What makes silver tarnish?

Silver tarnish -  it is the sulphur in the air, which often comes from coal-gas used for cooking and heating, that causes silver to tarnish or blacken. Silver combines with sulphur to form the black silver sulphide sometimes found on forks and spoons which have been in contact with egg yolk.
Silver is a precious metal which the Greeks called shining. In spite of its tendency to tarnish silver has long been used for the manufac­ture of coins, jewellery and other articles of value, on account of its comparative rarity, brilliant white colour and resistance to corrosion.
Sterling or "solid" silver is an alloy containing 92-5 per cent silver with 7-5 per cent copper to harden it. This alloy can be drawn into wire finer than a human hair, or beaten into sheets thin enough to be stacked 100,000 high in a one-inch pile. It is used for standard hall-marked silverware in the United States, Britain and the Commonwealth. But cupro-nickel, an alloy of copper and nickel, has largely replaced silver in the world's coinage.
Silver plate is a thin layer of silver put on to another metal. Various alloys and compounds containing silver serve many com­mercial purposes, especially in the photographic and electrical in­dustries. Silver is also very useful as an excellent conductor of elec­tricity.

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