Does gold rust?

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Pure gold is a noble precious metal, and the least reactive of all metals. It does not rust, nor will it tarnish.

However, because almost all gold manufactured items are not pure they will rust and tarnish. Pure gold is extremely malleable, and as such manufacturers (e.g. jewellers) generally alloy pure gold with harder metals. These harder metals will rust and/or tarnish and so will the resulting gold alloy.

The proportion and type of metal used in gold alloying greatly affects its resistance to corrosion and tarnish.

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Tarnished antique gold spoon and fork

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What is rust?
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Rust, or corrosion, is hydrated metal oxide. It is caused when a metal reacts with oxygen or water. This reaction is known as oxidising.

Metals that do not react to oxygen, such as pure gold, are termed 'noble'. Metals that will oxidise are known as base metals. Pure gold is the most noble of all metals.
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What is tarnish?
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Tarnishing is also a form of corrosion. The basic chemistry between the two phenomenon is the same. Tarnish, however, applies only to air dry surface corrosion.

Tarnish forms a thin film of oxidised metal over copper, brass, silver, aluminium and other similar metals. In some metals, such as silver, tarnish may be removed without seriously affecting the underlying metal.
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Does gold tarnish?
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The degree to which items will rust or tarnish is then dependent upon the alloy or purity of the gold. There are minimum legal requirements regarding the proportion of pure gold to other metals. In the UK, gold items must be over 37.5% pure gold, that is 9 carat or 375 fineness. Anything below this level in the UK cannot be called gold.

Carat, or karat in the US, measures gold purity in parts of twenty-four, where 24 carat is pure gold. Fineness measures purity in thousandths, where 999 is virtually pure gold.

In the US, 10 carat is the legal minimum and 14 carat is commonly used for gold jewellery. Along with the UK, in France, Austria, Portugal and Ireland, 9 carat is the minimum allowed. In Germany, Denmark and Greece it is even lower at 8 carat.

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Alloying gold with other noble metals such as platinum or palladium, as in white gold, will make it more rust resistant. Conversely, alloying with base metals like copper, as in rose gold, makes it less resistant to corrosion.

The skill of alloying gold is then to balance wearability, hardness and appearance, against softness and resistance to tarnish or corrosion.

Nine carat is affordable and very hard wearing, but lacks lustre and tarnishes easily. 18 carat is not so hard, but has a far greater lustre. Twenty-two carat is even brighter but is too soft for intricate items of jewellery. Pure gold, though totally rust resistant, is a soft metal, impracticable for virtually all general purposes. Instead pure gold is used as store of wealth, such as the gold bars and coins sold by companies such as ourselves.

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Cleaning gold
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If the contents of your jewellery box are looking a little tarnished it could be easy to return its sparkle. A simple wash in warm water with washing-up liquid may be all it takes. After washing, dry with a soft cloth and gently rub with a polishing cloth. A soft cotton swab can be used for hard to reach places.

Some people suggest using toothpaste or baking soda but to clean real gold a simple wash will usually suffice.