I always like to start a book off with a bang.
This is a book about bangs. Big ones, but also little ones. The tiny bang of a kernel of popcorn exploding into a white fluffy treat. The thousands of little bangs every second that power internal combustion engines. The little bangs of firecrackers and cap guns. The big bangs of thunder and lightning. The explosions of mining, warfare, volcanoes, and collapsing stars.
While many of those explosions are natural, this book is mainly about man-made explosions, and 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?
We can divide explosions into three groups: 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. We heat it until the water inside turns to steam. The pressure of the steam continues to increase as we keep heating it, 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 degrees Fahrenheit 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 that 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. 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 1960's.
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 the particles of each are tiny and homogenous. Later, mixing them so well that the oxidizer and the fuel are on the same molecule gave us explosives like guncotton and nitroglycerin.
As we learned to make many different types of explosives, we found a need to compare them against 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, we get more power. Likewise, if the same energy can be delivered in less time, we have more power. In explosives, this relates to the speed of the chemical reaction that generates the energy.
In history's first explosive, black powder, the chemical reaction happens at a speed lower than the speed of sound in the mixture. Black powder does not detonate. It deflagrates, which just means it burns rapidly. It still delivers quite a bit of energy, but it takes longer to do so. By containing the energy, and releasing it suddenly when the container bursts, we can increase the power quite a bit. But the total energy has not changed.
In many other explosives, the chemical reaction happens at a rate faster than the speed of sound in the material. A supersonic shock wave propagates through the explosive, releasing the energy faster than heat can be conducted. This is detonation, and the velocity of detonation relates to the power of the explosive.
Around the time of World War I, the explosive trinitrotoluene (known as TNT) was in widespread use. As more explosives were developed, people needed a way to determine how much of a new explosive they need to have the same effect they were used to getting with TNT. If an application used a pound of TNT to move rock in a mine, or launch a projectile, how much of the new explosive would they need to use?
By comparing to TNT, we get what is called the relative effectiveness of the explosive. TNT has a velocity of detonation of 6,900 meters per second (15,435 miles per hour). It has a density of 1.6 times that of water. The dynamite Alfred Nobel invented (a mixture of 75% nitroglycerin and 25% inert material) has a detonation velocity of 7,200 meters per second (16,106 miles per hour), which is a bit over 4 percent faster than TNT. It has a density 1.48 times that of water (so it is less dense than TNT). These factors lead to a relative effectiveness of 1.25. To get the same effect as a pound of TNT, you only need 1/1.25 (80%) as much dynamite by weight, or 12.8 ounces.
This concept goes further when we discuss nuclear weapons. Instead of relative effectiveness, we use terms like kilotons and megatons. These relate the effectiveness to thousands of tons of TNT, or millions of tons.
An explosion releases energy in the form of heat and the motion of the expanding gases. What is important is the rate at which the energy is released (the power). We measure energy in many different ways. The energy in a pound of TNT is 1.89 megajoules, or 453 kilocalories, or 527 watt-hours, or 1.4 million foot pounds. That's the same energy in a large candy bar. It is also about the same amount of energy you need to accelerate a car to 60 miles per hour. The difference is that the TNT releases all that energy in a microsecond or two, instead of the several seconds it takes to get the car onto the highway, or the hour it takes to run off the calories in that candy bar.
Power is energy per unit time, so it comes as no surprise that we measure power in as many ways as energy. Horsepower, watts, and foot-pounds per second are some of them. A pound of TNT would make a ball a bit over 3 inches in diameter. TNT's detonation velocity of 6,900 meters per second means that the whole pound would explode in less than six microseconds. 1.89 megajoules of energy in six microseconds 315 billion watts. By comparison, the largest U.S. nuclear power plant produces 3.3 billion watts, and serves four million people. Of course, it does it day in and day out, not just for six microseconds.
This is a book about the history of explosives. Not a lot of written information from the very earliest days has survived, and much of that is from people writing about things that happened years or even centuries before the writer was born. Teasing out what is real from what is conjecture is not always easy, and I try to avoid conjecture in favor of making it clear what we do not know.
Most of the history of explosives, however, coincides with the history of chemistry, and we do have excellent records of the original scientific papers on these subjects. Despite that, I have found in researching this book that there are numerous errors in what we normally think of as trustworthy sources. I have found encyclopedia entries that state something in one paragraph and contradict it on the next. This is not limited to Internet encyclopedias, although there is much misinformation there. It is said that a man with one watch knows what time it is. Reading two biographies of a notable scientist that disagree on dates or other particulars make a writer question everything.
Where I can, I try to find the original paper by the chemist who made the discovery. These papers have dates, and are often readily available (for example, the U.K's Royal Society, where many of the early scientists published their work, has digitized their journal, going back centuries, and made it available for free on the Internet). In other cases, there are patents, establishing a clear date for both the original filing of the patent, and its granting (often years later). This does not tell us how many years of work led up to the patent, but it does often establish priority, and indicates when other researchers might have known about a breakthrough or a process. To be sure of accuracy, I will generally cite a patent date, or the date of publication of a paper, rather than a second-hand account of when a discovery was made, no matter how good the credentials of the writer appear to be.
This is not a book full of footnotes, intended for scholarly research. However, given the name on a patent and the date it was filed, readers can quickly find the quoted material on-line. The same goes for papers mentioned with dates. For other information, the casual reader can use the names and molecules mentioned to search for books that mention them. Many of those books have been digitized, and in many cases, large portions of the book can be read on-line before purchase or a visit to the library.