Spontaneous combustion

Nothing caught fire, nobody smoked, not a spark. And yet the pile of rags in the garage is burning at three in the morning. Spontaneous combustion isn't magic: it's heat building up faster than it can escape.

The mechanism

Some materials heat up on their own, slowly: they oxidise in contact with air, or bacteria make them ferment. In a small, well-aired volume, this heat escapes and nothing happens.

But in a pile, a bag, a tightly packed bale, the heat stays trapped in the middle. The hotter it gets, the faster the reaction goes, so the hotter it gets. After a few hours or a few weeks, the core reaches its self-ignition temperature, and fire starts from the inside, with no outside flame.

Three conditions, always the same: a material that heats itself, a volume large enough to hold the heat, and just enough air.

Real cases, from most to least common

Rags soaked in drying oil. Linseed oil, teak oil, some stains and oil-based varnishes dry by absorbing oxygen from the air, and that reaction gives off heat. A rag balled up in a bin can ignite within hours. It's one of the best-known causes of fire for firefighters in workshops, building sites and garages.

Hay stored too damp. Above about 20% moisture, the bacteria in the hay ferment and heat the bale or the loft. The temperature climbs for days, then chemical reactions take over. Barns burn this way one to six weeks after haymaking.

Piles of compost, manure, sawdust, wood chips and coal. Large industrial piles are monitored with temperature probes.

Some metal powders and chemical residues ignite in air as soon as they come out of their packaging: they're called pyrophoric. That's the case with iron sulphide deposits in old oil tanks, which are opened with great care.

Damaged lithium batteries: an internal short makes them heat up until they run away. It isn't spontaneous combustion in the strict sense, but the result is the same, with no visible cause.

Spontaneous human combustion: myth and reality

The story has come back since the 18th century: a person is found almost entirely reduced to ash in a barely blackened room, feet or hands intact. It was long called spontaneous human combustion.

Today's accepted explanation is the wick effect. An ordinary heat source (a cigarette, a heater, a candle) sets fire to the clothes of a person who can no longer react: asleep, drunk, ill, already dead. Body fat melts, soaks into the clothing like a candle wick, and burns slowly for hours, with a small flame that barely spreads around it. Experiments with clothed pig carcasses have reproduced it. No case has shown a body catching fire without an outside source.

A few temperatures worth knowing

Diesel self-ignites at a lower temperature than petrol, even though it's much less volatile. That's what makes the diesel engine work: it ignites fuel by the heat of compression alone.

Firefighters' tips

• An oily rag never goes balled up in a bin. Spread it flat outside until it's dry and stiff, or drop it into a closed metal can full of water.
• A haystack that smokes or smells of caramel, tobacco or burning: don't open it, don't move it. The fresh air getting in would make the fire smouldering at the core flare up. Push a metal bar into the middle and leave it a few minutes: if it's too hot to hold, call the fire brigade.
• A compost heap that steams in winter is normal. A compost heap that turns black in the middle and smells burnt is too big and too dry: spread it out.
• Sunlight through a water bottle, a jar or a make-up mirror can focus onto a curtain, a pile of paper or a dry hedge: not spontaneous, but it looks like it, and it surprises people every summer. It's also a way to light a fire, see Allumer sans briquet.
• A swollen phone or scooter battery comes out of the house and goes on a non-flammable surface, outdoors if possible.

What to do about a fire that has started: House fire and Using a fire extinguisher.