Physical properties of gold

Gold (aurum, symbol Au) is the most malleable and ductile metal known, but it is also extremely durable and resistant to weathering and chemical action, as well as being thermally and electrically conductive. Gold melts at the relatively low temperature of 1,063 degrees Celsius.

Specific properties of investment gold and gold ingots

The hardness of gold on a scale from 1 to 10 (where 1 represents the softest substances, such as talc, graphite or lead, and 10 are diamonds) is about 2.5 to 3, which is the hardness between rock salt and copper.

The density of admixtures for gold is 15.5–19.3 g/cm³, which is almost twice as high as the density of lead and ten times higher than that of ice.

Malleability. One ounce of gold can be beaten into such a thin, translucent leaf that it can cover over 100 square feet and is only as thick as five millionths of an inch. It is so thin that we would need 1,000 wafers to reach the thickness of one sheet of paper. One ounce of gold can be stretched into a wire that is 50 miles long. With this ounce we would cover a wire thread measuring 1,000 miles.

Gold is a chemically very resistant metal. Of the common inorganic acids, it only reacts with aqua regia, which is a mixture of hydrochloric acid and nitric acid, and in cyanide solutions.

A special case is the dissolution of gold in elemental mercury. When gold comes in contact with mercury, a special solution of gold in mercury – an amalgam – is very easily formed, which remains liquid even at a relatively high content of gold. By heating the amalgam to a temperature above 300 °C, the mercury evaporates and only pure gold remains.

Recently, chemists in Japan have also discovered a mixture of organic compounds that allegedly dissolve gold. At boiling point (82 °C) they form a saturated solution. By reducing the solution to temperature below 20 °C, the pure metal precipitates out of the solution. Gold is also soluble in an aqueous solution of potassium iodide and iodine. Especially thin layers of gold can easily be dissolved with this solution.

Gold-coated transparent plastic films have excellent reflectance of light and thermal (infra) beams. Gold foil can protect against leakage of body heat (e.g. in obstetrics or in extreme natural conditions).

Colour Pure gold is golden yellow. In alloys with other metals, it takes on different shades – ranging from white to red. Mixtures with silver have rather lighter, even greenish shades, while mixtures with higher copper content have a reddish colour. Lately, white gold, which is a gold alloy with nickel and palladium, is far the most popular among items of jewellery. Very thin layers of gold have a blue-green colour.