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What is Tarnish?

heavily tarnished fork and spoon on a wooden surface

Tarnish Caused by Sulphur

Tarnish is a chemical reaction that causes a discolouration to the outer layers of certain metals.  It's caused primarily by sulphur-containing gases in the air.

Sulphur dioxide, which is a chemical compound of sulphur and oxygen, is a common air pollutant produced by burning fossil fuels.  It's also produced by most vehicles, domestic boilers and industrial machinery that burns fuel with high sulphur content.

The burning of fossil fuels provides about 80% of our energy.
An industrial power plant with tall towers blowing out smoke.  Photographed at dusk beneath a beautiful red skyThe name 'fossil' fuels came about because they formed from 'fossilised' remains of plants and animals.

The process of tarnishing, which is also known as oxidation, affects copper, brass, aluminium and several other metals.  Most people will have seen this discolouration on products made from sterling silver.

As the thickness of the tarnish increases, the colour changes.  What begins as a subtle yellowing, turns reddish brown, to blue, and finally black.  Interestingly the yolk of an egg which has a high sulphur content will cause a silver spoon to tarnish almost immediately.

Sulphur is usually the cause of 'rotten egg' smells.

Other factors that can speed up the process of tarnishing include humidity, perspiration, latex in rubber gloves, and wool.  Paper, cardboard and foam packaging can also increase the rate at which certain metals tarnish if moisture is present.  For this reason, silica gel sachets are often included in product packaging because they absorb moisture.

Tarnish is a fine layer that covers the top surface of the metal but doesn't cause any damage.  If anything, it forms a seal which protects deeper layers from becoming tarnished.  It's usually quite easy to remove.

silver polishing cloth can be used to remove tarnish from sterling silver jewellery.  Chemical dips are also available but should be used with caution.  Over time, these may leave marks that can be difficult or impossible to remove.

Pure silver is fairly resistant to tarnishing but many of the alloys used to improve strength and durability are not.  Copper which is the industry standard, does tarnish.

Article Photo

The photo in this article is the Cholla Power plant in Arizona.  Courtesy of Ron Wolf.  The image is clickable and redirects to the original photo.

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