Measurement System

• The purity of silver alloys is measured by millesimal fineness or the number of parts of silver per thousand. In other words, 800 silver would be 80% pure.
  
  

Coin Silver

• The United States government first started using coin silver in 1792, but at that time the purity could range from 750 to 950 per thousand. In 1906, the United States government passed a law defining coin silver as 900 parts per thousand.
  
  

Sterling Silver

• The term sterling silver was used to represent the legally defined purity of British silver coins. It is 925 parts per thousand with the rest usually made up of copper.
  
  

Contrast

• Sterling silver is usually slightly purer than coin silver (remember that old coin silver can vary in purity), and so may be worth a little more. The difference is fairly small, however, so they will often be identical in appearance.
  
  

Alloys

• Pure silver is soft and difficult to work with. By adding another metal to the alloy, the metal becomes harder and better able to hold a useful shape.