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An overview of gold -- weights & measures; physical & chemical properties.

Weights & Measures

The basic unit of weight used in dealing with gold is the troy ounce. One troy ounce is equivalent to 20 troy pennyweights. In the jewelry industry, the common unit of measure is the pennyweight (dwt.) which is equivalent to 1.555 grams.

1 troy ounce = 31.1034 grams
1 troy ounce = 1.0971 ounce avoirdupois (U.S.)
1 troy ounce = 480 grains
1 kilogram = 32.15 troy ounces
1 metric ton (1,000 kilos) = 32,151 troy ounces
10 tolas (Indian Subcontinent) = 3.75 troy ounces
5 taels (Chinese) = 6.02 troy ounces

Standard Bar Sizes

400 troy ounces (12.5 kilos)
32.15 troy ounces (1 kilo)
100 troy ounces (3.11 kilos)

Also, a wide variety of smaller-sized bars by various manufacturers are not deliverable to any exchange, but trade among makers in smaller markets.

Karat Gold Conversions

24-karat = .995 to .9999 pure (fine) gold
22-karat = .916 pure (fine) gold
18-karat = .750 pure (fine) gold
14-karat = .583 pure (fine) gold
10-karat = .4167 pure (fine) gold

The degree of purity of native gold, bullion (bars or ingots of unrefined gold), and refined gold is stated in terms of gold content. "Fineness" defines gold content in parts per thousand. For example, a gold nugget containing 885 parts of pure gold and 115 parts of other metals, such as silver and copper, would be considered 885-fine. "Karat" indicates the proportion of solid gold in an alloy based on a total of 24 parts. Thus, 14-karat (14K) gold indicates a composition of 14 parts of gold and 10 parts of other metals. Incidentally, 14K gold is commonly used in jewelry manufacture. "Karat" should not be confused with "carat," a unit of weight used for precious stones.

Geological & Mining Background

Gold is relatively scarce in the earth, but it occurs in many different kinds of rocks and in many different geological environments. Though scarce, gold is concentrated by geologic processes to form commercial deposits of two principal types: lode (primary) deposits and placer (secondary) deposits.

Lode deposits are the targets for the "hardrock" prospector seeking gold at the site of its deposition from mineralizing solutions. Geologists have proposed various hypotheses to explain the source of solutions from which mineral constituents are precipitated in lode deposits.

One widely accepted hypothesis proposes that many gold deposits, especially those found in volcanic and sedimentary rocks, formed from circulating ground waters driven by heat from bodies of magma (molten rock) intruded into the Earth's crust within about 2 to 5 miles of the surface. Active geothermal systems, which are exploited in parts of the United States for natural hot water and steam, provide a modern analog for these gold-depositing systems. Most of the water in geothermal systems originates as rainfall, which moves downward through fractures and permeable beds in cooler parts of the crust and is drawn laterally into areas heated by magma, where it is driven upward through fractures. As the water is superheated, it dissolves metals from the surrounding rocks. When the heated waters reach cooler rocks at shallower depths, metallic minerals precipitate to form veins or blanket-like ore bodies.

Two thirds of the world's supply comes from South Africa, and 2/3 of USA production is from South Dakota and Nevada. Other main mining areas are Canada, Russia, Australia and China. The metal is recovered from its ores by cyaniding, amalgamating, and smelting processes. Refining is also frequently done by electrolysis.

Occurrence of gold in the earth's crust = .005 parts per million

Estimated total mine production = 160,000+ tonnes since gold was first discovered
[Date of first gold coin = approx. 560 B.C. (minted by Croesus of Lydia)]

A metric tonne (equals 1,000kg) of gold has a volume of 51,762 cubic centimeters, equivalent to a cube with sides of only 37.27cm -- approx. only 1' 3'' !.

In fact, the total amount of gold in the world is a surprisingly small quantity. Here's how you can calculate the volume of the total quantity that has ever been mined.

The annual worldwide production of gold is approximately 80 million troy ounces per year. There are 32.15 troy ounces in a kilogram. Gold has a specific gravity of 19.3, meaning that it is 19.3 times heavier than water. So gold weighs 19.3 kilograms per liter. A liter is a cube that measures 10 centimeters (about 4 inches) on a side. Therefore, the world produces a cube of gold that is about 5.1 meters (about 17 feet) on each side every year. In other words, all of the gold produced worldwide in one year could approximately fit into an average living room and garage!

This annual production weighs 2,572,000 kilograms. A recent spot price for gold was $740 U.S. -- using that number, all of the gold produced in a year is worth $59.2 billion.

Similarly, it is estimated that all the gold ever mined in the world (160,000 tonnes as of 2007), could be placed in a single cube roughly 60 ft. on a side, with a value of $3.68 trillion.

Gold occurs in sea water to the extent of 0.1 to 2 mg/ton, depending on the location where the sample is taken. No method has been found for recovering gold from sea water profitably.

The name originates from the Old English Anglo-Saxon word 'geolo' meaning yellow. The Symbol Origin is from the Latin word 'Aurum' meaning "Glowing Dawn".


Chemical & Physical Properties

Chemical symbol for gold = Au
Atomic number = 79 (79 protons and electrons; 118 neutrons)
Number of naturally occurring isotopes = 1 (stable) (70 total possible)
Atomic radius = 0.1442 nm
Atomic mass = 196.96657 amu
Density = 19.3 g/cubic cm
Specific gravity = 19.32 (Gold is one of the densest of all the chemical elements, compare to 7.87 for steel, 14.0 for mercury and 11.4 for lead.)
Melting point = 1064.43 ºC degrees
Boiling point (liquid to gaseous state) = 2807 °C
Crystal structure: FCC (cubic)
Thermal conductivity = 310 W m-1 K-1
Electrical resistivity = 0.022 micro-ohm m at 20°C
Youngs modulus = 79 GPa
Hardness = 2.5 (Mohs), 25 Hv (Vickers)
Tensile stress = 124 MPa

Gold is extremely malleable (the extent to which a material can undergo deformation in compression before failure). In the annealed state it can be hammered cold into a translucent wafer 0.000013 cm thick. One ounce of gold can be beaten into a sheet covering over 9 square metres and 0.000018 cm thick.

Gold is extraordinarily ductile (degree of extension which takes place before failure of a material in tension). One ounce can be drawn into 80 km (50 miles) of thin gold wire (5 microns diameter) to make electrical contacts.

The ability of gold to efficiently transfer heat and electricity is bettered only by copper and silver, making it indispensable in electronics for semi-conductors and connectors in computer technology -- especially because gold is at the top of the series indicating its high corrosion resistance. In practise, gold dissolves only in aqua regia (a mixture of hydrochloric and nitric acids) and in sodium- or potassium- cyanide. The latter solvent is the basis for the cyanide process that is used to recover gold from low-grade ore. In everyday use gold does not tarnish.

Electronic Uses

The resistance to oxidation of gold has led to its widespread use as thin layers electroplated on the surface of electrical connectors to ensure a good connection. Gold-plated connectors are an integral part of plugs and sockets for cable terminations, integrated circuit sockets and printed circuit boards. In general, the more sophisticated the equipment and the greater the need for reliability, the greater the requirement to exploit the advantages of gold as a material. This means that in telecommunications, computers, automotive electronics and defense systems where safety is critical, gold is indispensable. Gold performs critical functions in computers, communications equipment, spacecraft, jet aircraft engines, and a host of other products.

Other Uses

Gold's efficiency as a reflector of heat  and infra-red radiation has led to liquid gold being used to reduce heat transmissions from aircraft engines and in the United States' Apollo space program. In the latter, reflective gold-coated plastic film was wrapped around parts of the lunar landing module and the moon buggy to protect sensitive parts from solar radiation.

Recordable compact discs depend upon gold's high reflectivity. Disc players require a high reflectivity of the laser beam and the only possible materials are gold, silver or copper. With the latter two metals, there is the inherent risk of tarnishing and oxidation. This is not a concern with gold. The gold surface is deposited on the recordable CDs by a process called sputtering.

Jewelry consumes around 75% of all gold produced. Gold for jewelry can be given a range of hues depending on the metal with which is alloyed (white, red, blue, green etc.).

· White gold (an alloy of gold with platinum, palladium, nickel, and/or zinc) serves as a substitute for platinum.

· Green gold (a gold/silver alloy) is used in specialized jewelry while gold alloys with copper (reddish color) are more widely used for that purpose (rose gold).

· Colloidal gold is added to glass to colour it red or purple.

· Gold can be made into thread and used in embroidery.

· Gold is used in restorative dentistry especially in tooth restorations such as crowns and permanent bridges as its slight malleability makes a superior molar mating surface to other teeth, unlike a harder ceramic crown.

· Colloidal gold (a gold nanoparticle) is an intensely colored solution that is currently studied in many labs for medical, biological and other applications. It is also the form used as gold paint on ceramics prior to firing.

· Chlorauric acid is used in photography for toning the silver image.

· Gold(III) chloride is used as a catalyst in organic chemistry. It is also the usual starting point for making other gold compounds.

· Gold is used as a coating enabling biological material to be viewed under a scanning electron microscope.

· Many competitions and honors, such as the Olympics and the Nobel Prize, award a gold medal to the winner.

· Since it is a good reflector of both infrared and visible light, it is used for the protective coatings on many artificial satellites and on astronauts' helmets to prevent blindness from the sun.

· Gold is applied as a thin film on the windows of large buildings to reflect the heat of the Sun's rays.

· Gold flake is used on and in some gourmet sweets and drinks. Called varak (or varaq) in India. Having no reactivity it adds no taste but is taken as a delicacy. Some use it as an excuse to create super-expensive delicacies ($1,000 cocktails). For similar reasons, it is also used as the basis for some superstitious, over-the-top health claims. Only the salts and radioisotopes (mentioned above) have any evidence of medicinal value.

Medical Uses

The claims for the medical benefits of gold date back many of thousands of years. Many ancient cultures, such as those in India and Egypt used gold-based medicinal preparations. Early applications of gold in China were in the treatment of ailments such as smallpox, skin ulcers and measles. In Japan, tradition suggests thin gold-foils placed into tea, sake and food are beneficial to health.

Apart from the obvious use of gold alloys in dental restorations, there are also a number of direct applications of gold in medical devices. As with dental applications, these are related to the excellent biocompatibility of gold as a material. Applications include wires for pacemakers and gold plated stents used in the treatment of heart disease.

Gold possesses a high degree of resistance to bacterial colonization and because of this it is the material of choice for implants that are at risk of infection, such as the inner ear. Gold has a long tradition of use in this application and is considered a very valuable metal in microsurgery of the ear.

Gold and gold compounds have also historically been used in drugs for the treatment of a wide range of ailments. This use of gold compounds in medicine is called chrysotherapy. The Frenchman Jacques Forestier reported in 1929 that the use of gold complexes was beneficial in the treatment of arthritis. Later work after the Second World War demonstrated conclusively that gold drugs are effective in treating rheumatoid arthritis patients. Two of the most commonly referred to gold compounds in such treatments are Myocrisin and Auranofin.

The isotope gold-198, (half-life: 2.7 days) is used in some cancer treatments and for treating other diseases.

Finally, it should also be borne in mind that without the reliability that gold provides in electronic components within medical devices such as pacemakers and ventilators,  many medical treatments would not be as effective as they are today.


The following reactions have now been shown to be effectively catalysed by supported gold catalysts:

· Carbon monoxide oxidation, including selective oxidation in a hydrogen stream
· Catalytic combustion of hydrocarbons
· Hydrochlorination of ethyne
· Hydrogen + oxygen reaction to give hydrogen peroxide
· Hydrogen sulphide and sulfur dioxide removal
· Oxidation of glucose to gluconic acid
· Oxidative decomposition of dioxins
· Oxidative removal of mercury
· Ozone decomposition
· Reduction of NOx with propene, carbon monoxide or hydrogen
· Selective oxidation, e.g. epoxidation of olefins,
· Selective hydrogenation, e.g. of alkynes and dienes to mono-olefins
· Vinyl acetate synthesis from ethene, acetic acid and oxygen
· Water Gas Shift

ABC's of Gold Investing p.138, V is for; www.abcs-of-gold-investing.com/
Crescent Chemical Company,
CRC Handbook of Chemistry & Physics,General Interest Publications of the U.S.G.S. - pubs.usgs.gov/gip/prospect1/goldgip.html
Lange's Handbook of Chemistry,
Los Alamos National Laboratory,
World Gold Council; www.gold.org

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