How Do Fireworks Get Their Shapes and Colors?

Whether you’re celebrating the Fourth of July or enjoying the grand finale at a theme park, firework displays always seem to have a few tricks up their sleeves. 

One moment, a burst of light blooms into a giant chrysanthemum overhead. Maybe an American flag even waves in three sparkling colors. These days, fireworks practically turn the night sky into an illuminated art gallery. Considering that fireworks are essentially controlled explosions, that’s pretty remarkable. Explosions aren’t exactly known for their precision. 

To shape and mold these wild bursts into recognizable configurations — to make the fire work, so to speak — technicians must keep this old maxim front of mind: Timing is everything.

You’re Literally Seeing Stars

The secret to a firework’s design begins long before the explosion leaves the ground. The large fireworks launched in professional displays are called aerial shells. Inside each shell are dozens — sometimes hundreds — of small pellets known as “stars,” the tiny pyrotechnic building blocks at the heart of the show. These stars are arranged around black powder (aka gunpowder) and contain various metals or chemicals that produce light and color when ignited.

After an initial charge fires a firework into the air, a time-delay fuse allows it to climb high into the sky before a second charge explodes. That explosion ignites the stars and sends them flying out into the night. If the stars are arranged randomly inside the shell, the result would be a simple spherical burst.

But pyrotechnicians (the experts who create fireworks) can also arrange the stars in specific patterns before sealing the shell. Stars placed in the shape of a heart, for example, will spread outward in a heart-shaped pattern when the shell bursts. The same principle can be used to create smiley faces, hearts, and more complicated designs, like ringed planets and butterflies.

The brilliant colors that make us “ooh” and “aaah” are carefully engineered as well. Different metallic salts and other chemicals in the stars produce different colors when heated. Copper compounds and chlorine make blue hues, which technicians consider the toughest color to create. That’s because the temperature needed to achieve the color has to be just right — burning not too hot, but not too low.

Magnesium makes white. Sodium creates yellow, and barium compounds go green. It’s not much different than how children’s chemistry sets use chemical reactions to create a rainbow of different-colored liquids, though it is more dangerous.

By combining stars made of different elements inside a mortar shell, designers can create multicolored patterns. Fireworks can even change color in mid-air when the stars are coated with different layers of chemicals — an outer layer of sodium, say, might burn off to reveal barium, changing from yellow to green as the different metallic colorings are revealed.

Modern fireworks shows, such as the epic bashes your city might mount for the Fourth of July, add yet another layer of choreography. Rather than lighting shells by hand, many professional displays use computerized firing systems. These systems can launch hundreds of aerial shells with split-second precision, allowing pyrotechnicians to synchronize explosions with music and create elaborate sequences across the sky.

New designs are innovated often. A butterfly-shaped firework isn’t just dazzling to look at — it’s a great metaphor for fireworks as a whole, patiently waiting inside its shell before emerging as something new and beautiful.