Sterling silver
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Sterling silver is an alloy of silver containing least 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.
Fine silver (99.9% pure) is generally too soft for producing large functional objects, and in Sterling the silver is usually alloyed with copper to give strength whilst preserving the ductility of the silver and a high precious metal content. Other metals can replace the copper, a recent development being the use of Germanium to reduce firescale in manufacturing and to give a high resistance to tarnish.
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Origin of the term
The term "Sterling Silver", in reference to the .925 grade of silver, emerged in England by the 13th century.
The terms "sterling" and "pound sterling", seem to have acquired their meaning over a period of time, and from several convergent sources. The first mention is that of "sterilensis" in 1078, and by the thirteenth century (by the 1200's) the term sterling had appeared.
"Easterling" Theory
It seems quite possible that Sterling Silver may have been known first as "Easterling Silver". The term "Easterling Silver" was used to refer to the grade of silver that had originally been used as the local currency in an area of Germany, known as "The Easterling".
This "Easterling" consisted of five towns in the eastern part of Germany which banded together in the 12th century under the name of the Hanseatic League. The Hanseatic League proceeded to engage in considerable commerce with England. In payment for English cattle and grain, the League used their local currency. This currency was in the form of 92.5% silver coins. England soon learned that these coins, which they referred to as "the coins of the Easterlings", were of a reliably high quality and hardness.
King Henry II set about to adopt the alloy as the standard for English currency. He imported metal refiners from the Easterling and put them to work making silver coins for England. The silver these refiners produced came into usage as currency by 1158 in the form of what are now known as "Tealby Pennies", and was eventually adopted as a standard alloy throughout England. The original term of "Easterling Silver" was later abbreviated to "Sterling Silver".
Though the coin weights and silver purity changed considerably in the intervening time (reaching a low point before the reign of Elizabeth I, who reinstated Sterling Silver coinage for the first time since the early 14th century), the pound sterling was used as currency in England from the 12th century until the middle of the 20th century. Specifically this was in the silver coins of the British Empire -- Britain, British colonies and some former British colonies. This sterling coin silver is not to be confused with the Coin silver standard.
Sterling silver, while no longer used in circulating currency anywhere in the world, is still used for flatware, jewellery and plate, and is a grade of silver respected for both relatively high purity and sufficient hardness to form durable objects in daily use.
Mint Mark Theory
Another credible theory is that, since mint marks on Sterling Silver pennies have included a star and a starling, this may be the origin of the word -- as a simple corruption of, for example, "starling silver" with common reference to the circulating coin.
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 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.
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 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.
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.ja:スターリングシルバー