The science of fireworks


Fireworks light up the night sky over Lake Mendota

Fireworks light up the night sky and reflect off the water over Lake Mendota//Photo by Jeff Miller, UW-Madison, 2012

It’s firework season in the United States. Independence Day. A celebration of our freedom and sovereignty as a nation.

Each year, we stand in awe beneath the brilliant light shows put on across the country, paying homage to our heritage and our history. But how often have you wondered how fireworks are made and how they work? How do we get the colors, shapes, sounds and more that manage to wow us year after year?

These are some of the questions that have also intrigued us, so we turned to the experts to learn more. Bassam Shakhashiri and Rodney Schreiner are chemistry professors who are passionate about science and for years have helped provide answers to these very questions to curious minds in Wisconsin. In fact, Shakhashiri – who founded the Wisconsin Institute of Science Literacy – has published a great piece on the chemistry of fireworks on his website, Science is Fun.

Here is what we learned:

  • Each firework launched into the sky is a precisely formed assembly of chemicals and fuel, carefully calibrated to produce a particular effect.

Fireworks produce three forms of energy: light, heat and sound. They are designed and built by professionals to rise into the air and explode at just the right time (or times!), producing the colors and sounds that will wow onlookers.

They consist of a fast-burning, low-explosion fuse that sends the firework skyward, exhausting the black powder (gunpowder) within the”charge lift” at the apex of the ascent. A slow-burning fuse simultaneously burns upward toward compartments within the firework that contain metal salts like sodium chloride (table salt!) and more black powder.

These compartments contain the stars, the 3-4 cm diameter clay-or-dough-like cubes packed with an oxidizing agent, a reducing agent, binding materials and the metal salts, which are responsible for giving fireworks their colors. When the fuse hits a compartment, it sets off a series of chemical reactions between the oxidizing and reducing agents, exciting the electrons within the metal salts and producing a colorful explosion. Each metal salt releases a specific amount of energy as their excited electrons relax, which is evident as visible light of particular wavelengths.

The energy and wavelengths of visible light

The energy and wavelengths of visible light

  • Firework technology hasn’t changed. Much.

Fireworks are “initially detonated electrically by a computer and there are fuses within each charge,” explains Schreiner. “The electrical spark sends the projectile up and simultaneously ignites a fuse that burns as it’s going up. It has the proper length so it reaches the center just when the charge reaches apex. There may be more fuses too; when that fuse goes off it sends stars flying out and each can have little fuses in them so when they get certain distance away, they also explode.”

“So, the technology has changed in the complexity. We now see things that actually create shapes, which, 25 years ago you wouldn’t have seen.”

Smiley face firework

Firework in the shape of a smiley face//Reddit

  • Building fireworks is dangerous, so robots now do some of the job.

“Many of the raw materials used requires blending chemicals together,” says Schreiner. “Much of this blending is now done robotically. A person puts the chemicals in and robots blend them together and put them into packaging. The final assembly is done by hand but can easily be set off by static electrical charge.”

Shakhashiri adds: “There are accidents that happen.”

“Approximately one person in fireworks manufacturing dies each year in the U.S., and there aren’t that many people doing it.” says Schreiner. “”It is dangerous.”

In fact, fireworks are best left to the professionals, who learn the trade through apprenticeship in a guild system. Shakhashiri’s website says that more than 8,000 people in the U.S. are injured by personal fireworks, nearly half of whom are children.

Sparkler

Sparkler//Wikimedia Commons

  • From a distance, fireworks always look like they’re coming toward you. If you could stand in the middle of a firework, it would be spherical.

“It has to do with how humans perceive distance,” says Schreiner. “When a firework explodes in the air against a dark sky, you have no cues in your vision to tell you distance.” Stars actually propel in all directions but the relative distance between you and them cannot be processed accurately by your brain, he adds. “So, your brain gives you the message that it thinks will be most valuable to you, and that is that there’s this bright stuff coming at you.”

“I’ve actually had the opportunity to be under fireworks while they’re going off and you can see it going out in all directions,” he says. “Newton’s Law says it has to.”

  • You can understand just how fireworks, well, work, and still be totally wowed by them.

Fireworks are becoming ever-more complex, Schreiner says, to create ever-more showy and elaborate displays. At Madison’s recent Independence Day celebration, Shake the Lake, he says he saw a variety of fireworks he’d never seen before, some of which he wondered: “How did they do that? I wouldn’t be surprised if there were professionals watching who said: ‘How did they do that?'”

The most impressive display he’s ever seen, though, was on January 1, 2000, at a show near James Madison Park in Madison. “They were setting off fireworks offshore and they were going up and we were at the top of the hill. We could look up at them and we could even feel how warm they were because it was January and it was cold outside.”

Chemistry professor Bassam Shakhashiri ignites a gas-filled balloon during a free demonstration about the science of fireworks held at the University of Wisconsin-Madison's Memorial Union Terrace

Chemistry professor Bassam Shakhashiri ignites a gas-filled balloon during a free demonstration about the science of fireworks held at the University of Wisconsin-Madison’s Memorial Union Terrace//Photo by Jeff Miller, UW-Madison, 2009

To learn more about Independence Day, the science of fireworks, and to witness cool fireworks-related experiments – like just how much energy in contained in a single flaming cheeto – attend Shakhashiri and Schreiner’s free, all-ages show: Science is Fun on Independence Day at the Memorial Union Terrace on Saturday, July 4, 2015 from 8:15 to 9:15 pm. The show will also feature music from Mike Leckrone and members of the UW Band, as well as special guests.

“It started a long time ago,” Shakhashiri says. “We do it because it’s part of the joy of sharing science and communicating science and further stimulating the senses and helping others and ourselves get a deeper appreciation of how combustion reactions can be done by professionals safely.”

“It’s fun in the best sense of the word.”