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The science behind the booms and blooms of fireworks for the 4th of July

Thousands watch the Fourth of July Fireworks Celebration in Boise, Idaho

The City of Boise's 10th Annual Fourth of July Fireworks Celebration was held at Ann Morrison Park in Boise, Idaho.
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The City of Boise's 10th Annual Fourth of July Fireworks Celebration was held at Ann Morrison Park in Boise, Idaho.

Fireworks were invented more than 1,000 years ago in China, shortly after the invention of gunpowder. Since then, the formula has changed a bit, but the basic principles remain the same: get something into the air and get it to explode in magnificent fashion.

Q: First things first: What is gunpowder?

Gunpowder, also known as black powder, is a key component in pretty much all fireworks. It is a combination of charcoal, sulfur and saltpeter. Historically, the best natural source of saltpeter, the common name for potassium nitrate, has been from caves caked with bat droppings. Luckily for us, we can now synthesize it.

Some of the earliest fireworks were simply gunpowder tightly bound so that it would explode with a loud pop when lit - the firecracker.

Q: Why do we have fireworks on July 4?

Fireworks are a patriotic tradition in many countries, but are closely associated with the American revolution. They were used in the very first Independence Day celebration, having been brought to America by the early settlers. No less than Founding Father John Adams declared that Independence Day “ought to be solemnized with Pomp and Parade. … Bonfires and Illuminations from one End of this Continent to the other from this Time forward forever more.”

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Founding Fathers John Adams (far left), Thomas Jefferson, Benjamin Franklin, Roger Sherman and Robert Livingston, from “Declaration of Independence” by John Trumbull Wikipedia

[ Your guide to Treasure Valley fireworks and other July 4 festivities ]

Q: Ok, we've established that fireworks are patriotic and cool. What’s inside them?

An aerial shell such as those used in large public displays of patriotism looks a lot like a cardboard cannon ball or a giant firecracker.

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Small aerial shells that are commercially available in some states. JOAN BARNETT LEE / jlee@modbee.com

Their internal structure is a source of fascination to many, especially scientists. “Anybody that’s a chemist always has a story in their past about explosions or fires or some kind of conflagrations that got them interesting in the field”, says Terry Gilton, clinical assistant professor of chemistry at BSU.

According to Gilton, a typical aerial shell is made of three basic parts, all tightly wrapped inside cardboard.

The main component is a dried paste made up of gunpowder or explosive resin and a flammable binding agent like dextrin, a carbohydrate made from starch. This material burns fast, creating the energy that blows the shell apart in the air.

Embedded in or around that paste -- like chocolate chips in cookie dough -- are the "stars." They are so named because these little pieces of explosive material are responsible for the vibrant colors you see when the shell explodes.

What starts the show is the bursting charge in the center of the shell. When it's set off by the timing fuse, it’s officially the 4th of July.

Q: So how does a firework launch and explode?

Fireworks are essentially low-tech missiles that are shot into the air with one charge, and then lit at their apex by another.

Aerial shells are launched from a mortar tube, triggered in modern times by a computer signal called a “cue.”

The process starts with the lifting charge. Once set off by the computer, that charge quickly generates a lot pressure due to the hot gas, propelling the shell into the air. The amount of gunpowder is specifically engineered to lift the shell to a predetermined height

The lifting charge has a second job, too: to light the timing fuse. This fuse is calibrated to ignite the bursting charge when the shell reaches its highest point.

The bursting charge ignites the explosive/binder mixture, rupturing the shell and sending the burning stars outward in all their glory.

Q: How do fireworks designers get those interesting patterns to form in the sky?

The shapes you see, from rings to blooms to more complex patterns, are all determined by how the stars are distributed inside the shell.

If they are evenly placed, for instance, you’ll get a big symmetric burst. If they’re placed in a ring around the bursting charge, they’ll create a glowing ring in the sky.

The patterns can also be influenced by adjusting the ratios of the chemicals involved, much like adjusting a recipe for a cake. This manipulation, says Gilton, can propel the stars out more or less quickly as well as changing how they burn. "You don't want it to explode, you want it to burn at a specific rate that you care about."

Q: What are these "stars"? How do they generate all those colors?

Those bright reds, stunning whites, and vibrant blues are generated by a variety of different chemical techniques, all of which use some kind of metal.

The stars, says Gilton, are typically made of an explosive and a metal salt. “If you want to see something, you need some sort of metal.” The metal salt most familiar to people is sodium chloride, normal table salt.

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James Neeley Western Display Fireworks

The most direct way of getting a color, especially a bright white, is by burning the metal, heating it and causing it to glow. This is called incandescence and is the way a traditional light bulb generates its light. For these whites, pyrotechnic chemists typically use something like titanium or magnesium.

To get the full range of colors, another process is needed. In this process, called luminance, the metal absorbs energy from the explosion, something called excitation, emitting a color of light specific to that metal. This is akin to how a firefly or a glow-stick generates its light.

Sodium chloride will emit a yellow color when excited. Replacing the sodium with other metals change the color. Calcium, for instance, will glow orange, barium green, and strontium red.

Blues and purples are notoriously difficult to create. Unlike warmer colors, there are not many materials that can be excited to yield blue light. You can get blue from copper, says Gilton, but moisture can ruin it. Care must be taken to shield it from moisture before launch. As with paint, purple can be created by mixing red and blue.

All sorts of interesting chemical mixtures have been tried, looking for new ways to generate color or to enhance and brighten existing colors. But in this ancient industry, cheaper is better and traditional metal salts are by far the most used. “Unless someone makes something that is as cheap and easily manipulatable … it’s probably not going to end up in fireworks,” says Gilton.

Q: What goes into making a firework show?

As much engineering goes into the display of fireworks as the fireworks themselves.

The first step in creating a show, according to Heather Gobet, owner of Western Display Fireworks that handles Boise's big show, is getting a musical score from the client. This score is analyzed for mood and fanfare. The entire show is then designed digitally, working out every effect.

The simplest effect is known as a "salute," a loud bang that is actually a sonic boom created by rapidly expanding gas. Firework manufacturers can enhance the sound by adding resin-soaked string to the outside of the shell, allowing more pressure to build. Some of the most complex effects are multi-layered aerial “cakes” with multiple explosions across the whole spectrum of color. Complex burn patterns can send the stars on twisted paths like crazed insects. These are called “go-getters” in the business. “Always seems to be a crowd-pleaser,” says Gobet.

Once the virtual design is completed, the software provides a video of the entire production for review.

As much engineering goes into the display of fireworks as the fireworks themselves. The first step in creating a show is getting a musical score. This score is analyzed for mood and fanfare and then the entire show is designed digitally.

Q: How is the show's design translated into what the crowd sees?

The design software provides the complex system of cues that will be used to launch the fireworks. Modern systems can coordinate thousands of cues essentially simultaneously.

The software also provides a series of physical tags that are used to mark each firework so that the technicians can assemble them on site. Technicians set up the mortars, place the shells and wire them to the computer control system based on the tags.

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Do not try this at home. Pyrotechnician Gary Louderback surveys his palletized mortars on a barge in Seattle. He led the crew that designed and performed the fireworks show at Tacoma Freedom Fair in 2014. Professional shows like Freedom Fair are the best way to enjoy July 4, as even legal fireworks shot off from your home driveway come with inherent dangers. Peter Haley News Tribune file photo

Q: What happens when a shell doesn’t launch?

According to Gobet, the “industry standard” is to expect about 3 percent of shells to be duds, especially in a complex firework show such as the City of Boise’s 12th Annual Fourth of July Fireworks Celebration in Ann Morrison Park.

After the show, a cool-down period of around 15 minutes is observed. After that, unlaunched shells are manually lit with a flare by technicians.

Fireworks are fun to watch but sometimes things can go wrong. Here are some examples.

Q: What precautions are taken to avoid fire?

The first line of defense here is in the design of the fireworks themselves. The rate at which the stars burn can be controlled by the chemical mixture, says Gilton. Part of the engineering of the show is making sure that the shells launch high enough that all the pyrotechnic reactions take place before the pieces reach the ground.

Western Display works closely with local fire officials, says Gobet, in the event that something goes wrong. Extinguishers and other equipment are available to the pyrotechnicians, and fire crews are on hand in case they have to do more than enjoy the show with the rest of us.

Kevin Davenport is an experimental physicist at the University of Utah and a 2018 AAAS Mass Media Science and Engineering Fellow with the Idaho Statesman: 208-377-6411, @tropnevaDniveK.

Things don't always go as planned. Fireworks accidents happen and can have serious consequences.

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