of a precious metal
refers to the ratio
of the primary metal
to any additives
. 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
, 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%.