What is Tarnish?
Tarnish Causes Discolouration
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 (a chemical compound of sulphur and oxygen) is a common air pollutant that comes from burning fossil fuels. It's also produced by most vehicles, domestic boilers and industrial machinery that burns fuel with a high sulphur content. Burning fossil fuels provides us with about 80% of the energy we use.
Fossil fuels which include oil, coal and gas are formed from the fossilised remains of plants and animals that lived millions of years ago. That's how the name came about. It's also produced naturally by active volcanoes.
The process of tarnish which is also known as oxidation affects copper, brass, aluminum and several other metals. Most people will have seen this discolouration on the surface of sterling silver.
As the thickness of 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 any 'rotten egg' smells.
Other factors can speed up the process of tarnishing some of which include include humidity, perspiration, the latex in rubber gloves and wool. Paper, cardboard and foam packaging can also increase the rate at which which certain metals tarnish if any moisture happens to be present. For this reason silica gel sachets are often included in packaging because they absorb moisture.
Tarnish is a fine layer that covers the top surface of the metal but doesn't cause any long term damage. If anything it forms a seal which protects deeper layers from becoming tarnished. It can usually be quite easily removed.
A 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 may be 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.