Fire

For other uses, see fire (disambiguation).

A large bonfire.

Fire, a form of combustion, is a chemical reaction involving two or more chemicals where the molecules will readily react with each other to form additional chemicals. Linguistically, the word fire refers to the combination of the brilliant glow and large amount of heat released during a rapid, self-sustaining burning of combustible fuel. Fire is not a state of matter: rather, it is an exothermic oxidation process by which heat and light energy are given out. Fire starts when a fuel with adequate supply of oxygen or other oxidizer is subjected to enough heat, and it is sustained by the further release of heat energy in the process, as well as a continuous supply of oxygen and combustible fuel. A match or lighter is usually used to start a fire, which can then propagate to other combustibles because matches and lighters are designed with materials of low burning point. Fire is extinguished when one or more elements of heat, oxidizer, or fuel is removed; this concept is used in the fire triangle. The unburnable solid remains of a fire are termed ash.

Flames can conduct electricity, as a small portion of any fire is ionized. This has been demonstrated in the laboratory and also in large wildfires that occur in the vicinity of power lines. This ability to conduct electricity is due to its partially plasmaic nature.

Controlling fire

A log burning in a fireplace.

Controlling fire for the purposes of providing heat and light was one of mankind's first great achievements. The ability of fire to generate heat and light made possible migration to colder climates and enabled people to cook food — a decisive step in the perennial fight against disease. Smoke signals were an early use of fire for communication, and fire soon enabled advancements in metallurgy such as smelting and forging. Archaeology 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.

By the time of the Neolithic Revolution, during the introduction of grain based agriculture, people the world over used fire as a tool in landscape management. These fires were typically controlled burns or "cool fires", as opposed to uncontrolled "hot fires" that damage the soil. Such hot fires destroy plants and animals, and endanger communities. This is especially problem in the forests of today where traditional burning is prevented in order to encourage the growth of timber crops. Cool fires are generally conducted in the spring and fall. They clear undergrowth, burning up biomass that could trigger a hot fire should it get too dense. They provide a greater variety of environments, which encourages game and plant diversity. For humans, they make dense, impassable forests traversable.

File:Gas burner on stove.JPG

A lit gas burner on a stove.

The modern applications of fire are numerous. In its broadest sense, fire is used by nearly every human being on earth in a controlled setting every day. Owners of internal combustion vehicles use fire every time they drive. Thermal power stations provide electricity for a large percentage of humanity. However, fire is also used more directly; many nomadic peoples still use fire for cooking. It is also used for smoking, and as a weapon.

In fact, the use of fire for warfare has a long history up to the present day. Homer detailed its use by Greek commandoes who hid in a wooden horse to burn Troy during the Trojan war. Later the Byzantine fleet used Greek fire to attack ships and men. In the Vietnam War, the Americans dropped a modern version, napalm, from the air. More recently many villages were burned during the Rwandan Genocide. Aerial bombing of cities, including firebombing, using incendiary bombs was also frequently used during World War II. Molotov cocktails are cheap to construct and are in common use as well.

Fire and religion

Vulcan of ancient mythology, using fire for his forge

Fires and burning have often been used in religious rites and symbolism. One reason may be that the smoke of the fire disperses upwards, into what may be considered into the heavens, considered by many religions to be the home of their supernatural deities.

Fire is one of the four classical elements, as well as one of the five Chinese elements. In Hinduism fire is one of five sacred elements of which all living creatures are comprised and is considered an eternal witness essential to sacred religious ceremonies.

Fire is a symbol of Ahura Mazda, or God, of the Zoroastrian religion. A Zoroastrian church is known as a Fire Temple. Fire is also an important part of Calcination, the fire operation in the art of alchemy.

In Roman mythology, Vulcan is the god of fire. The analogue in Greek mythology is Hephaestus. In Greek mythology, Prometheus is the Titan chiefly honored for stealing fire from the gods in the stalk of a fennel plant and giving it to mortals for their use.

In Judaism fire also has great significance. Candles are lit to usher in holidays and to separate Shabbat from the rest of the week, as well as to remember the dead. Another important fire symbol is the Eternal Flame, which was a fire kept in the First and Second Temples and will always be kept burning.

In Christianity, fire is a symbol of the Holy Ghost and is often used in descriptions of Hell.

Fire as a power source

A coal power plant in the People's Republic of China.

Fire has supplied much of the energy which has helped humans since ancient times. Wood was a prehistoric fuel. The use of fossil fuels such as petroleum, natural gas and coal in power plants supplies the vast majority of the world's electricity today. The International Energy Agency states it is nearly 80%. Mexico is typical with thermal energy providing 76% of all energy .

The burning of wood is often the first association to the word "fire". It is common in a developing country for wood to be the primary energy source as well. For instance, in Africa, 65% of the energy used comes from the burning of biomass. What is less obvious is that wood burning power stations are less environmentally destructive than the fired oil power station in two major respects. E.ON UK is soon to build a 44 megawatt wood fired power station in the United Kingdom for these reasons, as reported in the Guardian newspaper in October 2005: first, wood is a renewable resource, especially if trees are grown in a modern, sustainable way. Second, the carbon dioxide emissions are negligible because no more carbon dioxide can be produced by burning than would be produced by the natural rotting of wood. Thus, over a 100-year timescale, the effect is carbon-neutral. It is also claimed that this power station will be more efficient than coal: accelerants can be used to spread fire faster or have it burn hotter.

The fire in a power station is used to heat water, creating steam that drives turbines. The turbines are linked to an electrical generator.

Uncontrolled fire

A forest fire.

The self-sustaining nature of fire makes it extremely dangerous if uncontrolled. Fire can consume structures and trees and can severely injure or kill living beings through burns or smoke inhalation. Structure fires can be started by cooking accidents, electrical faults, fuel leaks, the misuse of lighters and/or matches, and accidents involving candles and cigarettes. Fire can propagate rapidly to other structures, especially where proper building standards are not met. Purposely starting destructive fires constitutes arson and is a criminal offense in most jurisdictions. The destructive capacity of fire has led most municipalities to offer fire fighting services to quickly extinguish fires. Trained firefighters use fire trucks, fire hydrants, and an array of other equipment to combat the spread of fires. Municipal buildings such as schools and government buildings often conduct fire drills to inform and prepare citizens on how to react to a building fire. Outside of urban settings, wildfires can consume large areas of forest and brush and often damage nearby settlements.

There are many different classification systems used for uncontrolled fires; in Europe and Australasia six groups are used:

Flammable gas warning.

• Class A: Fires that involve flammable solids such as wood, cloth, rubber, paper, and some types of plastics.
• Class B: Fires that involve flammable liquids or liquifiable solids such as petrol/gasoline, oil, paint, some waxes & plastics, but not cooking fats or oils.
• Class C: Fires that involve flammable gases, such as natural gas, hydrogen, propane, butane.
• Class D: Fires that involve combustible metals, such as sodium, magnesium, and potassium.
• Shock Risk (formerly known as Class E): Fires that involve any of the materials found in Class A and B fires, but with the introduction of an electrical appliances, wiring, or other electrically energized objects in the vicinity of the fire, with a resultant electrical shock risk if a conductive agent is used to control the fire.
• Class F: Fires involving cooking fats and oils. The high temperature of the oils when on fire far exceeds that of other flammable liquids making normal extinguishing agents ineffective.

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A house on fire.

In the U.S., fires are generally classified into four groups: A, B, C, and D.

• Class A: Fires that involve wood, cloth, rubber, paper, and some types of plastics.
• Class B: Fires that involve gasoline, oil, paint, natural and propane gases, and flammable liquids, gases, and greases.
• Class C: Fires that involve any of the materials found in Class A and B fires, but with the introduction of an electrical appliances, wiring, or other electrically energized objects in the vicinity of the fire.
• Class D: Fires that involve combustible metals, such as sodium, magnesium, and potassium.

A fifth group, Class K, is sometimes added. It refers to fires involving large amounts of grease or oil. Although, by definition, Class K is a subclass of Class B, the special characteristics of these types of fires are considered important enough to recognize.

Science of fire

A blacksmith's fire, used primarily for forging iron.

A flame is an exothermic, self-sustaining, oxidizing chemical reaction producing energy and glowing gas, of which a very small portion is plasma. It consists of reacting gases emitting visible and infrared light, the frequency spectrum of which is dependent on the chemical composition of the burning elements and intermediate reaction products.

In many cases such as burning organic matter like wood or incomplete combustion of gas, incandescent solid particles, soot produces the familiar red-orange 'fire' color light. This light has a continuous spectrum. Complete combustion of gas has a dim blue color due to the emission of single wavelength radiations from various electron transitions in the excited molecules formed in the flame. Usually oxygen is involved, but hydrogen burning in chlorine produces a flame as well, producing toxic hydrogen chloride (HCl). Other possible combinations producing flames, amongst many more, are fluorine and hydrogen, or hydrazine and nitrogen tetroxide. Recent discoveries by the National Aeronautics and Space Administration (NASA) of the United States also has found that gravity plays a role. Modifying the gravity causes different flame types.

The glow of a flame is somewhat complex. Black-body radiation is emitted from soot, gas, and fuel particles, though the soot particles are too small to behave like perfect blackbodies. There is also 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. The dominant color in a flame changes with temperature. The photo of the forest fire is an excellent example of this variation. Near the ground, where most burning is occurring, it is white, the hottest color possible for organic material in general, or yellow. Above the yellow region, the color changes to orange, which is somewhat cooler, then red, which is cooler still. Above the red region, combustion no longer occurs, and the uncombusted carbon particles are visible as black smoke.

The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a general flame, such as in a candle in normal gravity conditions, making it yellow. In microgravity or zero gravity, such as an environment in outer space, convection no longer occurs, and the flame becomes spherical, with a tendency to become more blue and more efficient. There are several possible explanations for this difference, of which the most likely one given is that the cause is the hypothesis that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs. Experiments by NASA in microgravity reveal that diffusion flames in microgravity allow more soot to be completely oxidised after they are produced than diffusion flames on Earth, because of a series of mechanisms that behaved differently in microgravity when compared to normal gravity conditions. Premixed flames in microgravity burn at a much slower rate and more efficiently than even a candle on Earth, and last much longer. These discoveries have potential applications in applied science and industry, especially concerning fuel efficiency.

Fire ecology is the study of the interaction of living things with fire.

See also

A car on fire. Cars are sometimes set alight after having abandoned by owners who are averse to paying for their removal.

• Explosion, a different kind of combustion
• Rust
• Pyromania
• A list of articles relating to fire
• A list of articles relating to specific fires
• A list of articles relating to firefighting

References

General references

• Dave Reay, (2005). Climate Change Begins at Home. Palgrave Macmillan. ISBN 1403945780

External links

• What exactly is fire? (from The Straight Dope)
• Early human fire mastery revealed BBC article on archeological discoveries
• Parts of a candle flame
• Flames in microgravity
• Spiral flames in microgravity
• moebuildingcontrol.co.uk - UK Guidance on fire safety codes and fire engineering