What Makes Fireworks Explode in Distinct Shapes?

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On a balmy summer evening, you're lying on a blanket in the grass, gazing up at the sky. The sounds of John Philip Sousa's "The Washington Post" march fill the air. Above, red, white, and blue lights shimmer and burst into shapes like fountains, flowers, and flags. It's Independence Day—a day that has been intertwined with fireworks since the birth of the United States in 1776. At that time, future president John Adams wrote to his wife Abigail, proposing that the day be "solemnized with ... illuminations" [source: APA History]. Today, we know these "illuminations" as fireworks.

But oh, how much fireworks have evolved since their inception, which is believed to have occurred around 1,000 years ago in China or India, alongside the discovery of gunpowder. The earliest fireworks came about when it was realized that packing gunpowder into rolled paper or bamboo tubes and lighting it would produce a deafening noise thought to ward off evil spirits. In the beginning, fireworks were nothing more than this loud bang, much like modern firecrackers, used to protect people during births, deaths, weddings, and New Year's celebrations, among other occasions [source: APA History].

When Italian explorer Marco Polo brought gunpowder from China to Italy in the late 13th century, Italian pyrotechnicians discovered how to launch firecrackers into the sky with the invention of aerial shells, which used gunpowder to propel the shells upward and cause them to explode. Italians were the first to incorporate metal pieces into the shells, creating sparkling gold and silver effects, and by the 1700s, they introduced metal salts to generate a spectrum of colors [source: APA History].

Those once terrifying bangs have long since transformed into spectacular skyward displays, with shapes and colors that become more intricate every year, thanks to the countless individuals—chemists, artists, firebugs, and even entire families—dedicated to perfecting a better, brighter show that offers not only more bang but also greater beauty for every dollar spent.

To craft these meticulously planned firework shows—far beyond the simple bangs and sparklers set off in backyards—chemists and pyrotechnic experts "bottle stars" in a sense. Let’s explore how they accomplish this feat.

To start, we're referring specifically to what's known as "display fireworks," not the consumer products purchased at local firework stands every Fourth of July (where allowed, of course).

Display fireworks are used in professional shows organized by cities and towns worldwide, often taking place on New Year's Eve. At these events, you might witness peonies—fountain-like bursts of color that form the most common shape—brocades, which create umbrella shapes with trailing stars, or even whimsical designs like hearts, smiley faces, and flags [source: APA Glossary].

The experts craft these shapes in a surprisingly straightforward manner, though that doesn't necessarily make the process easy. So, kids, don’t try this at home—if fireworks are mishandled, they can lead to serious injuries or even fatalities.

To create the aerial shells used in display fireworks shows, small tubes are packed with explosive chemicals to generate the desired colors, lights, and sounds. Gunpowder forms the main ingredient inside the shell, along with smaller explosive components known as stars, a bursting charge (similar to a firecracker), and a fuse. When the fuse is lit, it ignites the bursting charge, causing the shell to explode and light up the sky.

Aerial shells detonate in two phases. First, the shell must be launched into the air so that the lights explode above the crowd, not on the ground. This is like launching a rocket—an explosion is needed on the ground to send it upwards. To achieve lift-off, a tube called a mortar is positioned on the ground or partially buried in sand. Inside the mortar, gunpowder with an attached fuse is placed. The shell rests on top of the gunpowder. When the fuse is ignited, the gunpowder detonates, creating enough heat and gas to propel the shell into the sky [source: SciBytes].

In just a few moments, the shell reaches a sufficient height for the time-delay fuse inside to ignite, triggering the bursting charge to explode. This explosion sets off the gunpowder, causing the entire shell to burst, scattering the stars in all directions and creating the stunning shapes and lights of the fireworks we admire [source: SciBytes].

While every component of the aerial shell plays a vital role in producing the breathtaking fireworks displays we cherish, it is the stars that are responsible for the "oohs" and "aahs" of captivated onlookers. The chemical composition of the stars—an oxidizing agent, fuel, colorant, and binder—along with their arrangement in the aerial shell, forms the distinctive shapes and colors. Pyrotechnicians carefully select chemicals—strontium carbonate for red, calcium salts and calcium chloride for orange, salt for yellow, and barium compounds with chlorine for green. Blue, the most challenging color to achieve, is made from copper compounds and chlorine, crafting stars that will generate the specific shapes as the fireworks bloom in the sky [source: De Antonis].

To create the shapes, stars are positioned on a piece of cardboard in the desired pattern. For example, if the stars are arranged in a smiley face shape on the cardboard, they will explode into a smiley face in the sky. In fact, it’s possible to see several smiley faces simultaneously. With shaped fireworks, pyrotechnicians often launch multiple shells at once to ensure the shape can be seen from all angles. Spectators who have the cardboard aligned with their line of sight may only witness a burst of light or stars, rather than the full shape. However, with several shells fired at once, one should be oriented properly when it bursts, allowing the crowd to view the intended shape [source: Wolchover].

Julie Heckman, executive director of the American Pyrotechnics Association, states that the first fireworks with specific shapes were introduced in the early 1990s in Washington, D.C., to honor the returning Desert Storm troops. The inaugural shapes included purple hearts and yellow bows [source: Wolchover].

Shapes are also created for what are called set pieces—fireworks that remain on the ground and form images like an American flag. These set pieces are made using differently colored lances—tubes approximately four to five inches (10 to 12 centimeters) long—filled with chemicals that burn for about a minute. The lances are arranged into frames or boards and lit simultaneously to create any shape imaginable, from flags to words. Set pieces are costly to make and labor-intensive, which is why they are less common than aerial fireworks [source: Fireworks Alliance].

So, the next time you relax and enjoy the light display in the sky, think about the effort, creativity, and innovation behind those fireworks. And who knows? Perhaps next Fourth of July, you'll be treated to a unique, patriotic spectacle across the night sky—maybe even set to the stirring sounds of "The Stars and Stripes Forever" or the 1812 Overture.