Fire is a rapid, self-sustaining oxidation process of combustible gases ejected from a fuel. It starts by subjecting the fuel to heat or another energy source, e.g. a match or lighter, and is sustained by the further release of heat energy.
Controlling fire was one of humankind's first great achievements. It made possible migration to colder climes which otherwise would have remained out of reach for colonization. It also allowed for cooking food and using flame and heat to process materials. Archeology indicates that ancestors of modern humans such as Homo erectus seem to have been using controlled fire as early as some 790,000 years ago. The Cradle of Humankind site has evidence for controlled fire 1 million years ago.
Fires and burning have often been used in religious sacrifices, as the smoke of the fire disperses into the heavens. Fire is one of the four classical elements, as well as one of the five Chinese elements. In Christianity, fire is a symbol of the Holy Spirit, and is also often used in descriptions of Hell.
The burning of wood is often the first association to the word fire, and trees have since ancient times supplied much of the energy needed by humans. In the past, metal smelting and charcoal production consumed large quantities of wood for their production. Nowadays, large scale energy is usually not produced by fires of burning wood, but has been replaced by hydrocarbonoil and coal, and in some cases nuclear energy or renewable energy sources. Wood burning remains a heat source in many third world countries and where other sources of energy are unavailable.
The glow of a flame is somewhat complex, due to a mix of black-body radiation emitted from soot, gas, and fuel particles (though the soot particles are too small to behave like perfect blackbodies), and from photon emission by de-excited atoms and molecules in the gases. Much of the radiation is emitted in the visible and infrared bands. The color depends on temperature for the black-body radiation, and chemical makeup for the emission spectra.
An oxidizer (usually oxygen) is needed to react with the fuel. Sand, foam, or gases which do not support combustion (such as carbon dioxide) may be used to stop the flow of oxygen to a fire (smother the flames). In particularly violent fires, such as those of the Kuwaiti oil wells during the Gulf War, explosions may be used instead.
Heat is what allows liquid fuels to be vaporized, and solid fuels to undergo pyrolysis. Solids and liquids do not burn directly, they must first be converted into a gas through pyrolysis or vaporization. Removal of enough heat prevents fuels from burning. Water is particularly effective at removing heat due to its high specific heat capacity.
The chemical chain reaction is what perpetuates combustion; compounds such as halon extinguishing agents cause the chain reaction to be broken. The precise mechanism is not known, but it is thought that the halogen radicals end the reactions that support combustion.
If an uncontrolled fire is underway, the removal of these elements is the job of firefighters. Fire safety engineers provide fire departments and building designers with technical information about the best ways to remove enough of these four elements from modern buildings and industries in order to prevent fires.
