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  The surprisingly scientific flash behind the fireworks
  By David Ropeik
  3/19/2012 5:18:14 PM

  As you ooh and aah at the dazzling explosions of a fireworks display, there are three things going on that you probably wouldn’t guess: The chemists who made those pyrotechnics designed most of them so they wouldn’t explode, you’re actually seeing nature conserving energy, and most peculiar of all, when things are at their flashiest, you’re actually seeing the fireworks as they’re cooling down. The rockets' red glare, and all those bombs bursting in air, are the product of pyrotechnic chemistry that’s been refined ever since the Chinese first started using black powder for noisy fireworks to scare away evil spirits. The basic ingredients in black powder, and all fireworks, are the same as they’ve always been: a fuel source and an oxidizer. The fuel’s job, like the wax in a candle, is to provide heat. The oxidizer is there to provide more oxygen that the ambient air can supply, to accelerate the reaction — to speed up the burning.

But there’s more to making a basic firework than putting the ingredients together. Good visual effects come from a slower reaction. Pyrotechnic chemists, who are trying to create bedazzle instead of bang, don’t want their work to explode.They want it to burn for a bit so it gives a good visual show. To achieve the desired effect, the size of the particles of each ingredient have to be just right, and the ingredients have to be blended together just right. To slow down the burning, chemists use big grains of chemicals, in the range of 250 to 300 microns (the size of a small grain of sand), and they don’t blend the ingredients together very well. That makes it harder for the fuel and oxidizer to combine and burn, and produces a longer and brighter effect. For the really sparkly parts of fireworks, they use even bigger grains, roughly 1,000 microns in size, which are ignited by the black powder fire around them and combine with the air to burn with a spark effect. A good example of the fuel/oxidizer/sparkle combination is — duh — the sparkler. It’s made of medium-sized grains of fuel and oxidizer to get the fire going, mixed with even bigger grains of aluminum. When ignited, those grains burn in combination with the oxygen in the air, giving off the sparks. Aluminum burning at 2,700 degrees Fahrenheit (1,500 degrees Celsius) produces golden sparklers. At hotter temperatures, up to 5,400 degrees F (3,000 degrees C), the aluminum produces white sparks.

Well, so much for the basics. Now what about color? There are other chemicals used to produce colors, but they all do their dazzling thanks to the first law of thermodynamics: Nature conserves energy. Energy from the fire in the basic fuel is transferred to the atoms of the colorant chemicals. That excites the electrons in those chemicals into a higher energy state. The electrons literally orbit further away from the atom’s nucleus. Then, as they cool down, they move back to a lower state of energy. But remember, nature conserves energy. Energy is never lost, it’s just transferred somewhere else. As the electrons “calm down,” the energy they give up is converted into radiation. Light. That’s where the light of fireworks comes from. You actually see the colors in fireworks as they’re cooling down. The signature chemicals in fireworks each emit light at a specific wavelength, producing a specific color: strontium equals red ... copper equals blue ... barium equals green ... sodium equals yellow/orange. Just as you could combine crayon colors when you were a kid, combining the colorant chemicals can give you additional colors. Strontium (red) plus copper (blue) equals purple. The chemists produce little pellets of colorant chemicals, the size of sugar cubes, with a mixture of colorant and basic fuel blended to the right degree, and with the right-size particles so the pellet will burn at the desired rate. Then technicians can calculate how high they have to shoot their shells so they’ll be done burning before the pieces get back down to the ground.

Design artists then figure out how to get the fireworks to explode in shapes, and with sounds. The familiar whistling sound is easy. They pack some basic fuel into a cardboard tube, open on one end. As the fuel burns down inside the tube, the carbon dioxide it gives off rushes out the open end, making a whistling sound. It’s like when you whistle by blowing air out between your pursed lips. The shapes of the exploding lights depend on how the basic fuel and colorant pellets have been packed. If the explosive charge in the shell is in the middle, surrounded by a ring of pellets of sodium, when the timer fuse sets off the explosion, it ignites the sodium pellets and shoots them out into that familiar nice round yellow/orange circle. Two rows of colorant pellets around a central “bomb” gives you a double ring. If the inside of the shell is a mix of basic fuel and colorant all interspersed, the explosion ignites the colorant pellets that then spread out and fall down in a shower, producing a glowing willow tree pattern. To get the really tricky shapes, like stars or hearts, the colorant pellets are pasted on a piece of paper in the desired pattern. That paper is put in the middle of the shell with explosive charges above it, and below. When those charges go off, they burn up the paper, and send the ignited colorant pellets out in the same pattern they were in on the sheet of paper, spreading wider apart as they fly. Consumer-level fireworks, which are legal in most states, are made of the same chemicals the commercial shows use. The fireworks industry says that U.S. consumption of commercial fireworks (the big outdoor shows) plus consumer fireworks was nine times higher in 2007 than it was in 1976. Over that same time period, the estimated number of fireworks-related injuries actually went down by 12 percent. But then there was the guy who stuck a lit firecracker up his nose! Honest. Or the folks who burned their house down using fireworks indoors. So if you’re going to use “home” fireworks, remember: Knowing the science behind how they work doesn’t mean they’re risk-free. Be careful while you enjoy the show.





http://www.msnbc.msn.com/id/3077329/ns/technology_and_science-science/t/surprisingly-scientific-flash-behind-fireworks/



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