First controlled by prehistoric people, fire helped shape civilizations in many ways. Early people used fire to cook, keep warm, and protect themselves from wild animals. Fire also played a role in religious worship, hunting, and a number of other human activities. Not until the 18th century, however, did people understand the scientific nature of fire.

How fire burns

Ordinary fire comes about from a chemical process called oxidation. During oxidation, molecules of oxygen combine with the molecules of another substance. When oxygen combines with a substance such as coal, heat is produced. Heat is needed for a substance to combust, or burst into flames. When the oxygen is used up, the fire will go out. For example, when a candle's flame is covered, the flame will go out after it has used the remaining oxygen in the space beneath the cover.

The temperature at which something will catch fire and continue to burn is called its ignition point. Some substances need more heat to catch fire and burn than other substances do. Wood, for example, has a high ignition point; it needs a lot of heat to catch fire. Anyone trying to start a fire by rubbing two sticks together will find that it takes a long time to create enough heat to start a fire. To put out a fire, its temperature must be lowered. Firefighters spray water at fire, which lowers the fire's temperature and puts it out.

Fire often has flames, but fire can also burn without flames. A flame is created when heat forces gas from a burning substance and this gas combines with the oxygen in the air. When the gas from a candle's hot wax meets the air, for example, a flame is made. A substance that does not release gas when heated will burn without flames. One such substance is the form of coal called coke.

History of fire use

Humans did not learn how to start and control fires themselves until about 1.5 million years ago. Before then they got fire from trees struck by lightning or from volcanic eruptions. They tried to keep these naturally caused fires burning for as long as they could. This process continued for hundreds of thousands of years until early people finally learned how to start fires. They may have learned by accident as they noticed hot sparks flying when they chipped flint for their axes.

Eventually people learned to use fire for more than just keeping warm and cooking food. For example, they used fire to clear forests of smaller brush. By opening the forest, they were better able to spot the animals they were hunting. When people learned how to plant crops, they used fire to clear trees and brush from the land. This technique is called slash-and-burn cultivation. Slash-and-burn cultivation is still used in some tropical and temperate (mild) areas of the world. People use the ashes from the burned plants as fertilizer.

As people became more familiar with fire, it became more important to civilizations. People used fire to create pottery and, later, to make copper and tin from ore. In about 3500 BC people learned how to use fire to make bronze out of copper and tin. About 2,500 years later people used fire to obtain iron. As the centuries passed, people began to use fire to make such things as steam, rubber, and bricks. Today fire is an important part of many manufacturing processes.