Sterling silver is an alloy of 92.5% silver (by weight) and 7.5% other metals. Sterling silver items often bears the stamp 925, indicating the alloy.
Sterling Silver Composition
The only requirement for sterling silver is that it contains 92.5% silver. Usually, the other metal is copper because it increases the strength and hardness of the metal and isn’t toxic. Other elements found in sterling silver include zinc, nickel, germanium, silicon, platinum, and boron.
Difference Between Silver and Sterling Silver
Sterling silver is an alloy of 92.5% silver and 7.5% other elements. Pure or fine silver is 99.9% silver with 0.1% trace elements.
Fine silver is a lustrous metal, with high electrical and thermal conductivity, and good corrosion resistance. However, it’s soft, malleable, and ductile. Sterling silver is harder and more durable. It’s used in jewelry, musical instruments, silverware, currency, electronics, and ornaments. Fine silver is used for mirrors. It’s also used in jewelry, but usually as a coating over sterling silver rather to make entire items.
Does Sterling Silver Tarnish?
Both sterling silver and fine silver tarnish. What this means is that the silver reacts with oxygen or sulfur dioxide in air, forming a black or gray oxide or sulfide layer. Sterling silver tarnishes much more readily than fine silver does. This is due to the other metals in the alloy reacting with air. Neither sterling silver nor fine silver tarnish quickly in oxygen, but if sulfur is present (from smog or another source), it blackens fairly quickly. Silver also tarnishes more quickly when exposed to salt, as from sea air or when silver is used as a salt shaker. The good news is that tarnish actually protects the underlying silver from further attack. Some people intentionally darken silver using sulfur for an antiqued look. Natural tarnish often enhances an object’s value and is considered aesthetically pleasing.
However, you may wish to clean tarnish from an object. Silver is soft, so mechanical polishing to remove tarnish may scratch the metal. A non-destructive method is to place a silver item on a piece of aluminum foil in pot of boiling water that contains baking soda or salt. The electrochemical reaction lifts removes the tarnish. Rinse the item with clean water following the process.
Another common question is whether silver rusts. Technically, the answer is no, because “rust” is the name given to iron oxide and sterling silver doesn’t contain iron. However, rust is an oxidation product, just like tarnish.
How to Test Sterling Silver
The “925” stamp indicates sterling silver. But, anyone with a metal stamp can fake the mark. The only way to know for sure whether sterling silver is real is to test it.
Testing depends on whether the aim is to determine whether an item contains enough silver to be sterling silver or whether the goal is to tell sterling silver from fine silver.
Fine Silver vs Sterling Silver
Sterling silver is distinguished from fine silver based on its density and analytical tests that can indicate the presence of other elements. The best test is x-ray fluorescence, which gives the elemental composition of the sample. The simplest test is the “acid test.” This is a destructive test, usually performed using a commercial kit. The test involves scratching off a bit of metal from an inconspicuous part of the item. Acid, together with colored indicators, react with the scrapings. The resulting color gives an estimate of the purity of the silver.
Sterling Silver vs Base Metals
The acid test is also the prime test for distinguishing between sterling silver and either silver-plated items or those containing no silver at all. For this test, an item may be deeply scratched with a file to determine whether the silver color results from plating.
There are other quick and easy tests for silver, but they aren’t reliable on their own and don’t indicate the purity of the metal. Striking silver with a mallet produces a sweet chime compared with the sound produced by most other metals. Silver is not magnetic, so if an item is strongly attracted to a rare earth magnet, it’s not silver. It contains a significant amount of magnetic metals (steel, nickel, iron, cobalt). But, if the item isn’t magnetic, it’s not a sure sign it’s silver. The usual alloying metal is copper, which is not magnetic. An alloy containing 50% silver and 50% copper isn’t sterling silver, but it’s not magnetic, either.
Test that do not work or are not recommended include exposing silver to vinegar (acetic acid in vinegar isn’t strong enough to produce a definitive color change), exposing silver to bleach (unless you want tarnished silver), or estimating whether an ice cube melts quickly (too subjective).
References
- Brumby, A.; et al. (2008). “Silver, Silver Compounds, and Silver Alloys”. Ullmann’s Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_107.pub2
- Cinamon, D. S. (2007). All About Antique Silver with International Hallmarks. AAA Publishing, San Bernardino, CA.
- Rainwater, Dorothy T.; Redfield, Judy (1998). The Encyclopedia of American Silver Manufacturers (4th ed.). Schiffer Publishing Ltd., Atglen, PA.