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Sterling Silver
Trivia
Other Silver standards Fine silver is 99.9% silver or better. This grade of silver is used to
make bullion bars for international commodities trading. In the modern world
Fine Silver is understood to be too soft for general use.
Britannia silver is purer than sterling, at least 95.84% silver and up
to 4.16% copper. Its marks were
Britannia
and a lion's head in profile.The Britannia standard was a standard of plate obligatory in Britain between
1697 and
1720 to try to help
prevent British sterling silver coins from being melted to make plate. It became
an optional standard thereafter, and in the
United Kingdom and
Ireland is
now denoted by the millesimal fineness hallmark "958", with the symbol of
Britannia being applied optionally.Mexican silver
is also purer than sterling, usually 95% Silver and 5%
Copper.
Mexico
is the only country currently using silver in its circulating coinage, but these
coins are not minted from 95% "Mexican"
Silver.Coin silver
is 90% silver and 10% copper
as dictated by
United States
FTC
guidelines.Coin Silver is lower grade than sterling. The Coin Silver standard was established in the
US
in the
1820s.
This grade of silver was used in the silver coinage of the US as well as other
countries that used silver currency
minted
in the US, such as
Panama and the
Philippines.For industrial uses, for example in electronics, alloys such as
CuAg are favoured, which contains 72% silver and 28% copper and is known for
its
thermal conductivity.
Corrosion As the purity of the silver increases, the problem of
corrosion
or tarnishing lessens.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.However, silver is highly reactive toward
sulfur. Silver
tarnish is the formation of #333333
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.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.The #333333
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.
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. 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. 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.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). 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.
This article is licensed under the "GNU Free Documentation License". It uses material from the Wikipedia article "Sterling
Silver".
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