Gold is a metal prized for its beauty, usefulness, and scarcity. Throughout history, people have sought gold more eagerly than any other metal.
Gold has a lovely yellow color and a metallic luster. It is one of the easiest metals to shape. It can be beaten into thinner sheets than any other metal. Gold does not rust or tarnish. Gold deposits rich enough to mine profitably are found in many locations. But mining such deposits yields only small amounts of gold.
Gold was one of the first metals discovered. People have used gold for jewelry and money for thousands of years. Today, gold mining is an important industry in many parts of the world.
Properties of gold
Gold has valuable physical and chemical properties.
Physical properties.
Gold is one of the most lustrous (shiny) metals. Its chemical symbol, Au, comes from aurum, the Latin word for shining dawn. Gold is a soft metal. It is the most malleable of all metals. Malleability is a metal’s ability to be beaten or pressed into various shapes without breaking. Gold can be beaten into extremely thin, transparent sheets. Gold also has the greatest ductility of all metals. A metal’s ductility is its ability to be drawn into fine wires without breaking. One troy ounce of gold (31.1035 grams) can be drawn into a thin wire about 50 miles (80 kilometers) long. Gold is one of the densest metals. Gold weighs about 19 times as much as an equal volume of water at 20° C (68° F).
Gold conducts (carries) electric current better than all other metals except silver and copper. Gold also conducts heat well. In addition, gold reflects infrared (heat) rays better than any other metal.
Chemical properties.
Gold is a stable metal. It does not dissolve or corrode (wear away due to chemical action) in simple acids, such as sulfuric acid or hydrochloric acid, or in simple bases, such as sodium hydroxide or lime. Gold does not rust like iron because it is unaffected by water or oxygen in air. Gold does not tarnish like silver because it is unaffected by sulfur compounds in air. However, gold dissolves in aqua regia, a mixture of hydrochloric acid and nitric acid. In addition, gold dissolves in cyanide solutions when oxygen is present. Cyanide is a chemical compound of the elements carbon and nitrogen.
Chemists classify gold as a transition metal. For information on the position of gold on the periodic table, see the article Periodic table.
Gold alloys
Gold is rarely used in pure form. It is usually combined with one or more other metals to form an alloy. Gold alloy is less expensive than pure gold, yet the alloy retains the valuable properties of the gold. Copper is the metal most commonly alloyed with gold.
Manufacturers express the proportion of gold in an alloy by: (1) karats, or (2) fineness. The karat system, which is usually used for jewelry and ornaments, divides the alloy into 24 parts. One karat (sometimes spelled carat) is equal to one 24th part. Thus, 24-karat gold is pure gold. Jewelry made of 14-karat gold consists of 14 parts gold and 10 parts of some other metal or metals.
The fineness scale, used mostly for industrial products, expresses the proportion of gold in parts per thousand. Thus, 500 fine gold means the alloy consists of 500 parts gold and 500 parts other metal.
Uses of gold
Throughout history, people have used gold mostly as money and for jewelry. Today, gold also has many other uses.
Money.
All countries accept gold in payment for international debts, though such payments are not common. The world’s governments hold about 42,000 tons (38,000 metric tons or 38 million kilograms) of gold in their official stocks. Almost all this gold is in bricklike bars, which are also called ingots. The United States government stores its gold mainly at the Federal Reserve Bank in New York City and at a depository in Fort Knox, Kentucky. Gold bars are a form of bullion—that is, metal held for its value as a metal rather than as money.
Since the 1930’s, few gold coins have circulated as money. However, some countries continue to make gold coins for collectors and investors. Individuals and corporations hold more than 55,000 tons (50,000 metric tons) of gold, mostly as bullion, coins in collections, and jewelry.
Jewelry
accounts for about two-thirds of the gold used commercially each year. Gold jewelry can be various colors, depending on the metals used in the gold alloy. For example, white gold is a silver colored alloy of gold, copper, and such metals as nickel, palladium, silver, and zinc. In yellow gold, about equal amounts of copper and silver are mixed with a greater amount of gold, so that the alloy retains the color of pure gold.
In the United States, the metal in solid gold jewelry is mostly 10-karat to 21-karat gold. Gold-filled and gold-plated items consist of silver or another metal coated with gold. In gold-filled items, 5 percent or more of the object’s weight comes from gold. Gold-plated items contain less gold.
Other uses.
Many electrical and electronic devices, including computers, radios, and television sets have parts made with gold. Gold makes excellent electrical contacts because of its ability to conduct electric current, its high resistance to corrosion, and its ductility. Gold coatings are applied to samples to be examined with an electron microscope. An electron microscope is a device that uses a beam of tiny particles called electrons to magnify a specimen. The gold coating prevents the sample from taking on an electric charge, which could ruin the image.
Thin gold films on spacecraft reflect infrared rays from the sun, which can harm both people and equipment. A transparent gold film in windows of large office buildings also reflects infrared rays, helping to keep the buildings cool in summer. Dentists use gold for crowns because gold is easy to shape and will not corrode in the mouth. Artists use gold leaf, thin sheets of gold, for decoration and lettering.
Gold was used in some of the first examples of nanotechnology. Nanotechnology is the systematic production of structures that are slightly larger than atoms and molecules. The English scientist Michael Faraday created gold nanoparticles in colloids, (mixtures) in 1856. The German scientist Richard Zsigmondy developed one of the first synthetic reactions for gold nanoparticle dispersions in 1898. He used these nanoparticles to develop a special instrument called an ultramicroscope, which enables a person to see tiny objects that float in liquid or gas.
Gold nanoparticles exhibit unique properties compared to bulk gold. For example, they emit heat when exposed to certain wavelengths of light. Scientists are working to use this property to destroy cancer cells. Gold nanoparticles also exhibit fluorescence, a process by which some substances give off light or another form of electromagnetic radiation when they absorb energy. Scientists can use this property to label cells for biological studies. Gold nanoparticles can also exhibit magnetism not seen in bulk gold.
Unlike bulk gold, gold nanoparticles are excellent catalysts for some chemical reactions. A catalyst is a substance that increases the speed of a chemical reaction without being consumed by the reaction. Gold catalysts function at lower temperatures than other catalysts, enabling less energetic reactions to be catalyzed. Gold nanoparticles have also been used to create other nanomaterials, such as silicon nanowires.
Colloids of gold are also used as dyes. Tiny particles of pure or coated gold are mixed with one or more other substances to produce various colors. For example, pure colloidal gold mixed into normally clear glass produces a deep red color. A colloid that has gold particles coated with tin oxide is known as purple of Cassius for its rich color. The color of the gold colloids depends largely on the shape of the gold particles.
Recycling.
Because gold is so valuable, the gold used in many objects has been recycled throughout history. Today, gold refineries recover gold from discarded jewelry, dental materials, and other scrap that contains small amounts of gold.
Gold deposits
Gold is present in almost all rocks and soil. In addition, all the oceans contain dissolved gold. But most gold is too scarcely distributed to recover profitably. People recover gold only where nature has concentrated the metal in Earth’s crust.
Scientists known as prospectors or explorationists search for deposits of gold for mining companies. Explorationists have extensive training in geology, physics, and mining. They use a variety of instruments to analyze the chemical makeup of topsoil, water, and plants, and the rocks beneath Earth’s surface.
Types of deposits.
There are two main types of gold deposits: (1) lode deposits, and (2) placer << PLAS ur >> deposits.
Lode deposits, also called vein deposits, are concentrations of gold and other metals in cracks in rocks. The ore (metal-bearing rock) may be at or near Earth’s surface, or deep in the ground. Placer deposits are accumulations of loose gold in sand, gravel, or rock, usually in the bottom of a valley.
About 93 percent of the gold produced in the United States is extracted from gold ore. The remaining 7 percent is derived from other metal ores and placer deposits. Gold from placer mines amounts to less than half of one percent of total U.S. gold production.
Lode deposits.
Many scientists believe lode deposits form from hot water solutions that rise from far beneath Earth’s surface. Volcanic activity heats the underground water. As the water rises through cracks in rock, it dissolves gold and other metals in the rock. Rocks near the surface cool the water. Then the gold and other metals come out of the solution, forming solid deposits.
Deposits of pure gold—also known as native gold—are rare. In nature, gold is usually combined with silver in an alloy called electrum, or with one or more other metals. In addition, traces of gold often occur in the ore deposits of sulfides (sulfur compounds) of such metals as iron, copper, nickel, lead, and zinc.
In some cases, the sulfides and gold were once too scarcely distributed to be of commercial value, but the action of water has concentrated them. Water concentrates these materials by dissolving other substances and carrying them to lower levels under the ground. The water can do this because it is acidic. Before reaching the ground as rain or snow, the water dissolves carbon dioxide, which is always present in the atmosphere, forming carbonic acid. When the water soaks into the ground, it becomes more acidic by reacting with iron sulfide to form sulfuric acid.
Placer deposits
form when a lode deposit at Earth’s surface is eroded (worn away) by wind and water. Rain washes the eroded material downhill, in some cases into a stream. Gold in a stream sinks to the bottom, while the stream’s current carries particles of lighter materials away. Over millions of years, placer deposits may become buried and compressed into rocks. Much native gold is found in placer deposits.
Mining.
Lode deposits require hard-rock mining, in which miners drill and blast the rock to remove it from the ground. The exact method used depends on the location of the deposit. Open-pit mining recovers ore deposited at or near Earth’s surface. Underground mining recovers ore from deep beneath the surface. Trucks haul the mined ore to a mill.
To mine placer deposits, miners use machines to scoop up rock, sand, and gravel from the valley bottom. The miners then usually sift the ore at the site of the deposit. They place the ore in a container and wash it with water. The gold sinks faster than the other materials and collects at the bottom. Amateur miners use the sifting method when they pan for gold by hand.
Extracting gold from the ore
There are several methods of extracting (removing) gold from ores. The method used depends on the type of ore. However, all the methods are designed to expose valuable minerals to the surface of the ore, to extract gold and any other metals that may be in the resulting mixture, and to purify the metals that are extracted. Processing about 10 tons (9 metric tons) of ore yields as much as 1 troy ounce of gold.
Exposing the gold
beneath the ore’s surface is necessary so that the gold can be dissolved out of the ore. At the mill, machines called crushers reduce large chunks of ore to the size of road gravel. Then, ball mills grind the ore—mixed with water or a solution of cyanide and water—to the fineness of sand or flour. A ball mill is a rotating drum partially filled with metal balls about the size of baseballs.
Leaching
dissolves the exposed gold out of the ground ore using a chemical solvent. Today, most gold leaching processes use cyanide and oxygen. The most common process is known as carbon-in-pulp.
In this process, the ore from the crusher is mixed with a cyanide solution, then ground in the ball mill. The resulting pulp flows to a tank. Air is blown into the tank to supply oxygen. The cyanide and oxygen react chemically with the gold in the ore so that the gold can dissolve. Carbon grains the size of wheat or rice grains are added to the pulp to adsorb (collect) the gold as it dissolves. Filtering the pulp through screens removes the gold-bearing carbon. Treating the carbon with a hot solution of sodium hydroxide and cyanide releases the gold and any adsorbed silver or other metals. Screens remove the carbon grains.
Electrowinning
is a method of electroplating used to recover the metal from the leaching solution. First, the metallic solution is placed in a container called a cell. Also in the cell are a group of anodes (positive electric terminals) and a group of cathodes (negative electric terminals). Chemical reactions cause the metal to be deposited on the cathodes. The cathodes are made of steel wool. This material exposes a great deal of surface area to the solution, so the electrowinning step proceeds more rapidly than it would if the cathodes were steel plates. See Electrolysis.
Smelting
removes the gold from the steel wool cathodes. The steel wool is melted in a furnace along with a chemical mixture called flux. The flux combines with the steel wool and all impurities except silver. The flux floats on top of the gold. Workers pour off the flux and then pour the molten gold into molds to produce bars called ingots.
Refining.
The ingots are purified by one or more electroplating processes. The processes used depend on the amount of silver in the ingots. After refining, the gold is at least 99.9 percent pure. Workers then melt the gold and cast it into bars.
World gold production
China is the leading gold-producing country. Australia, Canada, Russia, South Africa, and the United States also produce large amounts of gold.
In the United States, Nevada supplies the most gold. In Canada, Ontario provides the most gold.
Much of China’s gold is mined in the eastern part of the country. In Australia, the largest gold deposit is in Kalgoorlie in Western Australia. Much of Russia’s gold is mined in the eastern half of the country. One of the world’s biggest gold fields is in the Witwatersrand, a district that is located in the Gauteng Province of South Africa.
Throughout history, about 225,000 tons (205,000 metric tons) of gold have been produced. Since about 1980, world gold production has sharply increased. The increase is due in part to a rise in the price of gold. At current rates, more gold will be produced in the next 70 years than in all of history.
History of gold
No one knows when people first discovered gold. Archaeologists have dug up gold jewelry dating from about 4000 B.C. near Varna, Bulgaria, on the coast of the Black Sea. Coins made of gold were produced in Lydia, a country in what is now western Turkey, during the 500’s B.C.
In the 1500’s, Spanish conquerors sought gold in Mexico and Peru. A legend grew about a land rich in gold called El Dorado, meaning the gilded. In that land, gold was supposed to be as common as sand. Through the centuries, many explorers set out to find El Dorado. The rich gold discoveries in California, Australia, Alaska, and South Africa were all believed by many people to be El Dorado at first, and each discovery began a gold rush. Probably the most famous gold rush was the one to California in 1849. The largest U.S. gold strike of the 1900’s was made near Carlin, Nevada, in the early 1960’s. An open-pit mine began operating there in 1965. The Carlin mine added about 10 percent to the annual gold production of the United States.
During the late 1800’s and early 1900’s, monetary systems of the United States and many other Western countries followed a gold standard. Under such a standard, a nation agrees to buy and sell gold at a fixed price and to exchange its paper money for gold on demand. The United States went off and on the gold standard several times and finally abandoned it in 1971. Since the 1930’s, gold has played a smaller and smaller role in world monetary systems. The United States has not minted gold coins as legal currency since 1933.
