Sterling silver

Sterling silver is the most common alloy of silver, used in for instance jewelry. It consists of 92.5 % silver and 7.5% of another metal, usually copper¹. In order to understand why people use sterling silver, we'll look into the properties of pure silver and sterling silver. After this, we'll take a look at the advantages and disadvantages of using sterling silver, and look at some exciting new discoveries.

When people hear the word "metal", they think of something strong, like an airplane or a bridge. However, pure metals are nothing like that. They are soft, with a Mohs hardness that is around 2 to 3 - almost soft enough to be scratched by a fingernail. Worse, they are quite ductile and malleable. The reason metals can be strong in practice is because they are alloys: combinations of multiple metals, or, rarely, of a metal and a non-metal. Steel, for instance, is iron alloyed with carbon.

Without going into too much detail, the reason that an alloy is stronger than the pure metal is the fact that the atoms are of a different size. In a pure metal, the atoms are organized in a nice, periodic lattice. Because everything is so nice and organized, moving one plane in this lattice with respect to another plane isn't very difficult: after the move, the microstructure didn't change. However, the layer is translated by the diameter of one atom. Of course, if we can do this once, we can do this many, many times. in effect pulling apart the metal. Now, imagine we have our normal metal, with occasionally, a bigger atom hidden in them, like a string of beads with one bigger bead. Now, the material around the bigger bead is strained, and moving this strained area involves relaxing the strain on one end and building it up on the other - a lot more difficult - not to mention the situation in which two big beads meet. Hence, the alloy is stronger, but also more difficult to bend and probably more brittle

Silver is no exception to this rule. With a hardness of 2.5². 99.9% pure silver, which is the highest purity commonly used, is too soft and ductile for use in jewelry. By alloying it with in particular copper, the hardness and strength is greatly improved. The standard percentage is 92.5% silver and 7.5% copper. This is useful for standardization; silver is fairly expensive - it is currently trading at $ 10 per ounce ³- and using a lower percentage saves money. By using a standard, customers are more easily guaranteed a certain percentage of precious metal. In the United States, 92.5 % is the lowest silver percentage that can still legally be called silver⁴; this means that sterling silver is the cheapest way of making a usable silver compound.

While alloying silver is a practical necessity, it is not without its disadvantages. In particular, the alloying metal (called the base metal), is less resistant to chemical than silver, and it will oxidize. This is sometimes counteracted by plating the sterling silver with pure silver, having the strength from its bulk material, but the chemical resistance from the surface. This does not work for items that wear, such as rings. Worse, silver has the property that it transports oxygen through its crystal lattice at elevated temperatures⁵. This means that the oxygen will penetrate deeply and causes deep oxidation of the copper. This is known as firescale⁵. It gives a difficult to remove, reddish purple stain to the surface. There are ways to prevent this from happening, but they are laborious.

In the 1990s, a solution to this problem was found by adding germanium to the mix, replacing part of the copper. On (patented) example of this is Argentium Silver⁶. This alloy does not tarnish as easily and does not exhibit firescale. Its physical properties are also more desirable. The main disadvantage appears to be the higher price of the alloy. First of all, germanium is expensive - in fact, it is typically even more expensive than silver⁷. Secondly, being patented, it can command a premium. At this point, it is about twice as expensive as regular silver. ⁸

In summary, sterling silver is the standard way of alloying silver for practical use. It is a good compromise between desired purity, hardness, durability and resistance to tarnish. That said, a modern replacement, using germanium, promises even better physical and chemical properties

Sources:

1. http://www.silverinstitute.org/silver_jewlery.php
2. http://www.mindat.org/min-3664.html
3. http://www.kitco.com/market/index.html
4. http://www.worldlygoodsbuttons.com/sterling_silver_information.htm
5. http://www.silversmithing.com/1fire.htm
6. http://www.argentiumsilver.info/
7. Gail Purvis Two-year time slot for solar III-Vs Review 19, Issue 4, (2006), Pages 25-28
8. http://www.argentiumsilverfindings.com/