Sterling silver is often characterized as pure silver, when in fact it is a silver alloy. Pure silver, also called fine silver, is defined as 99.9% pure silver, but is too soft for most uses. Sterling silver contains 92.5% silver, with the remaining 7.5% being another metal, most often copper. The addition of the less costly metal - often called a base metal - serves to harden the resulting alloy so that the metal can be cast into shapes that it will retain with use.