The fineness of a precious metal refers to the ratio of the primary metal to any additives or impurities. Most precious metal is used in the form of an alloy. Other metals are added to increase hardness, to make the metal more practical for use in such items as coins and jewelry, or to decrease the cost of the alloy. For example, copper is added to the precious metal silver to make a more durable alloy for use in coins, housewares and jewelry. A traditional measure for the fineness of silver in Britain is the mass of the amount of silver in 12 troy ounces of the resulting alloy. Britannia silver has a fineness of 11 troy ounces, 10 pennyweights, or about 95.83% silver, whereas sterling silver has a fineness of 11 troy ounces, 2 pennyweights, or about 92.5% silver. In other locations fineness is measured in units of mass per thousand. In the United States, silver coins often had a fineness of 900, meaning 90% silver and 10% copper. The fineness (or purity) of gold is commonly measured in carats or karats. The upper end of the scale is 24 carat for 100%. Thus 18 carat = 18/24% = 75%.
