Quick fact file: Gold
Atomic number: 79
Atomic weight: 196.967
Melting point: 1,064 degrees C
Specific gravity: 19.3 when pure
Gold is 19.3 times heavier than an equal volume of water
It’s rare, soft and unreactive
What is gold?
Gold is a rare metal. It has the chemical symbol Au, named after Aurora, the Roman goddess of the dawn. The purity of gold is described by its ‘fineness’ in parts per 1,000 or by the carat scale which is parts per 24. The word ‘carat’ derives from the Italian carato, Arabic qirat or Greek keration, all meaning the fruit of the carob tree. Ancient traders used carob seeds as the means to balance the scales in oriental bazaars. Pure gold is 24 carat or 1,000 fine.
The price of gold and other precious metals is quoted in terms of troy ounces. The term ‘troy’ is derived from Troyes, France, a major trading city of the middle ages. One troy ounce equals 31.1 grams.
What are the properties of gold?
Pure gold is soft and wears easily. It is often mixed with other harder metals. A mixture of metals is called an alloy.
Gold is very unreactive. This means it is resistant to corrosion and tarnishing. That is why a gold nugget can be buried in the ground for thousands of years and still come up looking shiny.
Gold is malleable (easily shaped) and ductile (can be drawn into very thin wire). A square lump of gold about the size of your thumb nail would weigh an ounce. That ounce of gold can be flattened into a sheet so thin that it would be thinner than a piece of tissue paper, and light could pass through it. It would cover an area about the size of a small bedroom. The same lump of gold can be drawn into a piece of wire 80 km long. That’s long enough to go around a rugby field 23 times.
Where is gold found?
Gold is found as a free metal in nature. It can be found as nuggets or bound up with rock and too small to see with the naked eye. It is sometimes found in association with other metals.
The Martha and Favona mines in Waihi are hard rock mines. This means the gold is mixed within the rock and too small to be seen. Ore averages a grade of three grams of gold per tonne of rock at Martha and around 10 grams of gold per tonne at Favona. Up to 25% of the precious metal produced in Waihi is gold.
What has gold been used for in the past?
Gold has been used for ornaments and decoration and as money for over 5,000 years. Gold leaf has been used for the decoration of tombs and statues, cathedrals and temples, fine books, and picture frames since Egyptian times. Many Egyptian burial cases, including King Tutankhamun’s (1352 BC), were gilded with beaten gold. Gold leaf is still often preferred for adorning the domes or ceilings of buildings (such as the Metropolitan Opera House in New York) because its resistance to corrosion means that it will outlast paint by many years.
Gold was made into jewellery long before it was used as currency. The earliest gold jewellery dates from the Sumeric civilisation around 3,000BC. The jewellery was worn by both men and women. Goldsmith’s skills that were understood and mastered at that time are still used today, although some of the techniques have been lost. Gold wedding rings, used in marriage ceremonies since the 9th century, date back to the ancient Egyptian times. The ring is placed on the third finger of the left hand because it was believed that this finger carried an artery leading directly to the heart.
Gold uses today
Gold reflective glass
Gold reflects heat. This property, and the fact that it is so malleable, means that it is used a lot to coat glass with a thin film which lets through light but not heat. One ounce of gold is enough to cover 93 square metres (approximately 1000 square feet) of glass. Gold-covered glass reflects heat off the outside of a building in summer, and helps to retain warmth in winter by reflecting the heat inside back into the room. The use of reflective glass has reduced cooling and heating costs by as much as 40% in some buildings.
Gold is a very good conductor of electricity. It is able to be drawn out into very thin wires, and it doesn’t corrode or tarnish at high or low temperatures. This means it is a great material for use in complex and small electronic applications.
Gold plating on contacts for switches, relays and connectors accounts for most of the 120 to 140 tonnes of gold required each year by the electronics industry. It is used in circuits in calculators, television sets, computers, telephones and lots of other products. Gold is also really important in satellites and computers.
Satellites and communications
Gold is used in satellites as part of their electronic circuits, and as a heat shield. We have come to rely on satellites for many things. They provide information about weather patterns around the world and help track the paths of storms. Satellites take photos of agricultural changes, such as diseases affecting crops, to predict production each year and help countries plan what they grow for food or trade. Satellites carry a large percentage of New Zealand’s international phone calls. Television companies transmit news, sports and entertainment programmes direct to viewers via satellite. Ships and aircraft use satellite tracking to determine their position. Fishermen trampers, and surveyors use global positioning systems (GPS) to accurately establish their location.
The space programme depends on the clean, non-corroding electrical performance of gold. Because the metal reflects heat it is used to protect astronauts, satellites and critical electronic components from damage by hazardous x-rays and solar radiation found in space.
Compounds of gold were first used experimentally in 1927 in the treatment of rheumatoid arthritis and are still used today. Radioactive gold is used to treat several types of cancer. Gold leaf is used to treat chronic ulcers and is used in surgery to patch damaged blood vessels, nerves, bones and membranes.
Each year dentists in the United States alone, use about 30 tonnes of gold. Gold alloys are used for crowns, bridges, gold inlays and dentures because of their high resistance to corrosion and tarnish.