When I was a small boy in Texas, summer meant more than school holidays: it meant fireworks. As Independence Day approached, from any one of hundreds of roadside stands that sprang up out of nowhere you could buy bottle rockets, Roman candles, sparklers, and, of course, Black Cat firecrackers (50 for $1).
One of the biggest drawbacks to our moving to Canada, in my opinion, was the lack of fireworks accessibility. I was somewhat mollified to discover that even Canadians celebrated the first week of July with large public fireworks displays, although their timing was off by a couple of days. But then, people have been celebrating national holidays with fireworks since long before either Canada or the U.S. existed.
The Chinese invented fireworks concurrently with inventing gunpowder, more than a thousand years ago. Gunpowder reached Europe during the Middle Ages, with many unpleasant results and one particularly pleasant one: by the 16th century it was being used to create spectacular displays marking important occasions.
Black powder’s basic formula, which has remained unchanged for centuries, is potassium nitrate (commonly known as saltpeter), charcoal and sulfur, mixed together in the ration 75:15:10. When gunpowder is ignited, the charcoal and sulfur burn–that is, they combine with oxygen. Unlike, say, a burning piece of paper, however, they draw their oxygen not from the air, but from the potassium nitrate. In pyrotechnic terms, the charcoal and sulfur are fuels, and the potassium nitrate is the oxidizer.
A pyrotechnic mixture is stable at room temperature. Applying heat to it, however, breaks down its chemical bonds, allowing fuel atoms to combine with oxygen atoms from the oxidizer. The new chemical compounds that are formed are actually more stable than the original compounds–they need less energy to maintain their structure than the original compounds did. This means there’s energy left over after the new compounds are formed, which is released as heat.
Of course, it’s not heat that makes fireworks spectacular, it’s light. Fireworks produce light three ways: incandescence, atomic emission and molecular emission. Incandescence is as familiar as the burners on an electric range: when substances get hot, they shed some of their excess energy as visible light. The hotter the substance, the brighter it glows.
The original fireworks displays consisted of nothing more than burning gunpowder, whose light comes from incandescent sulfur and charcoal. In the 19th century, however, fireworks got a lot brighter with the addition of metals such as aluminum, magnesium and titanium. These burn at very high temperatures, producing metal oxide particles that are, literally, white hot. A mixture of potassium perchlorate (an oxidizer) and fine aluminum or magnesium powder produces a flash of bright white light and, because it burns very quickly, a loud boom (caused by expanding gases colliding with the surrounding air.) In fireworks, this is called “flash-and-sound” powder.
Larger metal particles retain heat longer than powders and can continue to burn after the oxidizer is gone by drawing oxygen from the air. As a result, they create white sparks rather than a flash of light. The larger the particles, the longer the sparks last.
If you want coloured sparks in your starburst, you have to turn to atomic or molecular emission instead of incandescence. When atoms or molecules of various substances are heated, they are briefly boosted to a higher energy level. When they return to their normal energy level, they emit light of a specific wavelength. Sodium atoms, for instance, when heated above 1,800 degrees Celsius, give off a yellow-orange light (as in sodium-vapor street lights). This is atomic emission.
Other colours come from compounds, rather than pure elements: this is molecular emission.. Strontium compounds produce red; barium compounds produce greens and copper produces blue. The most commonly used compounds are strontium chloride, barium chloride and copper chloride.
All of these are unstable at room temperature, so they can’t be packed directly into the firework. Instead, chlorine-containing compounds such as polyvinyl chloride or chlorinated rubber are added, along with pure strontium, barium and copper. When the pyrotechnic is ignited, the chlorine-containing compounds burn, releasing free chlorine atoms which attach to the barium, strontium or copper and briefly produce the light-emitting molecules.
There are two kinds of fireworks shells. American-European style shells are typically cylinders seven to 30 inches in diameter. They’re launched from metal, plastic or cardboard mortar tubes. At the bottom of the shell is a portion of black powder. When it’s ignited, the hot gases it produces fill the tube and send the shell flying a few hundred meters into the air. At the same time, a time-delay fuse is ignited. When the shell is far above the ground, that fuse sets off another black-powder charge, called a “bursting charge,” which breaks the shell open and ignites metal or colour-composition pellets which are packed into the shell. These pellets, called stars, are expelled outward in a random pattern of light and colour. (Alternatively, the shell could contain flash-and-sound powder I mentioned earlier. Such a shell is called a “salute,” and often announces or concludes a fireworks display.)
Japanese-style “chrysanthemum” shells are spherical instead of cylindrical; the stars are arranged around a central black-powder bursting charge. This creates a round, symmetrical pattern when the shell explodes.
The stars may just flash and sparkle or produce extended trails, depending on their composition. Some stars have two layers of different colour-producing compounds, so that the starburst actually changes colour before it dies away.
Some shells contain several compartments, each with its own bursting charge and stars (or flash-and-sound powder), separated by nothing more exotic than cardboard. As each compartment explodes, it ignites a time-delay fuse that leads to the next compartment. The result is a shell that produces multiple bursts.
Some compositions don’t produce much heat and light, but do produce lots of gas, in spurts. Such a composition pressed into a narrow tube creates a whistling sound as the shell ascends.
It’s fascinating to know what’s going on inside a firework, but their appeal for most of us is far more visceral than intellectual. Which means that even I, while watching the Canada Day fireworks, did not say things like, “My, what a particularly fine exothermic oxidation-reduction reaction that was! And that molecular emission–precisely 672 nanometers, I’d say. Do you suppose they used strontium hydroxide or strontium chloride?”
Instead, like everyone else, I just said, “Ooh! Aah!”
