While many explosions are natural, I'm interested inhuman-made explosions — the discoveries that enable them, the people who made the discoveries, and the consequences for good and ill. Not just the who, when, and why but also the how. How did Chinese alchemists come to create black powder? What accidents led to the discovery of high explosives? How do explosives actually work?
There are three groups of explosions: mechanical explosions, chemical explosions, and nuclear explosions.
Mechanical explosions happen when pressure builds up in an enclosed space until the container suddenly bursts to release the pressure, like that kernel of popcorn. It is heated until the water inside turns to steam. The pressure of the steam continues to increase as it is heated until the tough little shell around it bursts. Lightning is a mechanical explosion. The electric current in the lightning bolt heats the air to over 50,000°F in less than ten microseconds. The resulting hot air expands at a rate faster than the speed of sound. In this case, the “enclosed space” is caused by inertial confinement, which just means the heating happens so fast, the expanding gas can’t get out of the way fast enough.
Chemical explosions involve fire. They can be reactions that are so fast they mimic lightning in their ability to heat gases faster than they can escape, or they can be slower (but still generally fast) reactions that build up pressure in a container.
Black powder in a musket or firecracker is one example of a reaction that explodes if contained but merely burns in the open. Not something that you'd want to do near children's outdoor playground equipment. Another is the explosions that happen in an internal combustion engine. It is difficult to make fuels in open air burn fast enough to explode. This is why fuel-air bombs (also called thermobaric weapons) were not invented until World War II and not used much until the Vietnam War in the 1960s.
Accidental chemical explosions happen when fuel and air mix in the right proportions. Coal dust explosions in the confines of a coal mine or flour dust explosions in mills are both deadly examples. Dust explosions are less common without an enclosing mine or building.
The key to chemical explosions is to mix the fuel and the oxidizer (such as air) very well, so that many fuel molecules are in contact with their respective oxidizer molecules. A pile of coal dust will not explode. The fuel and the air do not mix well enough. If the dust is not fine enough, an explosion is less likely. Again, too much of the fuel surrounds more fuel and is not close to the oxidizer.
A second key is the right proportion of fuel to oxidizer. If the fuel is surrounded by too much air, or vice versa, only a part of the mix will burn. The parts that remain merely soak up heat that could have gone into igniting more reactants and absorb some of the kinetic energy that could have gone into bursting the container wall. Ideally, each atom of fuel pairs with an atom of oxidizer, so the reaction goes to completion.
This second concept leads to the definition of lower and upper explosive limits. If there is not enough fuel, known as a “lean” mixture, a gas or vapor will not burn. If there is too much fuel, a “rich” mixture, again it will not burn. Some fuels have very narrow limits. Gasoline, for example, will only burn between about 1% to 7% fuel in air. Methane (natural gas), propane, and butane also have narrow combustion ranges. This is why they are safer fuels to use than, say hydrogen, which has a range of 4% to 75%, meaning that almost any amount of hydrogen in air will at least burn, and quite a large range will explode.
This recipe of the right proportions of oxidizer to fuel, mixed as closely as possible, is the hallmark of chemical explosives. The history of gunpowder is all about finding the right proportions of sulfur, charcoal, and nitrates and learning how to mix them so that the particles of each are tiny and homogenous. Later, mixing them so well that the oxidizer and the fuel are on the same molecule produced explosives like guncotton and nitroglycerin.
As humans learned to make many different types of explosives, they found a need to compare them to one another. Most people have a vague understanding that dynamite is more powerful than gunpowder, but what does it mean to be more powerful? Power is energy divided by time. If more energy can be produced in the same time, there is more power. Likewise, if the same energy can be delivered in less time, there is more power. In explosives, this relates to the speed of the chemical reaction that generates the energy.