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.
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 Nowadays, large scale energy is usually not produced by fires of burning wood, but has been replaced by hydrocarbon oil and coal, and in some cases nuclear energy or renewable energy sources. Wood burning remains a heat source in third world countries and where other sources of energy are unavailable.
There are four elements that maintain the combustion process, and the absence of any one of them will prevent a fire. The removal of these elements is the job of firefighters.
- Fuel may be removed from the site of a fire to curb its spread. In forestry, controlled burns are used to keep the available fuel supply low, so that intense fires do not occur. Gases that do not support combustion, such as nitrogen and carbon dioxide may be used to deprive an existing fire of its fuel.
- An oxidizer (usually oxygen) is needed to react with the fuel. Sand and foam may be used to stop the flow of oxygen to a fire. 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 fuels to be vaporized, which allows them to burn. Removal of enough heat prevents fuels from burning. Water is uniquely effective at removing heat; due to its high heat of vaporization, it removes a large amount of energy by simply boiling away.
- 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.
- Some materials are naturally fire-resistant. Either they are simply incapable of being oxidized , or they do not release enough energy in the process to sustain the fire.