Alan Shirley, age 15, of Tacoma, Wash., for his question:
WHERE DO WE GET TIN?
Tin is a bluish white metallic element that is very useful in our present day world. Tin resists corrosion, and ordinary atmospheric conditions or weak acids found in food do not affect it, making it useful as a container for canned foods.
Tin comes from deposits of the mineral ore cassiterite. Malaysia, Bolivia, Russia, Thailand and Indonesia together mine about three fourths of the worlds total. Among other tin mining countries are Australia, China, Great Britain, Nigeria and Zaire.
The United State has only small deposits of tin containing ore. Texas City, Texas, has the nations only tin smelter.
When you talk about the "tin cans" in which food can be purchased, you are talking about metal containers that are made of iron or steel, coated with a thin layer of tin. Tin alone bends easily and the iron or steel sheets give it strength. The tin in turn gives color and luster to the cans and keeps the metal underneath from rusting.
After the cassiterite ore leaves the mine, it goes through five important processes before pure tin is obtained.
Cassiterite contains arsenic and sulfur. These materials must be burned out of the ore in a furnace or a roasting machine. When copper sulfide is also contained in the ore, manufacturers draw it off by a process called leaching.
Then they heat the ore in a furnace for about six hours. This process is called smelting. The smelting causes slag or impurities lighter than the tin to float on the molten metal. The slag is drawn off and the metal is heated for about six hours more.
The tin, in liquid form, sinks to the bottom of the container and is drawn off into molds to cool as ingots. Later, the ingots are further purified by heat in another furnace.
The tin is run into a mold and the impurities, which have higher melting points, stay behind.
During a final processing of the tin, the liquid metal is stirred in order to let ail the gases escape, and then it is cooled. As the metal cools, the purest tin rises to the top of the kettles. Less pure tin sinks to the center.
The lower layer of metal during this final step of processing is so impure that it must be purified again before it can be used. The pure tin, taken from the top of the kettle, is cast into ingots and then sent to the rolling mill to be pressed into sheets for sale.
Tin is not ductile. This means that it cannot be drawn into thin wires. It has a melting point of about 449 degrees Fahrenheit.
Among the metallic elements, only lead has less ductility than tin. But tin is unusually malleable. It can be hammered into thin sheets, such as tin foil, and it can be used as a moisture proof wrapping.
This is so soft that it can be made into many different kinds of collapsible tubing.