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By Sharon Begley
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They were the original peace dividend: the first fireworks, blasted off after a victorious battle in 11th-century China when jubilant soldiers turned their rockets and gunpowder to less belligerent use. The 25 million Americans expected to ooh and aah at Fourth of July spectacles this week will see dazzling aerial displays of light and color that those ancient pyrotechnicians never dreamed possible. Rather than monochromic showers of sparks, the shows will splatter brilliant reds, greens and blues across the sky in tightly choreographed arrays of hearts, stars, letters and even bow ties. It's all courtesy of researchers who are unraveling "the physical processes [behind] the dramatic colors and special effects" of fireworks, writes chemist John Conkling of Washington College, in an article on firework science in the current Scientific American.
Some of the most brilliant advances in fireworks have been in colors. At the premiere of Handel's Fireworks Suite in 1742, the only hues were amber and silver, emitted by the burning black powder that both propels the firework skyward and ignites it. But modern pyrotechnicians have expanded their palette through sophisticated chemistry. They know that strontium chloride, for example, produces red and barium chloride makes green, explains Felix Grucci, president of Fireworks by Grucci in Bellport, N.Y., best known fAr it's centenary shows at the Statue of Liberty and Brooklyn Bridge. But theoretical knowledge isn't enough: these color-emitting compounds are unstable, and won't last in the shells until showtime. So pyrotechnicians are perfecting remote-control chemistry. They pack their shells with "stars"--pellets of color-producing compounds--made of pure barium, for instance, and nearby place chemicals that release chlorine when they get hot. When combined in midair, chlorine and barium produce green. Blue is the ultimate pyrotechnic challenge. The copper chloride that produces it decomposes if the firework is too hot. It takes precise control of the black powder to yield deep blue, a sign of an expert pyro-artist, says George Zambelli, who leads the family firm in Pennsylvania.
The real secrets in the $200 million-ayear American fireworks industry are recipes for packing the shells. "People would kill to know the chemistry end geometry of 'stars'," says Petr Spurney, managing director of Ruggieri USA, the American branch of one of the oldest fireworks makers. Until a few years ago pyrotechnicians simply stuffed stars into their paper shells randomly, and when black powder burst the shell open the result was the familiar pointillist pattern of bright sparkling color. Some new arrangements allow a single shell to yield sparks that change color in midflight (diagram). Others create images that seem to be painted on the sky.
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The Japanese have pioneered a bow-tie shape by packing stars only along the sides of shells so the explosion propels colorful sparks in lines radiating from a central-point. They have also figured out how to glue stars in a row around the circumference of a large round shell to produce a l single perfect circle. A Guy Fawkes Day I celebration in England last year dazzled I the audience with spiraling fireworks, created by packing stars into a paper tube with I an opening at one end and a bend in the I middle. And selectively weakening parts of the cardboard shell creates a five-pointed star: stars bursting through the weaker parts of the wall shoot farther than those crashing the stronger parts, explains Conkling. "It suggests that just about any geometry is possible," he says. In the works: the five-ring logo of the Olympics, which might be ready by the 1992 Games--if pyrotechnicians can keep their stars firmly hitched to the latest in chemistry.
The flash-sound mixture is released last.
Blue stars are released next
Red stars are released when bursting charge is activated by time-delayed fuse.
Black powder is ignited, propelling the shell skyward
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