Chinese alchemists seeking eternal life invented the first explosive
Taoist alchemists in China, searching for an elixir of immortality, produced something very different: gunpowder. It went to war by 1161 and remained the only chemical explosive for centuries, until nitroglycerin in 1847 opened a far more powerful and far more dangerous era.
An explosive stores a great deal of energy and releases it suddenly as heat, light, sound and pressure. Usually the energy is chemical, as in gunpowder or even grain dust, but a pressurised gas cylinder can burst explosively too. Chemically, an explosion is a reaction that, once started, sustains itself extremely fast because it both gives off large amounts of heat and turns compact solids or liquids into much more disordered gases.
The key distinction is speed. In low explosives the reaction spreads as a flame front slower than sound within the material, a process called deflagration. They are usually mixtures of a fuel and a separate source of oxygen, like gunpowder, and serve mainly as propellants, along with fireworks and flares. High explosives detonate: a shock wave races through them at supersonic speed, typically 3 to 9 kilometres per second. They tend to hold fuel and oxygen within the same molecule, as TNT does, and are used in mining, demolition and the military.
After nitroglycerin, too unstable for general use, came a run of safer and stronger materials: nitrocellulose and TNT from 1863, then smokeless powder and dynamite in 1867. TNT filled artillery shells in the First World War. Of the many chemicals ever made, only a few are suitable, because most candidates are too sensitive, toxic, costly or quick to degrade.
Mining is the biggest commercial user, since a charge set off in confined rock can break loose a fairly precise chunk while leaving the surroundings intact. Industry also uses explosions constructively in explosion welding, where a blast drives one metal against another so fast that they bond without melting, a method often used to clad steel with a different metal.
Source: Explosive
