The purity of many precious metals, including silver, is determined by the Millesimal Fineness System. This refers to a system of grading of precious metals by the amount of the pure metal present in the alloy which contains it. The purity is expressed as a percentage of the mass of the alloy. Pure silver may be stamped with the marking “999” or “.999 fine silver” to indicate that the silver is 99.9% pure. Such a system is necessary, due to the mixing of metals to form alloys.
Precious metals are mixed with other, less valuable metals in order to create qualities that are desirable for different applications in which the resulting alloys are used. In the case of silver, alloys are typically formed to obtain the quality of hardness, since pure silver is very soft and therefore, unsuitable for many of the applications for which it is used.
Silver is usually mixed with copper, which gives it additional hardness, but does not cause discoloration. Copper is the standard of the industry for silver alloy to be used in the creation of jewelry, silverware and many other common silver items.
An alloy made from 92.5% silver and 7.5% copper is called sterling silver. In the United States, this is the lowest amount of silver that can be formed into an alloy and legally identified and marketed as silver. This is not necessarily the case in other countries, where silver may commonly be mixed with metals other than copper.
Pure silver is often called “three nines fine” under the Millesimal Fineness system, while the silver used to make coins is only “one nine fine,” indicating that the alloy contains ten percent copper and ninety percent silver.
