The term sterling silver is used to describe silver with a purity of 92.5% which has been alloyed with 7.5% of another metal. Fine silver has a purity of 99.9% which is far too soft for use in household goods and jewellery hence it is mixed with another metal in order to strengthen it. In doing this it's important that the silver retains its appeal and doesn't lose quality or value so care must be taken when deciding what type of alloy to use. Although copper is the most popular choice other metals can be just as suitable and the reason for using different metals is to try to improve resistance to tarnish and to eliminate the risk of firescale. Interestingly pure silver does not tarnish.
The metals which can be used are far too numerous to mention and can also include a variety of other additives. Although new alloys frequently appear, one has not yet emerged to replace copper completely as the industry standard.
People with an allergy to sterling silver in the vast majority of cases are actually allergic to nickel which is often used in the alloy. Hypoallergenic silver is free from nickel. When a high percentage of copper is present in an alloy it will cause the sterling silver to tarnish more quickly.
Most countries around the world have their own system for hallmarking sterling silver and the main purpose for this is to indicate purity, to identify the silversmith or company by whom it was made or or to note the date and/or location of manufacture. 925 which is the most widely recognized international hallmark confirms the purity of the silver is 92.5% with an alloy of 7.5% copper and/or another mineral.