Assaying, << uh SAY ihng, >> is any process used to determine the amount of one or more metals or valuable minerals contained in substances. These substances include ore samples, ingots (molded masses of metal), and alloys (mixtures of metals). Assaying tests show the purity of precious metals and whether mineral beds will be profitable to mine. The method used depends on the substance analyzed. The two general methods are (1) dry, or fire, assaying and (2) wet assaying.
Dry assaying.
In this method, the sample is melted to separate the metals it contains. For example, an ore sample containing gold and silver is mixed with lead oxide and other substances. This mixture is put into a crucible of fire clay and set inside a furnace to be melted. The hot lead oxide produces lead, which picks up the gold and silver and carries them to the bottom of the crucible. The impurities and other substances float to the top. The remaining alloy of lead, gold, and silver is placed in a porous cup and melted in a furnace. The lead turns to lead oxide, which evaporates. The gold and silver remain as a mixture called dore metal. The silver is dissolved in nitric acid to separate it from the gold.
Wet assaying.
Assayers use various chemicals to dissolve and separate the metals contained in a sample. The amount of a metal in a sample is determined by weighing the compound the metal has formed with the added chemicals. In some tests, the metals can be extracted from these chemical compounds and then weighed in pure form. Another wet assaying method involves measuring the quantity of some agent that reacts with the chemical dissolved out of the ore or alloy.
An assay office is a laboratory where assaying is performed. Many mining and metallurgical companies have them. The United States Mint runs an assay office in New York City that assays and purchases gold and silver bullion. Canada maintains a similar bureau under the supervision of the Royal Mint.
