An explosive is a material, either a pure single substance or
a mixture of substances, which is capable of producing an explosion
by its own energy
It is not proper to define an explosive as a substance, or a
mixture of substances, which is capable of undergoing a sudden
transformation with the production of heat and gas. The production
of heat alone by the inherent energy of the substance
which produces it will be enough to constitute the substance an
All explosive substances
produce heat; nearly all of them produce gas. The change is invariably
accompanied by the liberation of energy. The products
of the explosion represent a lower energy level than did thc explosive
before it had produced the explosion.
An explosive perfectly capable of producing an explosion may
liberate its energy without producing one. An explosion may occur
without an explosive, that is, without any material which contains
intrinsically the energy needful to produce the explosion.
A steam boiler may explode because of the heat energy which
has been put into the water which it contains. But the energy is
not intrinsic to water, and water is not an explosive. Also, we
have explosives which do not themselves explode. The explosions
consist in the sudden ruptures of the containers which confine
require some stimulus, like a blow or a spark, to provoke
them to liberate their energy, that is, to undergo the change
which produces the explosion, but the stimulus which "sets off"
the explosive does not contribute to the energy of the explosion.
The various stimuli to which explosives respond and the manners
of their responses in producing explosions provide a convenient
basis for the classification of these interesting materials.
Classification of Explosives
Propellants or low explosives
are combustible materials,
containing within themselves all oxygen needful for their combustion,
which burn but do not explode, and function by producing
gas which produces an explosion.
Under normal conditions, low explosives undergo
deflagration at rates that vary from a few centimeters per second to approximately
400 metres per second.
It is possible for them to deflagrate very quickly,
producing an effect similar to a
detonation. This usually occurs when
ignited in a confined space.
Explosives of this class differ widely among
themselves in the rate at which they deliver their energy.
There are slow powders and fast powders for different uses.
Examples: black powder,
Primary explosives or initiators explode or detonate when
they are heated or subjected to shock. They do not burn; sometimes
they do not even contain the elements necessary for combustion.
The materials themselves explode, and the explosion results
whether they are confined or not. They differ considerably
in their sensitivity to heat, in the amount of heat which they
give off, and in their "brisance", that is, in the shock which they
produce when they explode. Not all of them are brisant enough
to initiate the explosion of a high explosive.
Examples: mercury fulminate,
the lead salts of picric acid and trinitroresorcinol,
mixtures of potassium chlorate with red phosphorus or with various
the tartarates and oxalates of mercury and silver.
- High explosives
detonate under the influence of the
shock of the explosion of a suitable primary explosive. They do
not function by burning; in fact, not all of them are combustible,
but most of them can be ignited by a flame and in small amount
generally burn tranquilly and can be extinguished easily. If
heated to a high temperature by external heat or by their own
combustion, they sometimes explode. They differ from primary
explosives in not being exploded readily by heat or by shock, and
generally in being more brisant and powerful. They exert a
mechanical effect upon whatever is near them when they explode,
whether they are confined or not. A high explosive compound
detonates at rates ranging from 1,000 to 9,000 meters per second,
and are, conventionally, subdivided into two explosives classes,
differentiated by sensitivity:
Some definitions add a third category:
- Primary high explosives are extremely sensitive to mechanical shock,
friction, and heat, to which they will respond by burning rapidly or detonating.
- Secondary high explosives, also called base explosives, are
relatively insensitive to shock, friction, and heat. They
may burn when exposed to heat or flame in small, unconfined
quantities, but detonation can occur. These are
sometimes added in small amounts to blasting caps to boost their power.
liquid oxygen mixed with wood pulp, fuming nitric acid mixed with
nitrobenzene, compressed acetylene and cyanogen, ammonium
nitrate and perchlorate,
- Tertiary high explosives or blasting agents, are insensitive to shock,
they cannot be reliably detonated with practical quantities of primary explosive,
and, instead, require an intermediate explosive booster, of secondary explosive,
e.g. ammonium nitrate/fuel oil mixture (ANFO) and slurry (wet bag)
explosives that are primarily used in large-scale mining and construction.
It is evident that we cannot describe a substance by saying
that it is "very explosive." We must specify whether it is sensitive
to fire and to shock, whether it is really powerful or merely
brisant, or both, whether it is fast or slow. We must distinguish carefully between
sensitivity, stability, and reactivity. A substance may be
extremely reactive chemically but perfectly stable in the absence
of anything with which it may react. A substance may be exploded
readily by a slight shock, but it may be stable if left to
itself. Another may require the shock of a powerful detonator
to make it explode but may be subject to spontaneous decomposition.
The three classes of explosive materials overlap somewhat, for
the behavior of a number of them is determined by the nature
of the stimuli to which they are subjected and by the manner
in which they are used.