Fire triangle and fuels

Wood does not really burn directly. When it gets hot enough, it breaks down and gives off flammable gases. The yellow flames you see are these gases burning together with oxygen from the air. This releases heat, which warms the next piece of wood, which in turn gives off gases. That is how the fire keeps itself going, as long as all three sides of the triangle are supplied.

Before wood can release these gases, it first has to heat up. If it is damp, much of the heat is spent evaporating the water first. That is why wet wood hisses, smokes heavily and burns poorly. Thin material heats up much faster than thick material because it has a large surface relative to its weight, where heat and air can reach it.

In the end, embers are left: almost pure charcoal that glows without big flames. Embers give off long, even heat. They are ideal for cooking, but they can also start a new fire hours later if they are not properly put out.

The most common reason a fire will not catch is not the match or the lighter but the material. It is too damp, too thick, or the in-between stages are missing. The proven order is: tinder (very fine, catches at once), then kindling in several sizes from matchstick-thin to thumb-thick, and only then fuel wood.

Air is just as important. A fire packed too tightly suffocates – even when there is enough wood. Lay the wood loosely so air can flow in from below. If a fire smokes heavily, that is often a sign of wood that is too damp or too little air.

The type of wood matters too, but less than dryness. Resinous softwood catches easily and burns fast and hot but can throw sparks. Denser hardwood burns more slowly and forms longer-lasting embers. So softwood is better for starting a fire, hardwood for cooking and warmth.

The fire triangle explains not only lighting but also extinguishing. Water removes heat from the fire. Sand or soil separate the embers from the air. Pulling pieces of wood apart takes away the fuel. Each of these removes one side of the triangle.

In practice, one measure alone is often not enough. Embers that were merely covered with soil can keep smouldering underneath for hours and flare up later. So for a campfire the rule is: drown it with plenty of water, stir, water again, and leave only when everything is cold.

Flammable liquids such as petrol, spirit or barbecue lighter fluid are off-limits. They evaporate, and the vapours can ignite suddenly – often right where the person pouring is standing. Good preparation of tinder and kindling replaces any “accelerant”.

Do not burn plastic, rubber, painted, coated or treated wood, chipboard or rubbish. They release toxic smoke and residues that you breathe in and that stay behind in the ash.

Before you make any fire at all, check the local rules and the current wildfire danger. In many areas open fires in or near forests, in protected areas or during dry spells are prohibited. A fire ban always applies, including to a “small” fire. For a fire that is out of control, there is only one answer: get to safety and call the local emergency number (112 in the EU).

The fire triangle is a simplified model. Real combustion is chemically more complicated, and fire-safety training sometimes extends the model with a fourth side, the chain reaction of combustion. For making fires outdoors, though, the three sides are quite enough to understand almost every problem.

A good exercise: when your fire is not burning properly, ask yourself in turn which side is missing. Is the material too thick or too wet (fuel)? Is the starting flame too small or too short (heat)? Is everything packed too tightly or lying on wet ground (air)? That way you usually find the cause quickly.