24 Karat Gold

Characteristics of Gold

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Gold is the most malleable and ductile metal; a single gram can be beaten into a sheet of one square meter, or an ounce into 300 square feet. Gold leaf can be beaten thin enough to become translucent. The transmitted light appears greenish blue, because gold strongly reflects yellow and red. Gold readily forms alloys with many other metals. These alloys can be produced to increase the hardness or to create exotic colors (see below). Native gold contains usually eight to ten percent silver, but often much more — alloys with a silver content over 20% are called electrum. As the amount of silver increases, the color becomes whiter and the specific gravity becomes lower.

Gold is a good conductor of heat and electricity, and is not affected by air and most reagents. Heat, moisture, oxygen, and most corrosive agents have very little chemical effect on gold, making it well-suited for use in coins and jewelry; conversely, halogens will chemically alter gold, and aqua regia dissolves it via formation of the chloraurate ion.

Common oxidation states of gold include +1 (gold(I) or aurous compounds) and +3 (gold(III) or auric compounds). Gold ions in solution are readily reduced and precipitated out as gold metal by adding any other metal as the reducing agent. The added metal is oxidized and dissolves allowing the gold to be displaced from solution and be recovered as a solid precipitate.

Recent research undertaken by Sir Frank Reith of the Australian National University shows that microbes play an important role in forming gold deposits, transporting and precipitating gold to form grains and nuggets that collect in alluvial deposits.

High quality pure metallic gold is tasteless, in keeping with its resistance to corrosion (it is metal ions which confer taste to metals).

In addition, gold is very dense, a cubic meter weighing 19300 kg. By comparison, the density of lead is 11340 kg/m³, and the densest element, Iridium, is 22650 kg/m³.

Gold as a Applications

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As the metal

Medium of monetary exchange
In various countries, gold is used as a standard for monetary exchange, in coinage and in jewelry. Pure gold is too soft for ordinary use and is typically hardened by alloying with copper or other base metals. The gold content of gold alloys is measured in carats (k), pure gold being designated as 24k.

Gold coins intended for circulation from 1526 into the 1930s were typically a standard 22k alloy called crown gold, for hardness. Modern collector/investment bullion coins (which do not require good mechanical wear properties) are typically 24k, although the American Gold Eagle and British gold sovereign continue to be made at 22k, on historical tradition. The Canadian Gold Maple Leaf coin contains the highest purity gold of any popular bullion coin, at 99.999% (.99999 fine). Several other 99.99% pure gold coins are currently available, including Australia's Gold Kangaroos (first appearing in 1986 as the Australian Gold Nugget, with the kangaroo theme appearing in 1989), the several coins of the Australian Lunar Calendar series, and the Austrian Philharmonic. In 2006, the U.S. Mint began production of the American Buffalo gold bullion coin also at 99.99% purity.

Today, gold has fallen out of use in coins made for general circulation.

Gold use in Jewelry

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Because of the softness of pure (24k) gold, it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties. Alloys with lower "k", typically 22k, 18k, 14k or 10k, contain higher percentages of copper, silver or other base metals in the alloy. Copper is the most commonly used base metal, yielding a redder metal. Eighteen carat gold containing 25% copper is found in antique and Russian jewelry and has a distinct, though not dominant, copper cast, creating rose gold. Fourteen carat gold-copper alloy is nearly identical in color to certain bronze alloys, and both may be used to produce police and other badges. Blue gold can be made by alloying with iron and purple gold can be made by alloying with aluminum, although rarely done except in specialized jewelry. Blue gold is more brittle and therefore more difficult to work with when making jewelry. Fourteen and eighteen carat gold alloys with silver alone appear greenish-yellow and are referred to as green gold. White gold alloys can be made with palladium or nickel. White 18 carat gold containing 17.3% nickel, 5.5% zinc and 2.2% copper is silver in appearance. Nickel is toxic, however, and its release from nickel white gold is controlled by legislation in Europe. Alternative white gold alloys are available based on palladium, silver and other white metals (World Gold Council), but the palladium alloys are more expensive than those using nickel. High-carat white gold alloys are far more resistant to corrosion than are either pure silver or sterling silver. The Japanese craft of Mokume-gane exploits the colour contrasts between laminated colored gold alloys to produce decorative wood-grain effects.

Other uses for Gold

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In medieval times, gold was often seen as beneficial for the health, in the belief that something that rare and beautiful could not be anything but healthy. Even some modern esotericists and forms of alternative medicine assign metallic gold a healing power. Some gold salts do have anti-inflammatory properties and are used as pharmaceuticals in the treatment of arthritis and other similar conditions. However, only salts and radioisotopes of gold are of pharmacological value, as elemental (metallic) gold is inert to all chemicals it encounters inside the body.

Gold leaf, flake or dust is used on and in some gourmet foodstuffs, notably sweets and drinks as decorative ingredient. Gold flake was used by the nobility in Medieval Europe as a decoration in foodstuffs and drinks, in the form of leafs, flakes or dust, either to demonstrate the host's wealth or in the belief that something that valuable and rare must be beneficial for one's health.
Gold solder is used for joining the components of gold jewelry by high-temperature hard soldering or brazing. If the work is to be of hallmarking quality, gold solder must match the carat weight of the work, and alloy formulae are manufactured in most industry-standard carat weights to color match yellow and white gold. Gold solder is usually made in at least three melting-point ranges referred to as Easy, Medium and Hard. By using the hard, high-melting point solder first, followed by solders with progressively lower melting points, goldsmiths can assemble complex items with several separate soldered joints.
Gold can be used in food and has the E Number 175. Goldwasser (German: "Goldwater") is a traditional herbal liqueur produced in Gdańsk, Poland and Schwabach, Germany and contains flakes of gold leaf. There are also some expensive (~$1000) cocktails which contain flakes of gold leaf. However, since metallic gold is inert to all body chemistry, it adds no taste nor has it any other nutritional effect and leaves the body unaltered.
Dentistry. Gold alloys are used in restorative dentistry, especially in tooth restorations, such as crowns and permanent bridges. The gold alloys' slight malleability facilitates the creation of a superior molar mating surface with other teeth and produces results that are generally more satisfactory than those produced by the creation of porcelain crowns. The use of gold crowns in more prominent teeth such as incisors is favored in some cultures and discouraged in others.
Gold can be made into thread and used in embroidery.
Gold is ductile and malleable, meaning it can be drawn into very thin wire and can be beaten into very thin sheets known as gold leaf.
Gold produces a deep, intense red color when used as a coloring agent in cranberry glass.
In photography, Gold toners are used to shift the color of silver bromide black and white prints towards brown or blue tones, or to increase their stability. Used on sepia-toned prints, gold toners produce red tones. Kodak publish formulae for several types of gold toners, which use gold as the chloride (Kodak, 2006).
Electronics. The concentration of free electrons in gold metal is 5.90×1022 cm-3. Gold is highly conductive to electricity, and has been used for electrical wiring in some high energy applications (silver is even more conductive per volume, but gold has the advantage of corrosion resistance). For example, gold electrical wires were used during some of the Manhattan Project's atomic experiments, but large high current silver wires were used in the calutron isotope separator magnets in the project.
- Though gold is attacked by free chlorine, its good conductivity and general resistance to oxidation and corrosion in other environments (including resistance to non-chlorinated acids) has led to its widespread industrial use in the electronic era as a thin layer coating electrical connectors of all kinds, thereby ensuring good connection. For example, gold is used in the connectors of the more expensive electronics cables, such as audio, video and USB cables. The benefit of using gold over other connector metals such as tin in these applications, is highly debated. Gold connectors are often criticized by audio-visual experts as unnecessary for most consumers and seen as simply a marketing ploy. However, the use of gold in other applications in electronic sliding contacts in highly humid or corrosive atmospheres, and in use for contacts with a very high failure cost (certain computers, communications equipment, spacecraft, jet aircraft engines) remains very common, and is unlikely to be replaced in the near future by any other metal.
- Besides sliding electrical contacts, gold is also used in electrical contacts because of its resistance to corrosion, electrical conductivity, ductility and lack of toxicity. Switch contacts are generally subjected to more intense corrosion stress than are sliding contacts.
Colloidal gold (Colloidal sols of gold nanoparticles) in water are intensely red-colored, and can be made with tightly-controlled particle sizes up to a few tens of nm across by reduction of gold chloride with citrate or ascorbate ions. Colloidal gold is used in research applications in medicine, biology and materials science. The technique of immunogold labeling exploits the ability of the gold particles to adsorb protein molecules onto their surfaces. Colloidal gold particles coated with specific antibodies can be used as probes for the presence and position of antigens on the surfaces of cells (Faulk and Taylor 1979). In ultrathin sections of tissues viewed by electron microscopy, the immuno gold labels appear as extremely dense round spots at the position of the antigen (Roth et al. 1980). Colloidal gold is also the form of gold used as gold paint on ceramics prior to firing.
Gold, or alloys of gold and palladium, are applied as conductive coating to biological specimens and other non-conducting materials such as plastics and glass to be viewed in a scanning electron microscope. The coating, which is usually applied by sputtering with an argon plasma, has a triple role in this application. Gold's very high electrical conductivity drains electrical charge to earth, and its very high density provides stopping power for electrons in the SEM's electron beam, helping to limit the depth to which the electron beam penetrates the specimen. This improves definition of the position and topography of the specimen surface and increases the spatial resolution of the image. Gold also produces a high output of secondary electrons when irradiated by an electron beam, and these low-energy electrons are the most commonly-used signal source used in the scanning electron microscope.
Many competitions, and honors, such as the Olympics and the Nobel Prize, award a gold medal to the winner.
As gold is a good reflector of both infrared and visible light, it is used for the protective coatings on many artificial satellites and in infrared protective faceplates in thermal protection suits and astronauts' helmets.
Gold is used as the reflective layer on some high-end CDs.
The isotope gold-198, (half-life: 2.7 days) is used in some cancer treatments and for treating other diseases.
Automobiles may use gold for heat insulation. McLaren F1 uses gold foil in the engine compartment.

Carat

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Carat (purity)

For other uses of the word "carat", see Carat.
The carat (abbreviation ct or K) is a measure of the purity of gold and platinum alloys. In the United States and Canada, the spelling karat is used, while the spelling carat is used to refer to the measure of mass for gemstones (see Carat (mass)). As a measure of purity, one carat is purity by mass:

where

X is the carat rating of the material,
Mg is the mass of pure gold or platinum in the material, and
Mm is the total mass of the material.
Therefore 24-carat gold is pure (100% Au w/w), 18-carat gold is 75% gold, 12-carat gold is 50% gold, and so forth.

Historically, in England the carat was divisible into four grains, and the grain was divisible into four quarts. For example, a gold alloy of fineness (that is, 99.2% purity) could have been described as being 23-carat, 3-grain, 1-quart gold.

The carat system is increasingly being complemented or superseded by the millesimal fineness system in which the purity of precious metals is denoted by parts per thousand of pure metal in the alloy.

The most common carats used for gold in bullion, jewellery making and by goldsmiths are:

24 carat (millesimal fineness 999)
22 carat (millesimal fineness 916)
20 carat (millesimal fineness 833)
18 carat (millesimal fineness 750)
15 carat (millesimal fineness 625)
14 carat (millesimal fineness 585)
10 carat (millesimal fineness 417)
9 carat (millesimal fineness 375)