Explosives

There are four general categories of chemical explosives: blasting agents, primary, low, and high explosives. Blasting agents such as dynamite are relatively safe and inexpensive. Construction workers and miners use them to clear rock and other unwanted objects from work sites. Another blasting agent, a mixture of ammonium nitrate and fuel oil, ANFO, has been used by terrorists around the world because the components are readily available and unregulated. Ammonium nitrate, for instance, is found in fertilizers. One thousand pounds of it, packed into a truck or van, can devastate a large building.

Primary explosives are used in detonators, small explosive devices used to set off larger amounts of explosives. Mercury fulminate and lead azide are used as primary explosives. They are very sensitive to heat and electricity.

Low, or deflagrating, explosives such as gunpowder do not produce as much pressure as high explosives but they do burn very rapidly. The burning starts at one end of the explosive and burns all the way to the other end in just a few thousandths of a second. This is rapid enough, however, that when it takes place in a sealed cylinder like a rifle cartridge or an artillery shell, the gases released are still powerful enough to propel a bullet or cannon shell from its casing, though the barrel of the rifle or cannon toward a target hundreds or thousands of feet away. In fact this relatively slow burning explosive is preferred in guns and artillery because too rapid an explosion could blow up the weapon itself. The slower explosive has the effects of building up pressure to smoothly force the bullet or shell out of the weapon. Fireworks are also low explosives.

High, or detonating, explosives are much more powerful than primary explosives. When they are detonated, all parts of the explosive explode within a few millionths of a second. Some are also less likely than primary explosives to explode by accident. TNT, PETN (pentaerythritol tetranitrate), and nitroglycerin are all high explosives. They provide the explosive force delivered by hand grenades, bombs, and artillery shells. High explosives that are set off by heat are called primary explosives. High explosives that can only be set off by a detonator are called secondary explosives. When mixed with oil or wax, high explosives become like clay. These plastic explosives can be molded into various shapes to hide them or to direct explosions. In the 1970s and 1980s, plastic explosives became a favorite weapon of terrorists. Plastic explosive can even be pressed flat to fit into an ordinary mailing envelope for use as a "letter bomb."