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
[19] [20] .
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 explosive.
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 them.
Explosives commonly 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


Classification

  1. Propellants or low explosives Black Powder 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, smokeless powder.


  2. Tetracene 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, lead azide, the lead salts of picric acid and trinitroresorcinol, m-nitrophenyldiazonium perchlorate, tetracene, nitrogen sulfide, copper acetylide, fulminating gold, nitrosoguanidine, mixtures of potassium chlorate with red phosphorus or with various other substances, the tartarates and oxalates of mercury and silver.


  3. 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: Dynamite
    • 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.
    Some definitions add a third category:
    • 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.
    Examples: dynamite, trinitrotoluene, tetryl, picric acid, nitrocellulose, nitroglycerin, liquid oxygen mixed with wood pulp, fuming nitric acid mixed with nitrobenzene, compressed acetylene and cyanogen, ammonium nitrate and perchlorate, nitroguanidine.


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.