![]() Gold is a good conductor of heat and electricity and does not tarnish when it is exposed to the air, so it can be used to make electrical connectors and printed circuit boards. One karat is equal to one part in twenty-four, so an 18 karat gold ring contains 18 parts pure gold and 6 parts alloy material. The amount of gold in an alloy is measured with a unit called a karat. Gold alloys are used to make jewelry, decorative items, dental fillings and coins. Pure gold is soft and is usually alloyed with other metals, such as silver, copper, platinum or palladium, to increase its strength. One sheet of gold leaf can be as thin as 0.000127 millimeters, or about 400 times thinner than a human hair. Thin sheets of gold, known as gold leaf, are primarily used in arts and crafts for gilding. A single ounce of gold can be beaten into a sheet measuring roughly 5 meters on a side. Gold is the most malleable and ductile of all known metals. It has been estimated that all of the gold that has currently been refined could be placed in a cube measuring 20 meters on a side. There is roughly 1 milligram of gold dissolved in every ton of seawater, although extracting it currently costs more than the gold is worth. Gold is sometimes found free in nature but it is usually found in conjunction with silver, quartz (SiO 2), calcite (CaCO 3), lead, tellurium, zinc or copper. An attractive and highly valued metal, gold has been known for at least 5500 years.
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