An explosive isn't something that burns hard. It's a reaction that travels faster than sound through the material itself. The whole difference between a quarry and an accident lies in that sentence.
Burning, deflagrating, detonating
Combustion needs oxygen from the air. It advances as fast as the air arrives: a wood fire.
A deflagration carries its own oxygen. The reaction spreads from grain to grain through heat, at a few metres to a few hundred metres per second. Confined, it builds pressure until the container bursts. That's black powder, and the powder in a cartridge.
A detonation travels as a shock wave through the material, at 2,000 to 9,000 metres per second. It doesn't need confinement: it shatters what it touches. That's dynamite, TNT, military explosives.
The families
Primary explosives detonate at the slightest shock, friction, spark or heat. They're used in tiny amounts, in detonators, to start the chain. They're also the most dangerous to handle, and the ones that most often kill amateur makers.
Secondary explosives are stable: you can often burn them without them detonating. They need the shock of a detonator. Almost everything used in practice belongs to this family.
Hence the explosive train, the same everywhere: a detonator (primary, a few tenths of a gram), sometimes a booster, then the main charge (secondary). Each stage wakes the next. Without a detonator, a block of military explosive is an inert material you can set down, cut, or even burn, which doesn't mean you should.
The explosives that mattered
| Name | Era | Family | What to know |
|---|---|---|---|
| Black powder | China, 9th century | Deflagrating | Charcoal, sulphur, saltpetre. Made cannons and mines for 800 years. |
| Nitroglycerin | 1847 | Secondary, very sensitive | A liquid unstable to shock. Unusable as is. |
| Dynamite | Nobel, 1867 | Secondary | Nitroglycerin soaked into a porous earth. Made tunnels and railways possible. |
| TNT | 1863, military from 1902 | Secondary, very stable | Melts without detonating, is poured into shells. Used as a unit of measure. |
| ANFO | 1950s | Secondary | Ammonium nitrate and fuel oil. The most used civil explosive in the world, in mines and quarries. |
| Emulsions | 1960s | Secondary | Nitrate-based pastes, water-resistant. They replaced dynamite. |
| RDX, PETN | Second World War | Secondary, powerful | The bases of military plastic explosives (C-4, Semtex) and detonating cord. |
Ammonium nitrate deserves a line of its own: it's first of all a fertiliser, stored by the thousand tonnes. Badly stored, it caused the disasters of Toulouse (AZF plant, 2001, 31 dead) and Beirut (port, 2020, more than 200 dead).
What they're used for
- Mines and quarries: by far the first use. Every day, thousands of tonnes of explosive break rock.
- Tunnels and mountain roads.
- Demolition of buildings and structures: the load-bearing columns are cut in the right order, and the building falls in on itself.
- Preventive avalanche release, at ski resorts.
- Seismic surveys: small charges send waves into the ground to look for oil or study what lies beneath.
- Airbags, ejection seats, explosive bolts on rockets: tiny charges that act in milliseconds.
- War, of course.
The people who use them, shotfirers or blasters, are trained and licensed. In France as almost everywhere, making, holding or transporting an explosive without authorisation is a serious offence.
The effects
An explosion injures in three ways:
- The pressure wave, which tears lungs, eardrums and intestines. It weakens very quickly with distance.
- Fragments, which carry much further than the wave. They're what sets safety distances.
- Heat and falls: burns, being thrown, collapses.
The US National Counterterrorism Center publishes indicative evacuation distances, which give the order of magnitude:
| Device | Amount | Evacuate buildings within | Outdoors, move away to |
|---|---|---|---|
| Pipe bomb | 2 kg (5 lb) | 21 m (70 ft) | 366 m (1,200 ft) |
| Bag, suitcase | 23 kg (50 lb) | 46 m (150 ft) | 564 m (1,850 ft) |
| Car | 230 kg (500 lb) | 98 m (320 ft) | 457 m (1,500 ft) |
| Van | 450 kg (1,000 lb) | 122 m (400 ft) | 732 m (2,400 ft) |
What people who've seen it remember: it isn't the metres that protect you, it's what stands between you and the charge. A thick wall, an earth bank, the corner of a building. And windows: most of the injured in an urban explosion are hurt by glass, sometimes hundreds of metres away. Stay away from windows.
Field tips
- Old explosive is more dangerous than new. Old dynamite "sweats": nitroglycerin seeps out and forms crystals or oily drops on the sticks or the cardboard. At that stage, a knock is enough. Don't touch it again, call bomb disposal.
- "Turn off radio transmitters" signs near blasting sites are no joke. Some electric detonators can be set off by a powerful transmitter close by. Walkie-talkies and phones off near a blast.
- A smell of almonds or solvent, or a headache near an old stock: some nitrate-based explosives are absorbed through the skin and cause violent migraines, the "dynamite headache" well known to miners.
- A wire sticking out, a new box in an abandoned place, an object that has no reason to be there: you don't know, so you don't touch. Mark the spot and leave the way you came.
- Wartime leftovers still kill farmers and walkers every year, in France and elsewhere. See Ammunition and unexploded ordnance.
If you find some
- Don't touch, don't move, don't cut anything.
- Walk away the way you came, and keep others away.
- Mark the spot from a distance, with something visible.
- Call the police or the fire service (112 in Europe). Bomb disposal comes out for free.
- No radio or phone next to the object: call once, from a distance.
Firearm ballistics are in La balistique and Ammunition and calibers.