Explosives are substances or mixtures of substances which burn very rapidly or detonate to
produce large volumes of combustion gases which expand violently.
Gunpowder is the explosive which has been longest used by man - fired in a confined space, it's
explosion can create pressures of more than 6,000 atmospheres.
High Explosives such as dynamite, guncotton, mercury fulminate, nitroglycerine, and T.N.T.
(trinitro-toluene), can produce explosions which have up to six times the power of gunpowder.
The most violent of these which is in practical use is mercury fulminate - under suitable
conditions, the pressures generated by the detonation of mercury fulminate can reach 200
tons per square inch.
THE NATURE OF EXPLOSIVES
Most explosives which are in military or commercial use are solids or liquids which occupy a minute
volume in comparisson to that occupied by their combustion products which are gasses. Explosives either
burn very rapidly or decompose, almost instantaneously when detonated.
While some explosives require a detonator or hot flame to initialise the explosion, others such as
nitrogen iodide are so unstable that they will detonate at the slightest vibration or touch.
Broadly, explosives can be separated into two types; rapid burning, such as gunpowder, where the
flame spreads through the explosive material very rapidly; and detonating, such as mercury fulminate
and nitroglycerine, where all parts of the charge decompose as a shock wave passes through them - almost
instantaneously!.
Some explosives, dynamite and guncotton among them, may be lit with a flame and burn without extreme violence;
if subjected to a shock, such as that from a detonator, they will explode violently.
Many combustible materials, when given suitable conditions for rapid burning, will cause an explosion
(see explosive mixtures).
GUNPOWDER
Although the Chinese alchemists had experimented since the 10th century with mixtures which burnt fiercely,
the invention of gunpowder owes much to the work of Roger Bacon
who found that it's effectiveness as an explosive was very much dependant on the purity of the saltpetre used.
Bacon achieved high purity in his saltpetre by perfecting a technique of successive crystalisation. Bacon
published his work on gunpowder in a coded form.
HIGH EXPLOSIVES
In`low' explosives, such as gunpowder, the reaction is progressively
ignited by heat although the ignition may travel at speeds of over 300 metres per second.
High explosives are characterised by the fact that once they are detonated, the reaction spreads by means of
a supersonic shock wave which decomposes the explosive immediatly in it's wake.
EXPLOSIVE MIXTURES
Combustible materials burn by combining with the oxygen which is present in and comprises some 20 per cent
by volume of, air. Provided that the materials which are necessary for combustion are supplied to the flame
at a steady rate, the combustion will take place in an orderly manner. It is worth noting that air will
burn equally well in an atmosphere of, for example, natural gas.
An explosive mixture is one in which the materials necessary for combustion are mixed in ideal, or near
ideal, proportions. When such a mixture is ignited, the combustion takes place at a furious rate to
cause an explosion.
