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Explosives, chemical compounds or mixtures that undergo rapid burning or decomposition with the generation of large amounts of gas and heat and the consequent production of sudden pressure effects. The chief use of explosives in peacetime is for blasting and quarrying (see Quarry and Quarrying), but explosives are also used in fireworks and signaling apparatus (see Fireworks) and for setting blind rivets and forming metals. Explosives are used as propellants for projectiles and rockets and as bursting charges for demolition purposes and for projectiles, bombs, and mines (see Bomb; Mine; Nuclear Weapons; Projectile; Rocket).

The first explosive known was gunpowder, also called black powder. In use by the 13th century, it was the only explosive known for several hundred years. Nitrocellulose (see Cellulose) and nitroglycerin, both discovered in 1846, were the first modern explosives. Since then nitrates, nitrocompounds, fulminates, and azides have been the chief explosive compounds used alone or in mixtures with fuels or other agents. Xenon trioxide, the first explosive oxide, was developed in 1962.

II Characteristics of Explosives
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Explosives are grouped into two main classes, low explosives, which burn at rates of inches per second, and high explosives, which undergo detonation at rates of from 914 to 9140 m per sec (1000 to 10,000 yd per sec). Explosives vary in other important characteristics that influence their use in specific applications. Among these characteristics are the ease with which they can be detonated and their stability to conditions of heat, cold, and humidity. The shattering effect, or brisance, of an explosive depends upon the velocity of detonation. Some of the newer high explosives with a detonation rate of 9140 m per sec are extremely effective for military demolition and certain types of blasting. On the other hand, for quarrying and mining, when it is desirable to dislodge large pieces of rock or ore, explosives with a lower detonation velocity and lower brisance must be employed. Explosives used as propellants in rifles and cannon should burn still more slowly, as they are required to deliver a steadily increasing push to the projectile in the barrel of the gun rather than a sudden shock which, if strong enough, might break the gun. Special types of explosives that are sensitive to heat or shock and have a medium-high brisance are used to initiate the detonation of less sensitive high explosives. High explosives are often mixed with inert materials to reduce sensitivity and lower brisance, as in the case of dynamite.

III Propellants
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Two types of explosive are in general use for the propulsion of projectiles in firearms and rockets, and both are commonly called by the generic name of smokeless powder. The term is properly applied to the low explosive, gelatinized nitrocellulose. The other type of smokeless powder, which consists of a mixture of nitrocellulose with a high explosive such as nitroglycerin, is known correctly as double-base powder or compound powder. A common double-base explosive is cordite, which contains 30 to 40 percent nitroglycerin and a small quantity of petroleum jelly as a stabilizer. The term smokeless powder applied to either type of explosive, however, is misleading, because neither is free from smoke when exploded, and neither takes the form of a true powder.


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