Does gold rust?

We're often asked if gold rusts or tarnishes. Discolouration or spots can sometimes cause concerns over the authenticity of gold bullion or old jewellery. Pure gold is a noble precious metal, and the least reactive of all metals. Simply put, pure gold does not rust, nor will it tarnish.

However, because almost all gold manufactured items are not 100% pure they can potentially rust and tarnish over time. 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.

Tarnished antique gold spoon and fork

What is rust?

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 which is why it does not rust.

Red spots on gold

As mentioned above, pure gold is nonreactive, it will not rust. However, that doesn't mean that gold coins and bars – made in 24 carat / 999.9 fashion – won't ever get any red spots.

These spots are indeed rusting, but they are not caused by the gold or damaging the gold. Instead, they are almost always caused by one of two things:

1. Copper
2. Silver

Pure gold or 24 carat gold is 999.9 parts gold per 1000. This means it is 99.99% pure, with 0.01 being made of another metal. That 0.01 part of the metal is typically copper or silver in a bullion coin, and in some cases these tiny particles can react to oxygen or other chemicals – causing a rust spot when located on the surface of a coin or bar.

An example of a red spot or rust spot, as discovered by our own Vault team.

Some bullion coins, such as the Krugerrand, US Eagle, and Sovereign, are also struck in 22 carat gold. As such they have 91.6% gold, with the remainder made up from other metals, with copper once again being a popular choice. This improves the durability of the coin, but can also make them more susceptible to rust/tarnish, and it isn't uncommon to see the colour of older Sovereigns vary considerably depending on how they have been stored over the years.

Alternatively, sometimes a similarly tiny amount of another metal can get onto the dies before a coin or bar is struck. The best refiners usually clean their minting equipment immaculately, but in periods of heavy demand they perhaps might reduce the regularity of cleaning to help speed up production. Maintaining a purely sterile environment is far easier these days, but still difficult in a busy refinery or coin mint.

There have been famous cases, such as Roubles discovered by the Russian Central Bank to have rust spots, but metallurgists quickly cleared up any fear of counterfeiting or dubious behaviour. Indeed, despite any red spots, your coin or bar is still worth its weight in gold. The gold itself hasn't been damaged, so it's still a solid bullion investment.

Experts do warn investors and collectors not to try and clean off any rust spots on their gold, as the cleaning is abrasive and will likely do damage to the item greater than the imperfection you are trying to remove.

What is tarnish?

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.

Does gold tarnish?

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.

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, impractical 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.

Cleaning gold

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. As mentioned above however, when it comes to gold coins (particularly collectable coins), it is generally advised to save yourself the tedious job, and not clean old coins.