SCOTT SIMON, host:
A fireworks display is usually the centerpiece of Fourth of July celebrations. NPR's Science Desk has prepared a primer on fireworks. Elaine, a little fireworks music, please.
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SIMON: Nah, that's not what I meant. What about something for Independence Day?
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SIMON: Better, better. So here then with a short course on the chemistry of fireworks is NPR's science correspondent Joe Palca.
JOE PALCA: If you want to talk to an expert on the chemistry of fireworks, you can't do much better than John Conkling. He's an adjunct professor of chemistry at Washington College in Maryland, and the author of a textbook on pyrotechnics.
Conkling says to make fireworks you need two chemicals.
Professor JOHN CONKLING (Washington College): One of your chemicals has to be oxygen-rich and give up that oxygen when it's heated to high temperature.
PALCA: You need oxygen for things to burn. The other chemical you need is a fuel to burn up. Or in the case of fireworks, explode. Gunpowder is popular is a popular fuel.
Prof. CONKLING: The fireworks that shoot up in the air and function have a bursting charge, which is usually traditional gunpowder, black powder, in the center of the canister or the sphere that you're shooting up in the air.
PALCA: And surrounding that bursting charge are pellets of more fuel and those oxygen-containing compounds called oxidizers. When the bursting charge explodes, it ignites these pellets, turning them into bright displays of stars that light up the night sky.
Prof. CONKLING: Finding the perfect combination of an oxidizer and a fuel, and then finding the perfect weight ratio of those two chemicals to give the exact type of burning effect youre looking for can take a lot of time in the lab to come up with.
PALCA: And thats where the artistry comes in. Conkling says certain elements are almost always used to produce certain colors. For example, yellow.
Prof. CONKLING: Sodium.
Prof. CONKLING: Barium or boron.
Prof. CONKLING: Strontium.
Prof. CONKLING: Copper.
PALCA: Cool. What am I leaving out?
Prof. CONKLING: Purple?
Prof. CONKLING: It's a combination of the strontium and copper, the red and the blue combined to create a purple color.
PALCA: I've always wondered how they get those displays that burst in one color and then change to another. So I asked Conkling.
Prof. CONKLING: That's one of the real secrets of the industry.
PALCA: The secret is you make a pellet with two layers; an inner layer of, say, sodium, and an outer layer with boron. The outer layer burns first, giving green stars. And you get the yellow stars produced by the sodium when the outer layer burns away.
Should have thought of that. And what about those really loud fireworks?
Prof. CONKLING: It's a fireworks shell called a Salute.
PALCA: It usually has a different kind of fuel than the colorful displays.
Prof. CONKLING: A lot of metal fuel: aluminum powder.
PALCA: Aluminum powder with the right oxidizer burns very fast and very, very hot. Put that in a fireworks casing, ignite it, and it explodes violently.
Prof. CONKLING: Much, much more violently than gunpowder explodes. So the concussion you get from the salute is definitely in a class by itself.
PALCA: Imagine a job where you get to explode things for a living. Conkling says being a pyrotechnic chemist is the best deal going.
Prof. CONKLING: Experiments are so much fun that it made other types of chemistry a little bit dry as a result.
PALCA: Well, I dont see why Conkling should have all the fun.
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PALCA: Joe Palca, NPR News, Washington.
(Soundbite of sirens)
PALCA: Oh-oh. We're in trouble now.
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SIMON: You're listening to WEEKEND EDITION from NPR News.
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