Gold , symbol Au (from Latin aurum,”gold”), soft, dense, bright yellow metallic element. Gold is one of the transition elements of the periodic table (see Periodic Law); its atomic number is 79.
Pure gold is the most malleable and ductile of all the metals. It can easily be beaten or hammered to a thickness of 0.000013 cm (0.000005 in), and 29 g (1.02 oz) could be drawn into a wire 100 km (62 mi) long. It is one of the softest metals (hardness, 2.5 to 3) and is a good conductor of heat and electricity. Gold is bright yellow and has a high luster. Finely divided gold, like other metallic powders, is black; colloidally suspended gold ranges in color from ruby red to purple (see Colloid).
Gold is extremely inactive. It is unaffected by air, heat, moisture, and most solvents. It will, however, dissolve in aqueous mixtures containing various halogens such as chlorides, bromides, or some iodides. It will also dissolve in some oxidizing mixtures, such as cyanide ion with oxygen, and in aqua regia, a mixture of hydrochloric and nitric acids. The chlorides and cyanides are important compounds of gold. Gold melts at about 1064° C (about 1947° F), boils at about 2808° C (about 5086° F), and has a specific gravity of 19.3; its atomic weight is 196.97.
Gold is found in nature in quartz veins and secondary alluvial deposits as a free metal or in a combined state. It is widely distributed although it is rare, being 75th in order of abundance of the elements in the crust of the earth. It is almost always associated with varying amounts of silver; the naturally occurring gold-silver alloy is called electrum. Gold occurs, in chemical combination with tellurium, in the minerals calaverite and sylvanite along with silver, and in the mineral nagyagite along with lead, antimony, and sulfur. It occurs with mercury as gold amalgam. It is generally present to a small extent in iron pyrites; galena, the lead sulfide ore that usually contains silver, sometimes also contains appreciable amounts of gold. Gold also occurs in seawater to the extent of 5 to 250 parts by weight to 100 million parts of water. Although the quantity of gold present in seawater is more than 9 billion metric tons, the cost of recovering the gold would be far greater than the value of the gold that could thus be recovered.
The metal has been known and highly valued from earliest times, not only because of its beauty and resistance to corrosion, but also because gold is easier to work than all other metals. In addition, gold was easier to obtain in pure form than the other metals. Because of its relative rarity, gold became used as currency and as a basis for international monetary transactions (see Dollar; Gold Standard). The unit used in weighing gold is the troy ounce; 1 troy ounce is equivalent to 31.1 grams.
The major portion of the gold produced is used in coinage and jewelry (see Metalwork). For these purposes it is alloyed with other metals to give it the necessary hardness. The gold content in alloys is expressed in carats (see Carat). Coinage gold is composed of 90 parts gold to 10 parts silver. Green gold used in jewelry contains copper and silver; white gold contains zinc and nickel, or platinum metals. Gold is also used in the form of gold leaf in the arts of gilding and lettering. Purple of Cassius, a precipitate of finely divided gold and stannic hydroxide formed by the interaction of auric chloride and stannous chloride, is used in coloring ruby glass. Chlorauric acid is used in photography for toning silver images. Potassium gold cyanide is used in electrogilding. Gold is also used in dentistry. Radioisotopes of gold are used in biological research and in the treatment of cancer (see Isotopic Tracer).
The simplest process used for mining gold is panning, using a circular dish often with a small pocket at the bottom. The prospector fills the dish with gold-bearing sand or gravel, holds it under a gentle stream of water, and swirls it. The lighter parts of the gravel are gradually washed off and the gold particles are left near the center of the pan or in the pocket.
As gold mining developed, more elaborate methods were introduced and hydraulic mining was invented. The hydraulic method consists of directing a powerful stream of water against the gold-bearing gravel or sand. This operation breaks down the material and washes it away through specially constructed sluices in which the gold settles, while the lighter gravel is floated off. For mining on rivers, elevator dredges are generally used. The elevator dredge is a flat-bottomed boat that uses an endless chain of small buckets to scoop up the material from the river bottom and empty it on the dredge into a trommel (a container built of screening). The material is rotated in the trommel as water is played on it. The gold-bearing sand sinks through perforations in the trommel and drops onto shaking tables, on which it is further concentrated. Dredging can also be used in dry beds of ancient rivers if ample water is within a reasonable distance. A pit is dug, and the dredge is moved in and floated on water pumped from the adjacent source.
Extensive underground deposits of gold-bearing rocks are often discovered by a small outcrop on the surface. Shafts are sunk, as in coal mining, and the ore is brought to the surface. It is then crushed in special machines. Gold is extracted from gravel or from crushed rock by dissolving it either in mercury (the amalgam process) or in cyanide solutions (the cyanide process). Some ores, especially those in which the gold is chemically combined with tellurium, must be roasted before extraction. The gold is recovered from the solution and melted into ingots. Gold-bearing rock with as little as 1 part of gold to 300,000 parts of worthless material can be worked at a profit.
The rarest form of gold is a nugget. The largest known nugget, the Welcome Stranger, weighing about 70.8 kg (about 156 lb), was turned up accidentally, just below the surface of the ground, by a wagon wheel in Victoria, Australia, in 1869.
Gold production dates from the Etruscan, Minoan, Assyrian, and Egyptian civilizations, when placer gold was derived from alluvial sands and gravels by simple processes of washing or panning. Gold was produced in this manner at an early period in India, central Asia, the southern Ural Mountains and in the regions bordering the eastern Mediterranean. With progress in mining technique, primary auriferous (gold-bearing) veins were exploited; this type of gold mining attained some importance before the Christian era. During the Middle Ages little progress was made in gold production and mining.
At the time of the discovery of the Americas the value of the total gold stock of Europe was probably less than $225 million. During the succeeding 350 years, from the end of the 15th century to about 1850, the world gold output totaled about 4,665,000 kg (about 150 million troy oz). South America and Mexico became large producers of gold during this period. Spain’s domination in South America resulted, in the 16th century, in a large increase in gold produced in the New World; some resulted from simple seizure of gold from the Native Americans, who had long mined the metal.
Placer deposits of gold were discovered on the Yukon River in Canada and Alaska in 1869. The discovery in 1896 of a rich deposit in the Bonanza Creek, a headwater of the Klondike River, which in turn is a tributary of the Yukon, led to another gold rush. In 1910 discoveries were made on Bitter Creek, near Stewart, British Columbia. In 1911 gold was also discovered in Alaska, some 32.2 km (some 20 mi) from the Canadian boundary at the source of the Sixty Mile River, which rises in Alaska and flows into the Yukon River. Since 1911 the production of gold in Ontario, in the Porcupine and Kirkland lake districts, has gone ahead rapidly.
On December 31, 1974, the U.S. federal government lifted a 41-year ban on the private ownership of gold. Around that time gold was being traded on the London bullion market at record highs approaching $200 an ounce. After subsequent sharp decreases, prices rose to a high of $850 in January 1980. They then dropped considerably and in the early 1990s settled at about $370 an ounce.