Tina Harper, age 12, of Patterson, N.J., for her question:

WHERE IS TIN FOUND?

Tin is a bluish white metallic element that has been used by man since ancient times. The metal comes from deposits of the mineral ore cassiterite.

Cassiterite lies in veins in rock and in gravel. The world produces about 275,000 tons of tin each year, with only an extremely small percentage coming from the United States. The nation's only tin smelter is located in Texas City, Texas.

About three fourths of the world's total tin production comes from ore that has been mined in Malaysia, Bolivia, Russia, Thailand and Indonesia: Most of the rest comes from ore that has been mined in Australia, China, Great Britain, Nigeria and Zaire.

After cassiterite comes out of a mine, it goes through a series of processes before pure tin is produced.

Cassiterite contains arsenic and sulfur. These minerals must be burned out of the ore in a furnace or a roasting machine.

Next the ore is heated 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.

Tin in liquid form sinks to the bottom and is drawn off into molds to cool as ingots.

Cooled 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. The liquid tin is stirred in order to let all 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 is so impure that it must be purified again before it can be used.

The pure tin is cast into ingots and then sent to a rolling mill to be pressed into sheets for sale.

Tin will melt at about 449 degrees Fahrenheit, which is a rather low melting point. It is a lightweight metal and is extremely useful.

Tin cannot be drawn out into thin wires. Thus, scientists say that the metal is not ductible. But tin is extremely malleable, which means that it can be hammered into thin sheets, such as tin foil. Tin is also so soft that it can be made into many different kinds of collapsible tubing.

When you talk about "tin cans" you actually mean containers of iron or steel, coated with a thin layer of tin. Tin cans 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.

Tin resists corrosion. Ordinary atmospheric conditions or weak acids found in food do not affect tin. As a result, it proves useful as a container for canned foods.

Tin plate, or terneplate, is used for roofing. It is a sheet of iron coated with lead tin alloy.