Control over fire is one of the most important discoveries ever made by mankind, with applications ranging from cooking food to the internal combustion engine. It has also served as inspiration, been praised and cursed, preserved in music and myth, art and alchemy - and, very often, has been feared. This is because fire lacks one thing - an obvious off switch!

What Is Fire, Anyway?

Technically speaking, a fire is a combustive process that produces heat (and possibly light) during a series of complex exothermic¹ chemical reactions in which a combustible fuel is oxidised - and there are many more types of fire than most of us are aware of. However, those of us who aren't chemists, firefighters, or rocket scientists will usually deal with only one type of fire, the combustive reaction of an organic material that produces lots of heat and a visible flame², which we will deal with below. A fire can seem like a living thing, moving, growing, and duplicating itself if it gets the chance - and like a living thing, and a voracious one at that, it needs fuel, oxygen, and warmth to come into being and to survive.

Dramatis Personae

Three elements make up the so-called fire triangle. All three have to be present for the fire to burn - remove any of them, and congratulations, you've managed to extinguish it!

Oxidants: The most obvious oxidant, oxygen, is all around us, with molecular oxygen (O₂) making up roughly 21% of the atmosphere. Water also consists largely of oxygen, but it's tied up with hydrogen so we humans can't breathe it. When it's not reacting with hydrogen, oxygen gladly combines itself with anything and everything it can get its electrons on, and if nothing else is available, combines with itself to form O₂ molecules, which are what we breathe. However, it's ready to pounce on anything else as soon as the opportunity presents itself, oxidising the other material. This is a good thing - without oxidation, we wouldn't be able to use the oxygen in our bodies - but can also cause problems. Rust, for example, is oxidised iron.

Other common oxidants include ozone (O₃) and other oxygen compounds such as nitrous oxide(N₂O) and hydrogen peroxide (H₂O₂), as well as halogens like fluorine, chlorine, bromine, iodine, and their compounds.

Fuel: Carbon is found in great quantities in every living thing, and in quite a few dead ones! Everything organic³ is, per definition, carbon-based. That includes oil, methane, butane, propane, wood, straw, coal... The things we're used to burning. There are millions of known organic compounds, with millions more theoretically possible, and carbon is the fourth most common element in the Universe. Even alone, it can take very different forms, from diamonds to graphite to buckyballs. Carbon is an ideal basis for more complex molecules, because it has four electrons available for sharing with other atoms, forming strong covalent⁴ bonds. Because these bonds are so strong, it takes energy to make them let go - but when they do let go, the molecules recombine with others, releasing more energy in the process⁵. Not all fuels are organic, of course - hydrogen gas, for example, is highly flammable - but organic fuels are the kind you're most likely to encounter day-to-day.

Heat: This is, of course, the method of choice for adding that extra bit of energy to the mix. Throwing your campfire off the top of a cliff won't make it light! Heat gets all those molecules jumping around and anxious to share electrons, ready for action.

A Brief Plot Synopsis

Whether your fuel is solid, like coal, liquid, like oil, or a gas already, what actually burns will always be vapour. The initial heat is required to make whatever is vapourise, and to make it ignite. Every substance has a different flash point, at which it will catch fire, and slightly after that, the fire point, when it produces enough heat to make more vapour and sustain the reaction.

The process of combustion takes the materials involved apart atom by atom and reassembles them into something totally different - carbon dioxide, carbon monoxide, water, a bit of plasma and a handful of other compounds formed with the other gasses in the atmosphere and whatever else was in the fuel. Since the average fire isn't hot enough to burn everything completely, there will also be some pure carbon in the form of ash or soot.

Oddly enough, what makes fire so valuable to us is the 'waste' energy emitted in the form of heat and light. Control over fire allowed mankind, for the first time, to stockpile heat and light, ultimately the sun's energy, stored in plants through photosynthesis. Really, it's just saving up the summer sunshine for when the sun isn't around! It's also hotter than mere sunlight, making it suitable for a variety of tasks like baking bread, smelting ore, or making clay pots. And while we've perfected the art of making fire so that it's now available at the push of a button, the basic tools required are really quite simple and can even be made without access to other tools to make them with.

Various Endings

Before you even think about starting a fire - for whatever reason - you should know how to put it out. In theory, this is quite simple, because all you have to do is take away one of the three components: the fuel, the oxygen, or the heat. Most likely, you've already used each of those three options - but did you know what it was you were doing, and why it works?

Removing The Heat

If there's not enough heat to sustain the reaction, it will stop, without your having to interfere either with the fuel or with the oxygen. And after all, the heat is what threatens us...

Blow it out:

This, presumably, is the method each of us uses most often now that homes are no longer heated primarily by building a fire in the hearth. When you blow out a candle, the narrow, concentrated jet of air does three things:

1. The flame is physically moved away from the wick, so it can't melt, draw up, and vapourise more wax.
2. The fast-moving air cools the wax in the wick to stop it vapourising. The already-vapourised wax will still drift up into the air, though, and if you're quick, touching a lit match to this vapour will re-ignite the candle!
3. The extra oxygen can help the vapour already in the air burn more quickly, so it will run out of fuel faster.

This only works on very small flames, like candles, matches, or oil lamps. Blowing on a fire larger than that will just give it extra oxygen, and make it burn all the hotter!

Douse it:

Water is the classical way to put out a classical fire. Besides cutting it off from the air, the water rapidly cools the fuel, stopping the reaction and preventing it from spreading. The water is vapourised in the process - rising as (dangerously hot) steam - but water vapour is not flammable, and will push the oxygen out of the way as it rises. If you only put a little water in the fire - if you've built it out of green branches, for example - it will still burn, but not as hot. Because the fuel is not entirely consumed before convection carries it beyond the flame, this means a wet fire will produce more smoke and soot.

Water is a good way to deal with fires burning dry, organic matter for fuel, which is why it's found in fire extinguishers classified for this type of fire. Unfortunately, there are many types of fire where water does not help, or can even make the situation worse. Burning oil or other grease will spit and scald whoever is trying to put it out, and, like petrol, may float on top of the water, still burning and spreading easily. Other chemicals may give off noxious fumes when combined with water, and some, such as sodium, will ignite! Electrical fires, likewise, should never be put out with water until the power has been turned off at the mains, or there is a very real risk of electrocution. The sudden cooling may also rupture gas tanks, suddenly adding a lot of already-vapourised fuel to the mix and most likely causing a huge explosion.

Unless you know what you're doing and you know what's burning and why, don't try to fight a fire with water except as a last recourse!

Scatter the embers:

Whether by kicking them, poking them with a stick, or pushing them apart with a jet of water, scattering the embers will help the fire burn out faster. More surface is exposed to the air, so they burn faster, but also cool faster, especially since they're not keeping eachother warm. Charcoal is actually a fairly good insulator, which is why thick wooden beams can hold out so long in a burning house, and a banked fire can survive for days.

If you've built a fire and let it burn down, pull the embers or ashes apart and let them cool completely before binning them or getting them near anything flammable - just be sure the surface on which you're scattering them is fireproof!

Removing The Oxygen

Fire, like human beings, requires oxygen to sustain itself. Part of what makes a fire so deadly - quite apart from the heat - is this competition for resources. A burning fire quickly uses up the available oxygen in a given space and replaces it with incompletely oxidised carbon - CO, or carbon monoxide, a gas which is highly toxic⁶ and hard to detect. Fortunately, it's 'toxic' to fire, too - once all the available oxygen has burned up, the fire will go out.

Since it's impractical to physically remove the oxygen from around the fire - unless it's a fire inside a vacuum pump or the airlock of a spaceship - a fire is usually best put out by creating a smaller, separate air pocket around the fire and letting it use up just the limited amount of oxygen inside.

Snuff it out:

When you snuff out⁷ a candle, you enclose it in a small, air-tight bell where it will quickly run out of oxygen and suffocate. The same principle can be applied to most small to medium-sized fires. It's especially useful to extinguish burning fat or oil - if a frying pan catches fire in the kitchen, put a lid on it. If a candle burns out of control, cover it with an inverted pot.

Smother it:

This means inserting a layer of something non-flammable between the fuel and the oxygen, sealing them off from one another. Most modern fire extinguishers rely on this method, coating whatever is burning in non-flammable foam or powder, which is why they should be aimed at the base of the fire, not the flames! Unfortunately, they'll also leave a mess. The more traditional bucket of sand dumped on the fire or heavy blanket⁸ used to beat it out rely on the same principle.

In the unfortunate event that a person's clothing or hair has caught fire, smothering the flames is also the best way to put it out. First, don't panic! Drop and roll on the floor - even if it's made of wood - to smother the flames between yourself and the ground. If someone else is burning, trip them if necessary, and roll them, or wrap them in a heavy blanket! If it's a loose garment that's burning, take it off - just be careful if the fire's already eaten through it closer to the body, because the material may have fused with the skin. In this case, don't try to remove it yourself, just administer first aid and call an ambulance!

Care should be taken using other methods to extinguish a burning person, because many of them can cause injury. The foam or powder from a fire extinguisher can be harmful especially if inhaled or sprayed in the eyes - keep it away from the face! Water seems like a good option, but the jets from fire hoses (which are, after all, also used to disperse crowds) or water-based fire extinguishers can be strong enough to cause real damage. Immersing a burning person completely in cold water - a pond, a fountain, or a pool - can cause or worsen life-threatening shock. You should also be careful with CO₂ extinguishers, because the cold can be enough to damage living tissue if it's exposed too long. After all, CO₂ is also used to burn off warts!

Push the oxygen aside:

A carbon dioxide extinguisher contains liquid CO₂, which is very cold⁹ and is under a lot of pressure. When it's sprayed, the CO₂ will quickly expand, pushing aside the oxygen, helping stop the fire, and it won't leave any residue. At the same time, it will quickly cool down the fire without the danger of spattering, as it leaves no moisture as it evaporates - though it may make the moisture in the air cool down enough to make it snow! CO₂ extinguishers aren't much use for fires with dry, organic fuel, as the material tends to reignite, and can be dangerous with chemical fires!

Replace all the oxygen:

You can't do this if there are people in the same area as the fire - they still need to breathe, too! But some spaces are predestined for this way of fire-fighting, if they're sealed off and the atmosphere is controlled, because it's very effective! Engine rooms of ships or server rooms, for example, where fire is always a possibility and conventional methods are dangerous and would cause more damage because of the residue, are sometimes equipped to be flooded with a non-flammable gas such as CO₂ at the touch of a button. However, this isn't a method the average person will ever use!

Make the oxygen unavailable:

A halon extinguisher is a very efficient way of putting out a fire, as the halon reacts with the oxygen before it can get to the fuel. However, it has a high environmental impact, destroying the ozone layer, and hanging around in the atmosphere for a good 400 years! Use of halon extinguishers is illegal in most of Europe and in Australia, except with special permit, and the production of new halon extinguishers has been banned except for a few specific applications. These include use by the military and in certain types of aircraft, where the benefits are considered to outweigh the environmental damage.

Blow it out by blowing it up:

This method is rare, but can be done where nothing else will work. Oil rig fires, for example, are extremely difficult to extinguish because the fuel supply is, for the usual timeframe desired for putting out a fire, unlimited, and can't be cut off. Water won't work on these flames, and they grow so high and hot that smothering them is extremely difficult. Explosive charges are sometimes placed next to the fire and detonated. This creates a cloud of very rapidly expanding gas, which produces a shockwave that leaves a partial vacuum. The fire is extinguished like a giant blowing out an enormous candle, with much the same mechanisms involved.

1. The flame and the fuel still coming up through the pipeline are physically moved apart.
2. There's a lot of fast-moving air - during the explosion, when the hot gas pushes the air aside, but also when the air rushes back to equalise the pressure in the 'hole' that's left behind by the explosion. This cools everything down a little, helping to prevent reignition.
3. The chemical reactions that take place during the explosion use up much of the oxygen in the surrounding air before the fire can get to it, meaning it simply isn't available for combustion.

Obviously, this is a rather desperate method, because it has the potential to destroy both the surroundings and the people involved! In general, putting out a fire with explosives¹⁰ is not recommended. However, this method can yield good results when used by professionals who know what they're doing, even extinguishing fires that have been burning for years.

It's important to be very sure the fuel has sufficiently cooled down before you re-expose it to the air, or with all three elements present, it will spontaneously start up again! So either leave it for a good long time, or cool the outside with water, if you can do so without risk. Remember that a very hot container may burst if suddenly cooled!

Removing The Fuel

If there's nothing to burn, the fire will go out - simple, no? Unfortunately, countless things are flammable, especially when the fire is already very hot.

Cut off the fuel supply:

There isn't much you can do immediately about do about the fuel in a wood fire, but if you have something like a gas fire, the most obvious solution is to turn it off at the main or otherwise interrupt the supply before the point where it's ignited.

With other types of fuel, you can simply prevent the fire from finding anything new to burn - this means removing all flammable things around the fire. The bigger the flames, the wider the clear zone will have to be. If you're making a campfire, you're already using this method¹¹ by digging a fire pit and making a ring of stones to build it in and staying away from overhanging branches.

This method is usually also the only successful way to fight an out-of-control wildfire. Firebreaks - clear areas between patches of vegetation or other flammable things - are made in advance, or the available fuel toward which the main fire is heading is burned away beforehand in a series of smaller, deliberate fires so that the main one will starve. This practice gave rise to two expressions which we now usually use in a more metaphorical sense. We literally 'fight fire with fire' using controlled 'back-fires'. Sometimes, however, they're not as easy to control as we'd assumed, in which case the plan backfires!

Let it burn out:

This isn't really very good way of putting out a fire, because presumably, there's a good reason you're trying to put it out right now. But if all else fails, just let it burn until it's run its course - just be sure to remove all nearby new sources of fuel you can get at, including yourself, and observe it carefully. Maybe you can put it out entirely once it's burned down a bit. Otherwise, it won't stop by itself until all the fuel or all the oxygen is gone!

A Final Word Of Advice

Fire is a terrible, destructive force and can be insanely difficult to extinguish once it's out of control. Don't play with fire, no matter how old you are! Learn anything to do with open flames, whether it's fire-breathing or building a campfire, from someone who knows what they're doing, and know what you're doing. If there's any doubt that you can put the fire out quickly, call the fire department sooner rather than later, because lives are at stake! If it's more than you can handle, don't even try. Just get yourself and anyone else who could be in danger out of harm's way, and leave the extinguishing to the professionals.