Module 3 - Arson and Explosives
Lesson 3 - Explosives
The Fuse
A Burning Fuse
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An explosive device has an ignition source or a fuse that when ignited causes a reaction between the compounds in the metal casing. A fuse is simply a length of cord either filled with combustible material or made from combustible material. The length of a fuse determines its burn time. For example, a fuse 30 cm long will take 60 seconds to burn and then detonates the explosive. Some fuses detonate after set periods by using mechanical, electronic, or chemical timers. Other fuses are point-detonating fuses; they combust upon impact such as when dropped or thrown.
Some explosives have remote detonators that use wires or radio waves to detonate the explosive device from a distance. Bombs hidden in containers such as packages, suitcases, boxes, or portable stereos are usually triggered by battery-powered ignition switches (such as clocks) that are activated by opening the containers or by time-delay switches. Car bombs are usually detonated when the vehicle is started by the ignition switch. Usually, most of the time and effort that goes into making an explosive device is spent on the ignition source.
The burning fuse is the simplest and oldest type of fuse. This simple fuse, invented in tenth century China, consisted of lightweight paper filled with loose gunpowder. This type of fuse was used by the Chinese to ignite fireworks, and it is still found in fireworks today.
Inside the casing of an explosive, there is either a pure compound or a mixture containing an oxidizer and a fuel source. An example of a pure explosive compound is nitroglycerin, a highly unstable, heavy, colourless, oily liquid. Nitroglycerin is faster to ignite and more powerful than gunpowder. Nitroglycerin combined with sawdust makes dynamite. Dynamite is used for controlled blasting of dam sites, canal beds, and mines or demolishing large buildings by imploding their foundations.
Nitroglycerin
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An oxidizer is a molecule that releases some atoms of one or more oxidizing elements allowing the fuel source of the explosive to continue burning. The fuel source of an explosive is an unstable chemical compound that when ignited produces an explosion. Two examples of oxidizer/fuel source mixtures found in explosives are the following:
- Black powder = potassium nitrate + charcoal and sulfur
- Flash powder = potassium nitrate + aluminum or magnesium
Nitroglycerin is used as a medication for a heart condition called angina pectoris. When nitroglycerin is consumed, the body converts to nitric oxide, which is a natural vasodilator causing widening of the blood vessels allowing increased blood flow.
Plastic explosive material can be molded into various shapes because it has a consistency similar to play dough. Most plastic explosive material contains the explosive compound cyclotrimethylenetrinitramine (RDX). Plastic explosives are expensive and require powerful detonators.
Flash Point
- The lowest temperature at which the vapour of a combustible liquid can be made to ignite momentarily in air