The science of fireworks: Explaining the ‘oohs’ and ‘aahs’

Ever wondered how pyrotechnicians make explosions in the shape of hearts or smiley faces? Or what makes Fourth of July fireworks red, white and blue?

To learn about the science of fireworks, WHYY Health and Science reporter Carolyn Beeler spoke to John Conkling, an emeritus professor of chemistry at Washington College in Maryland and former executive director of the American Pyrotechnics Association.

Anatomy of a firework

A professional-grade firework shell consists of three main components: the container, a cardboard tube or sphere ranging in size from a baseball to a basketball, a bursting charge that explodes the shell, and pellets packed with chemicals that produce color and sparkle.

The shell is attached to a cup at the bottom containing the propelling charge and a fuse or wires coming out of it.

If you cut the shell in half, you would see the large ‘bursting charge’ in the center, usually made of black gunpowder in a paper wrapper, surrounded by pellets ranging in size from a pea to a golf ball.

An internal timing fuse runs from the propelling charge on the bottom of the firework up through the bursting charge. When the external fuse is lit and the propellant shoots the firework into the air, that second internal fuse is ignited at the same time. When the shell reaches several hundred feed in the air, the internal fuse burns into the center of the shell, activating the bursting charge and launching ignited pellets into the sky.

A pyro’s palette

The color and sparkle of fireworks are created as compounds in the pellets cool down.

As energy from fire is transferred to the atoms of certain chemicals, the energy excites their electrons into higher energy states, moving them farther from the nucleus. As the electrons cool down and return to their normal position, that energy is converted to radiation, producing visible light.

Strontium makes red, barium green, and a copper compound creates blue.  Conkling called that color “elusive” in the world of fireworks.

“Blue is tough, you have to have just the right temperature of your flame while that material’s burning to be able to get a blue,” Conkling said. “If it’s not hot enough you don’t get the blue, if it’s too hot the blue washes out.”

Conkling said pyrotechnicians use a relatively established color palette, as a limited number of materials meet requirements for environmental impact, toxicity and cost.

“It’s very rare that a new chemical or color comes along,” Conkling said.

Smiley faces, hearts, stars, oh my!

In a traditional firework, the pellets are scattered symmetrically around the bursting charge within the shell to explode into a sphere.

To create fancier shapes, firework manufacturers paste the pellets on a sheet of paper in the correct pattern. Then they put a bursting charge above and below the paper. When the charge goes off, it burns off the paper and sends the ignited pellets off in the same, ever-widening pattern.

Creating a crowd favorite – the weeping willow

The weeping willow-like firework is created by using relatively small pellets packed with long-burning chemicals. The pellets doesn’t just flash, they burn for a few seconds creating a long, sparkly tail. Magnesium-aluminum alloys are mixed in with copper oxide to get the crackling noise created as the sparks shower to the ground.

“It works beautifully in that willow, it just overwhelms your senses as you see that just falling and crackling as everything comes to the ground,” Conkling said.

Making a firework whistle

To make a firework whistle as it flies up into the air, firework manufacturers pack chemicals (usually several potassium compounds) into a pencil-thin tube. As each layer ignites and burns, it releases a puff of gas, mostly carbon dioxide, that is forced out of the tube to create the whistling sound.

Conkling said the combination of materials originally was studied for use as a new propellant until researchers discovered it worked better as a noise-maker.

New trends in firework displays

The next big thing for fireworks shows is increasing coordination with musical scores, said Conkling. 

Computer firing technology has allowed fireworks artists to synchronize fireworks with dips and crescendos in music, or specific lyrics. For example, a round of red fireworks can be timed for the ‘rockets red glare’ line in the Star-Spangled Banner.

Conkling believes this kind of choreography will get better and better as technologies evolve.