Chad Anderson, age 15, of Great Falls, Mont., for his question:
HOW ARE ASSAYING TESTS MADE?
Assaying is the procedure used to find out the amount of one or more metals or valuable materials contained in substances. There are two general types of methods used: the dry or fire process and the wet process.
In the dry process, the sample to be assayed is melted to separate the metals it contains. An ore sample of gold and silver, as an example, is mixed with lead oxide and other substances. This mixture is then 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 and is removed. The gold and silver remain as a mixture called dore metal. The silver is dissolved in nitric acid to separate it from the gold.
In the wet process of assaying, assayers use certain 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. A large number of mining and metallurgical companies maintain such offices.
In the United States, the Bureau of the Mint operates an assay office in New York City that assays and purchases gold and silver bullion. Canada has a similar bureau under the Royal Mint's charge.
An alloy is a mixture of two or more metals, or of a metal and some other material. Most alloys contain a large amount of one metal, called the base metal, and then smaller amounts of one or more other metals.
Many pure metals are too soft, rust too easily, or have other mechanical or chemical disadvantages to be useful by themselves. But often these disadvantages can be overcome if the metals are combined with other metals into alloys.
The number of possible alloys is almost endless.
Alloys are made up of extremely tiny particles called grains. Each grain is a small crystal that is tilted differently from those next to it. Most alloys contain many grains.
The rate at which alloys are heated and cooled affects the grain size. Usually, the faster they are cooled from a high temperature, the smaller the grains will be.
The most desirable grain size depends on the intended use of the alloy. Heat treating and annealing, or heating followed by slow cooling, are important processes for making the desired kinds of alloys.