Most metals are metallic grey or silvery white whereas gold is characteristically a metallic yellow colour, in other words gold-coloured. Caesium is also gold coloured. The gold colour seems related to relativistic effects of the outermost gold orbitals.

Small amounts of other metals alloyed with gold change the colour as well as mechanical properties such as hardness. White gold for jewellery is formed by mixing palladium, silver, or nickel with gold, although the result is green gold with certain proportions of silver. White gold is commonly used for wedding rings in the USA. Addition of some copper gives "rose gold", a soft pink colour. Remarkably other colours such as purple (a gold:aluminium alloy), blue (a gold:indium alloy) and even black (a gold:cobalt alloy) may be formed.

Gold is usually alloyed in jewellery to give it more strength, and the term carat describes the amount of gold present (24 carats is pure gold). It is estimated that all the gold in the world, so far refined, could be placed in a single cube 60 ft. on a side. It is metallic, with a yellow colour when in a mass, but when finely divided it may be black, ruby, or purple.

It is the most malleable and ductile metal; 1 ounce (28 g) of gold can be beaten out to 300 square feet. It is a soft metal and is usually alloyed to give it more strength. It is a good conductor of heat and electricity, and is unaffected by air and most reagents.

Gold is readily available commercially and its price changes day by day and is one of the most widely tracked commercial prices.

The most common gold compounds are auric chloride (AuCl 3 ) and chlorauric acid (HAuCl 4 ). A mixture of one part nitric acid with three of hydrochloric acid is called aqua regia (because it dissolved gold, the King of Metals). It is unaffected by air and most reagents. It is found free in nature and associated with quartz, pyrite and other minerals. Two thirds of the world's supply comes from South Africa, and 2/ 3 of USA production is from South Dakota and Nevada. Gold is found in sea water, but no effective economic process has been designed (yet) to extract it from this source.

Gold Assay

It is critical from the public's perspective that there is confidence in the claimed purity of any particular item made from precious metals. This confidence is provided by an 'assay' (test and assess) of the precious metal content of that item. It is impossible to tell the precious metal content of any item simply by looking at it. Precious metals such as old, silver, and platinum are too soft to use alone for making jewellery, cutlery and other goods. Quite properly they must be alloyed with base metals (which happen to be cheap) for manufacturing. The assay protects the consumer by ensuring sure that not too much base metal was used. It also safeguards responsible manufacturers by providing an independent assessment of quality and content that in which the public has confidence.

There has been an "Assay Office" at Sheffield in England since 1773 when local silversmiths won the right from Parliament to assay silver in Sheffield. The 1773 Act of Parliament appointed 30 local men as 'Guardians of the Standard of Wrought Plate in the Town of Sheffield' to supervise the work of the Office. In 1773 Sheffield already had an established tradition of fine silverware production and the number of Guardians who were also silversmiths was restricted to just ten to ensure that the Assay Office offered an independent and impartial service. This safeguard was to ensure the Office was run for the benefit of the consumer as well as the manufacturer. Once a piece was assayed, it was marked using a "hallmark", making the hallmark perhaps the oldest mark of consumer protection. The first UK Assay Office was and is based at Goldsmiths' Hall in London. It founded around 1300, and is from where the term "hallmarking" originates, meaning "marked in Goldsmiths' Hall".

While there are assay offices in the USA, there is no hallmarking scheme.



Cartoon by Nick D Kim ([Science and Ink], used by permission).