Sterling silver

Sterling silver is an alloy of silver containing 92.5% pure silver and 7.5% other metals, usually copper. The millesimal fineness is 925. Sterling silver objects are usually stamped with either the word "Sterling" or ".925", or the Lion Passant mark as part of the Hallmark.

Corrosion

As the purity of the silver increases, the problem of corrosion or tarnishing lessens.

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Chemically, silver is not very active — it does not react with oxygen or water at ordinary temperatures, so does not easily form a silver oxide. However, the other metal in the alloy, usually copper, may react with oxygen in the air.

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However, silver is highly reactive toward sulfur. Silver tarnish is the formation of black silver sulfide on the surface of the metal, and is caused by sulfur and sulfides which attack the surface of the silver. Because of this, the rate of discolouration of silver by tarnish is worse with higher levels of these airborne pollutants.

Related Topics:
Sulfur - Silver sulfide - Sulfide - Pollutant

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Eggs, which contain a considerable quantity of sulfur as a constituent of protein, tarnish silver extremely quickly. Small amounts of sulfide occur in the atmosphere naturally, but another major man-made source is hydrogen sulfide (H2S), which is added to natural gas used domestically. Hence a gas flame can also tarnish silver.

Related Topics:
Egg - Protein - Hydrogen sulfide - Natural gas

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The black silver sulfide (Ag2S) is among the most insoluble salts in aqueous solution, a property that is exploited for separating silver ions from other positive ions.

Related Topics:
Silver sulfide - Insoluble - Aqueous - Solution - Ion - Positive ion

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Removing tarnish

A very popular technique for removing tarnish involves the creation of an electrochemical cell. If the other metal is anodic relative to silver, then the silver in the sulfide will revert to metallic silver. The metal at the anode will be oxidized. The sulfide ions can travel to the anode via the electrolyte (solution). Metals that will work are iron, zinc, aluminium and magnesium. Aluminium foil is cheap.

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A typical procedure is to line a pyrex glass dish with alumimium foil so the bright side of the foil will contact the solution. Add one litre of water, and heat until near boiling.

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Add one tablespoon of sodium chloride and one tablespoon of sodium bicarbonate, and gently stir to dissolve. Ensure that the silverware has been washed in warm dishwashing detergents, and well rinsed in warm water to remove dirt and grease.

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Carefully add the silverware to the dish, ensuring that each item is in contact with the aluminium foil, and boil until the tarnish has disappeared, turning the silverware if necessary. The continuous boiling is required so that the aluminium oxide continually exposes the unreacted aluminium surface beneath, otherwise the reaction will not occur. The aluminium will gradually be converted to aluminium oxide. The hydrogen that is generated in the reaction will combine with the sulfide ions to produce some hydrogen sulfide gas (rotten egg gas).

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It should be noted that the process is not converting the silver back to the original hard, lustrous surface, but into a soft, white powder that can be removed easily by rubbing with a little bicarbonate of soda paste. The silver powder will be easier to remove than the tarnish would have been.

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