We continue to review some of the most important materials in heat treatment and metallurgy.
Gold (chemical symbol: Au)
Gold is a soft, yellow metal (Fig. 1) with a lustrous sheen that has long been sought after since prehistoric times. Gold has a very high density and is an excellent conductor of both heat and electricity. It is unaffected by air, water and alkalis, meaning it will not rust or tarnish. Furthermore, gold is resistant to attack by almost all acids; the exception being aqua regia (a mixture of hydrochloric and nitric acids), which will dissolve the element.
The most malleable and ductile of all known metals, a single ounce of gold can be formed into a sheet of gold leaf (400 times thinner than a human hair) measuring 5 meters wide. Gold is one of the few elements that occurs in its natural state and is found in ore deposits that are mined to produce about 1,500 tons per year worldwide.
Gold has been known since prehistoric times and was one of the first metals to be worked by humans since nuggets and flakes could be found in and around waterways. Around 2000 B.C., the Nubians and Egyptians began crude mining operations to obtain more of the precious metal to be used as jewelry, in ceremonial artifacts and for ornamentation used to decorate shrines and temples. The minting of gold into coins (Fig. 2) dates back to around 640 B.C. in what is now modern-day Turkey. For decorative applications, the amount of gold in an alloy is measured in carats. One carat is equal to one part in 24. Therefore, 18-carat is 18 parts gold and 6 parts alloy.
The majority of gold mined is stored as bullion, to be minted into coins or smelted into bars and valued as weight. It also sees extensive use in the making of jewelry and in forms of art, either in in its pure form (24 carat) or as an alloy.
While pure gold is extra lustrous, it is very soft. Therefore, 18-carat or 9-carat alloys are more common for use in jewelry applications since they are much more durable. Gold is a common element for electroplating, and a very thin layer can be used to cover another metal. Typical applications that use electroplating are watch gears, artificial limb joints, electrical contacts, dental fillings and crowns, and electrical copper components.
Here are a few important facts about gold.[2,5]
- Atomic number: 79
- Atomic weight: 196.966569
- Melting point: 1337.33 K (1064.18°C or 1947.52°F)
- Boiling point: 3129 K (2856°C or 5173°F)
- Density: 19.282 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 6
- Group number: 11
- Group name: none
- Electron configuration: [Xe] 4f145d106s1