What happens in combustion

Chemically, combustion is rapid oxidation. Wood, gas or spirit fuel consist largely of compounds of carbon and hydrogen. They react with the oxygen in the air – air contains about 21 per cent of it – to form carbon dioxide and water. This releases the energy stored in the fuel as heat and light.

Solid fuels such as wood first have to be broken down. When wood is heated, the water in it evaporates first. Then its components decompose into flammable gases and vapours; this process is called pyrolysis. Only these gases mix with air and burn as a flame. The flame in turn heats the wood beneath it, which gives off new gases – a self-sustaining chain reaction.

What remains is charcoal, which consists mostly of carbon. It burns at its surface without a large flame and glows as it does. These embers give even heat, which is why cooking over embers works better than over tall flames.

Thin material ignites more easily because it has a lot of surface relative to its mass. A match can light a dry blade of grass, but not a thick branch – the heat spreads through too much mass in the branch. That is why every fire lay builds up from thin to thick: tinder lights kindling, and kindling provides enough heat to pyrolyse thicker wood.

Moisture costs heat. Before wet wood can give off gases, its water has to evaporate first, and that takes a lot of energy. The fire cools down, smokes heavily and goes out easily. Dry, dead wood therefore burns much better than fresh or wet wood.

Air must reach the flame. The flame is burning gas, and that gas needs oxygen. If the fire is packed too tightly or covered too early with wood that is too thick, it suffocates. Air from below and gaps between the pieces keep it alive.

In incomplete combustion there is not enough oxygen, or the flame is too cool to convert all the gases fully. Soot (tiny carbon particles), unburned gases and carbon monoxide are then produced. Lots of white to grey smoke often indicates damp wood or a fire that is too cool; part of it is water vapour. A hot fire with good airflow smokes less.

The yellow colour of a wood or candle flame comes from glowing soot particles in the flame. A blue flame, as on a well-adjusted gas stove, shows that the gas is well mixed with air and burns more cleanly. If a stove soots heavily or burns yellow and unsteadily, something is usually wrong with the air supply.

Any combustion in enclosed spaces can produce carbon monoxide, a gas you can neither see nor smell. No barbecues, stoves or fires in tents, living spaces, basements or garages. Accelerants such as petrol never belong near a fire: their vapours spread invisibly and can ignite explosively.

Open fires are restricted or banned in many places. Check the local rules and the current wildfire danger beforehand. Never leave a fire unattended, and put it out completely before you leave: water on it, stir, more water, until nothing is warm any more. Even embers that no longer visibly glow can stay hot for a long time.

Some textbooks extend the fire triangle into the fire tetrahedron: the chemical chain reaction – the cycle in which the flame keeps releasing and igniting new gas – is added as a fourth side. Some extinguishing agents work exactly here by interrupting this chain reaction.

Putting out a fire always follows the same logic as lighting it, only backwards: water cools and removes heat, sand or a fire blanket removes the air, and clearing away fuel takes away the fire's food. Anyone who understands why a fire burns also understands how to put it out safely.