gold.

Throughout history, jewelers and goldsmiths have selected gold as their precious metal of choice because of its unique properties. Gold is naturally beautiful and resists corrosion and tarnishing. It is also soft and malleable, which allows artists to shape the metal into almost any design. Most jewelry is still made that way -- by individuals using craft skills and simple tools that have been around for centuries. Factory production of gold jewelry, however, is becoming more common.

Electronics manufacturers also use gold extensively to take advantage of its high conductivity. Gold conducts electricity better than all other metals except silver and copper. And it doesn't corrode easily. This makes the metal an ideal choice for plating contacts, terminals, printed circuits and semiconductors. A typical computer, for example, uses gold in both the display and the circuit board. Each computer holds only a small amount -- less than 0.1 kilograms (3.5 ounces) of a 27-kilogram (60-pound) machine [source: SWICO Recycling] -- but the numbers add up. According to the U.S. Geological Survey, one metric ton (1.1 tons) of circuit boards can contain 40 to 800 times the amount of gold contained in gold ore mined in the U.S. [source: Grossman].

Other industries that rely on computers and microelectronics, such as aerospace, use large amounts of gold. According to the World Gold Council, NASA used more than 40.8 kilograms (90 pounds) of gold in the construction of the Columbia space shuttle. Much of it found its way into electrical contacts and circuit boards, but a large amount was used in thin-film applications. Sheets of gold 0.15 millimeters (0.006 inches) thick are highly reflective and make effective radiation shields. Similar gold films are now being used to coat the windows of large office buildings, deflecting the sun's rays and controlling passive heating.

Other industries have founds ways to put gold to work, too.

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One big use we didn't talk about last section was gold's role as a form of currency. You can read much more in How Currency Works, but it's worth mentioning here because gold and money are practically synonymous. Gold coins have been around for centuries, probably since King Croesus, the ruler of ancient Lydia, issued pure gold coins on a large scale in 640 B.C. As we mentioned, the Greeks and Romans also minted gold coins, like the widespread Roman aureus coins. It would be many years before another coin would be so popular. That coin was the ducat, introduced in Venice in 1284. Great Britain issued its first major gold coin, the florin, the same year. Ephraim Brasher, a goldsmith, struck the first U.S. gold coin in 1784.

­In addition to making coins out of gold, governments also hold gold in reserve in case they need to make payments for intern­ational debts. In fact, the world's central banks hold about 20 percent of the aboveground supply of gold [source: World Gold Council]. The U.S. government stores its reserves in two locations -- the Federal Reserve Bank in New York City and the United States Bullion Depository at Fort Knox, Ky. Walk into either facility, and you would see bricklike bars, known as ingots, stacked like firewood. Each bar is 7 inches by 3.625 inches by 1.75 inches and weighs 400 ounces, or 27.5 pounds. In metrics, that comes out to a bar roughly 18 centimeters by 9 centimeters by 4 centimeters weighing a little more than 11 kilograms. Fort Knox currently holds 147.3 million ounces of gold (4.2 million kilograms). With a book value of $42.22 per ounce, that makes the Fort Knox holding worth $6.2 billion [source: United States Mint]!

­The demise of the gold standard has led to the rebalancing of those reserve portfolios. You can read more about the gold standard in this HowStuffWorks article on the New Deal, but here are a few basics. Countries on the gold standard will exchange paper currency for gold and will buy and sell gold at a fixed price. In 1900, with the passage of the Gold Standard Act, the United States formally adopted the gold standard, only to abandon it in 1971.

If gold is relatively rare and most of it has already been mined, does it have much of a future? One thing to remember is that a significant portion of the world's supply of gold is held in reserve by central banks or by individuals who keep gold as an investment. By some estimates, so-called "bar hoarding" accounts for nearly 236 tons (214 metric tons) of gold [source: National Geographic]. Recycling also plays an important role. Approximately 85 percent of all the gold ever found is still being used today, which means that the gold in your favorite jewelry could have once glittered from the headdress of an Incan or Aztec king [source: Temescu].

Some scientists are looking to the heavens to find more gold. In 1998, the Near Earth Asteroid Rendezvous (NEAR) spacecraft passed close to the asteroid Eros and sent back data indicating that the potato-shaped space rock was a vast warehouse of metals. If Eros is typical of stony meteorites that crash to Earth, then it contains about 3 percent metal. Given the dimensions of the asteroid, ­NASA scientists estimate that Eros may house 20 billion tons (18.1 billion metric tons) of gold and similar amounts of other metals, such as aluminum and platinum [source: BBC News].

Back on Earth, prospectors continue to search for new gold deposits using new, highly sensitive methods of detection. These new methods greatly increase the odds that once-overlooked gold will be discovered. For example, a gold mine near Carlin, Nev., is producing gold from a large low-grade deposit that was opened in 1965 after intensive scientific and technical work had been completed. Other such deposits most certainly exist.

Wherever new mines are established, mining companies will need to focus more on the environmental impact of their operations. Extracting a single ounce of gold requires the removal of 250 tons (227 metric tons) of rock and ore [source: National Geographic]. Then there's the effluent -- the cyanide-laced liquid from the extraction and refining processes -- that's most often dumped offshore. "No Dirty Gold," a campaign run by the nonprofit Earthworks, seeks to raise the environmental standards of the global mining industry. The campaign has enlisted the support of 30 of the world's leading jewelry companies, persuading them to stop selling gold from mines with questionable practices.

­Such a campaign is unlikely to stop the global demand for gold. But perhaps it will make us realize that the gold rings on our fingers have a life and a history far beyond our own.