Introduced by Bernard Kelly, Member, on 22 October 1999

An explosion is a sudden release of energy; its power is related to the rate at which energy is released _ compare letting the air out of a balloon to bursting it. The energy is normally stored as chemical energy which is released by oxidising the chemical to produce heat. For maximum efficiency, there needs to be a balance between the amount of fuel and of oxygen.

The first explosive _ gunpowder _ was known to the Chinese in the first century but it was not until the 13th century that Roger Bacon wrote, as an anagram for secrecy, the formula of it: 10% sulphur, 15% carbon (charcoal) and 75% saltpetre (sodium nitrate). The finer these components are ground before mixing, the faster the explosion develops. Gunpowder was first used commercially in Hungary in 1627 and by 1689 was in use in Cornish tin mines.

In weapons, the `black powder' was ignited by a spark _ the flintlock musket _ but for tunnelling a trail of the powder was woven into a rope, which was covered with gutta percha (rubber) to make it waterproof, as a `safety fuse'. Holes drilled into the tunnel face were filled with gunpowder and fired in turn from the centre in concentric circles; in quarrying, a line of charges was used.

By 1846, after the atomic and molecular combination theories had been formulated, nitro-glycerine was invented , a molecular explosive. This combined the fuel and the explosive in one molecule, the most intimate mixture possible, but unstable. It burns much faster than gunpowder. The formula is:


where the nitrogen (N) separates the fuel (CH2O) from the oxidiser (O2). Nobel found this would detonate `sympathetically' by an impact and unreliably from a flame but to make it transportable be mixed the explosive with an inert earth _ kieselguhr _ to produce dynamite.

At much the same time, nitro-cellulose was developed using cotton as the fuel. Nobel mixed the two to produce cordite.

Using coal as the raw material, the nitro-toluenes (especially tri -nitro-toluene, TNT) were produced. All these were used as commercial explosives in increasing quantities. In the 1980s dynamite was found difficult to handle in the bulk loads then required .

In warfare, TNT, which burnt incompletely and so produced smoke was mixed with ammonium nitrate to give `amatol', moving back from molecular to mixed explosives, but using liquids which mixed intimately into a slurry. For commercial use, ammonium nitrate was mixed with aluminium and ethylene glycol in `powergel'. This was transported as separate components and mixed on site in such a way as to incorporate air bubbles into the mix. The shock wave from the detonator caused the air bubbles to collapse, heat the aluminium and start the explosion. Recently, emulsion explosives have been developed in which the fuel (oil) is a stable dispersion coating the oxidiser droplets, resulting in an explosive which is 92% oxidiser and 8% fuel, thus achieving a very fast reaction. It has a shelf-life of about a year.

Initiating the explosion has been achieved by a variety of methods: a naked flame, a smouldering `fusee', mercury fulminate, an electrically heated wire and a shock tube _ the cheapest and most recent method. This consists of a very thin layer of explosive (18 g / km of tube) on the inside surface of a plastic tube ignited at one end by a spark.

In the first World War, it is reported that 750,000 tons of explosive were used over the four years. This weight is now used weekly for commercial purposes world-wide.

Don Lovell