Explosives

For detonating charges of comparatively insensitive high explosives, compounds are used that will themselves detonate under a moderate mechanical shock or heat with sufficient force to explode the main charge. For many years mercury fulminate, Hg(ONC)₂, was the compound chiefly employed for this purpose, either alone or mixed with other substances such as potassium chlorate. Its manufacture, however, is hazardous and it cannot be stored at high temperatures without decomposition. In addition, mercury may be difficult to obtain in time of war. As a result the fulminate has been replaced almost entirely in commercial and military detonators by lead azide, PbN6, diazodinitrophenol, and mannitol hexanitrate. These initiators are used in conjunction with a charge of cyclonite or PETN, which have largely replaced the tetryl (trinitrophenylmethylnitramine) used previously. These sensitive explosives have high brisance and explosive strength values. They are frequently used also as booster charges between the detonator and the major charge of high explosive in large shells and bombs. A blasting cap or exploder is a small charge of a detonator designed to be embedded in dynamite and ignited either by a burning fuse or a spark.

In coal mining the use of ordinary high explosives is hazardous because of the danger of igniting gases or suspended coal dust that may be present underground. For blasting under such conditions several special types of safety explosives have been developed that minimize the danger of fires or explosions by producing flames that last for a very short time and are relatively cool. The types of safety explosives approved for work in coal mines are chiefly mixtures of ammonium nitrate with other ingredients such as sodium nitrate, nitroglycerin, nitrocellulose, nitrostarch, carbonaceous material, sodium chloride, and calcium carbonate. Another kind of blasting charge for use in mining has grown in favor, because it produces no flame whatsoever. This charge is a cylinder of liquid carbon dioxide that can be converted into gas almost instantaneously by an internal chemical heating element. One end of the cylinder contains a breakable seal through which the gas can expand. The carbon dioxide charge is not a true explosive and absorbs heat rather than evolving it. It has the additional advantage that the force of the explosion can be directed at the base of the bore hole in which the charge is placed, thus lessening the shattering of the coal.

See also Nuclear Energy.