Assaying, a process for determining the amount of a certain metal contained in an ore or alloy. Assaying is used to evaluate mineral deposits, to check the composition of ores and alloys purchased by industrial firms, and to determine the metallic content of coins and jewelry. Many commercial laboratories specialize in assay work.
In the broader meaning of the term, an assay can be an analysis for determining the amount of almost any substance in a sample. Plants can be assayed for their alkaloid content, for example. Assaying is also used to determine the concentrations of chemical elements in industrial chemicals. A bioassay is a chemical analysis used to evaluate the strength of a substance, such as a drug, by determining its effect on a living organism.
Assay reports of ores usually give the percentages of the desired metals contained in the ores. For gold and silver, however, the number of troy ounces of the metal that would be found in a ton of ore is given. The contents of most alloys also are given in percentages, but gold and silver again are exceptions. The gold content of an alloy can be stated in terms of fineness (the number of parts of gold in 1,000 parts of alloy) or in karats (the number of parts of gold in 24 parts of alloy). Thus, an ornament that is 75 per cent gold would be 750 fine or 18 karats. The purity of silver is expressed in fineness, but not in karats.
The first step in assaying is to obtain a representative sample of the mixture to be tested. In the case of a solid alloy, holes are drilled at predetermined points in the material, and the shavings are used as a sample. For a deposit of ore, the material is crushed and thoroughly mixed, and a sample of the required size is removed for testing.
Once the sample has been prepared, there are several methods of assaying it. The method chosen depends both on the type of material being assayed and the degree of accuracy required. The methods include:
used mainly on gold and silver ores. The ore is heated with chemicals called fluxes, which combine with most of the impurities in the ore to form a slag. The metal separates and settles to the bottom of the mixture. The slag is chipped off the top of the mixture after cooling. After further processing, the remaining metal is weighed, and its weight is compared with that of the original ore sample. This comparison is used to determine the weight of metal per ton of ore.
The sample is dissolved in appropriate solvents and the resulting solution is chemically analyzed. With some substances, the metal can be separated from the solution by electrolysis or by the addition of salts.
The sample is made to emit light by passing an electrical discharge through it. The amount of metal in the sample is determined by measuring the intensity of light emitted.
The sample is made to emit X rays by being irradiated with an intense X-ray beam. The amount of metal in the sample is related to the intensity of the X rays emitted.