Fireworks: A field day for applied science

How chemistry, physics and engineering light up the skies on the Fourth of July

jbordsen@charlotteobserver.comJuly 1, 2012 

When you get all choked up watching Fourth of July fireworks, save a little of that ooh-ahh emotion for chemistry and other scientific disciplines.

Bassam Z. Shakhashiri knows all about this: He’s a professor of chemistry at the University of Wisconsin and is the president of the American Chemical Society. Shakhashiri is also an entertainer by choice, giving lectures and programs around the world that help better connect people with the often obtuse world of science.

This interview with him is augmented with information from his engaging and reader-friendly Web site, http://scifun.chem.wisc.edu.

Q: How many sciences are involved in fireworks?

There’s chemistry, physics, engineering and the art in putting the rocket packets together; there’s computer science in firing off each packet. There’s the science that goes on inside our brains as we design them and set them off; there’s science in how we’re captivated by the loud sounds and beautiful things we see.

Each firework launched into the sky is a precisely formed assembly of chemicals and fuel, carefully calibrated to produce a particular effect – a red chrysanthemum spray accompanied by a powerful explosion, or a blue strobe, for example. Understanding how the contents of a firework produce the impressive variety of colors, forms, and sound intensities requires only a simple understanding of chemical reactions.

Q: How old is fireworks science?

It goes back to the Chinese, who mixed charcoal with different chemicals to produce loud sounds more than 1,000 years ago. They had recipes for black powder – coal – and frankly, that hasn’t changed much.

This little-changed formulation explodes at a rate of about 3 meters (9.84 feet) per second. When it burns in the open air, black powder’s heat and gas dissipate quickly. The key to fireworks’ success is to trap the heat and gas in the bottom of the shell, which is positioned in a launch tube or mortar, until the trapped gas pressure builds to such a force that, when it escapes, it hurls the firework high into the air.

Making fireworks one by one is still an art based on ancient science. But setting them off in a choreographed manner, to control the enjoyment of vivid colors in the sky or to set the explosions to music, shows a different aspect of ingenuity.

Q: How large are the firework shells used in public displays?

Some are about the size of your fist, some three or four times that large.

Q: How exactly do they work?

The shells hold a combination of charges.

One, with slow ignition, lifts the firework to the sky; it propels the firework but does not detonate it. You want slow ignition so the firework will explode farther from the ground.

Then you have the “star” components – they’re about the size of big marbles and hold metal salts that give off the different colors. Red, for example, can come from strontium salts; orange from a combination of red salts and sodium chloride – ordinary table salt.

The black powder lift-charge is calculated to exhaust itself precisely when the slow-burning, time-delay fuse reaches the first compartment packed with light-producing stars and black powder. This happens when the firework is at the very apex of its upward flight. Simultaneously the fuse sets off sound-producing explosives and detonates the stars, initiating color emission.

So you have a chemical ignition taking place in the sky that releases energy in the form of sound and light.

Energy is being released faster than it can be dissipated in the air; this creates a small shock wave, and that’s what you hear.

An explosion is a combustion reaction that has gone out of control – and that’s what this is. Compare that with lighting a candle or using a propane torch: Those are controlled combustion reactions.

Q: How a firework looks when it explodes is controlled, right?

You can control that to some extent by the ration of ingredients you put in it.

When an aerial firework explodes, its component stars fly off in all directions. However, when viewed from a distance, these aerial fireworks seem flat, as though they were displayed on a screen.

We see the explosions much sooner than we hear them. That happens because light travels about a million times as fast as sound.

The speed of light is 300,000,000 meters per second, but the speed of sound is only about 340 meters per second.

If you are watching fireworks that are about a kilometer (0.62 miles) away, the light takes only 3 millionths of a second to reach you. The sound takes about 3 seconds.

You can tell how many kilometers away fireworks are exploding by starting to count seconds as soon as you see an explosion. Stop counting when you hear the explosion and divide the count by three. This gives the distance away in kilometers.

Q: How does weather factor into fireworks?

With rain, fireworks won’t lift off and won’t explode in the sky. When you strike a wet match you can’t make it light, right? That’s part of the reason they’ll call off a fireworks show due to rain. The other issue is lightning or thunder that could impact the audience.

Clouds make a difference only if they obstruct your view, that’s all.

Q: How does safety factor into fireworks?

Safety is paramount; that’s why only professionals should set them off.

That’s where computers come into play: The combustible materials are set off electrically, by computer protocol. These are explosives, and should not be set off by people standing right there. That’s why we sit away from where they are set off.

There’s also a bit of fallout from fireworks – the smoke clouds. All these hold metal salts, and we have to be careful not to be where they’re falling because they could have undesirable health effects. The red strontium – which gives fireworks a red color – is something nobody should breathe.

Bordsen: 704-358-5251

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