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WHAT'S THAT STUFF?
More than 1,000 years ago, most likely in China, someone made the serendipitous discovery that a mixture of sulfur, charcoal, and saltpeter (potassium nitrate) burned with startling speed and flash. The mixture, which eventually came to be known as gunpowder, was a Chinese mainstay for centuries, used in ceremonies to scare off evil spirits and even in military rockets.
Gunpowder made its way to Europe, probably during the early 1200s. During the Middle Ages, gunpowder-based creations--the precursor to modern fireworks--were limited to booms and a few sparkles, aided by a few iron filings or some copper or zinc. The repertoire of colors was that found in most campfires: oranges, yellows, and the occasional white-hot.
It wasn't until the 1800s that chemists began to use then-recently synthesized compounds that, in the right mixtures, burned in reds, greens, even blues and purples, and that the colors we traditionally ascribe to fireworks began to show up in night skies.
But with the exception of minor formula improvements, your fireworks colors have been the fireworks colors of your great-grandparents--until recently. Connoisseurs of Fourth of July displays may have noticed that over the past two decades, colors have gotten markedly more vivid. Even blue, the most difficult color of all to produce, has evolved from an anemic bluish white to something approximating an honest-to-goodness blue.
"People may think colors look brighter--well, they're correct," says John A. Conkling, technical director of the American Pyrotechnics Association and adjunct chemistry professor at Washington College in Chestertown, Md.
THE NEW BREEDS of fireworks colors operate by the same principles as fireworks colors in general. Certainly, one would never suspect that dazzling colors can spew forth when the dull gray pellets of mixtures are set aflame. So what is that stuff, anyway
The orangish hues of ancient fireworks are largely produced by black- or gray-body radiation--the glow of very hot solid particles. By contrast, the striking greens and reds in modern fireworks are the spectral emissions of excited gas-phase molecules. A few metal chlorides, which fluoresce strongly in the visible wavelengths, are the basis for almost all the colors in modern fireworks.
Barium chloride produces green; strontium chloride produces red; copper chloride produces blue. The problem is, these compounds are so hygroscopic that they render any mixture damp, unburnable, and even unstable. The solution to this problem has been to bring metal and chlorine together in a vapor during the burning process, where the energy from the burning can then excite the molecules' electrons, producing the colorful emissions.
A typical fireworks color burning mixture consists of, in addition to the requisite fuel and oxidizer, a compound containing one of the metals and a chlorine-donating compound. The mixture is wetted down to bind it together, then cut into flammable chunks known as stars--the colored dots that burst from a fireworks shell into the sky.
Old books on pyrotechnics are chock-full of recipes for stars, formulas that enthusiasts have been continually refining over the decades. In fact, Conkling says, little professional research is devoted to fireworks: Most developments in fireworks formulas stem from experiments by amateurs.
During the early days of fireworks colors, stars were made with potassium chlorate, KClO3, which serves as both an oxidizer and a chlorine donor. But KClO3's unfortunate propensity for forming friction-sensitive compounds when it comes in contact with sulfur, metal powders, ammonium salts, or moisture has caused more than a few deadly explosions. Consequently, it's rarely used in display fireworks anymore. Nowadays, most star formulas use potassium perchlorate (KClO4). Such stars are harder to ignite but are not nearly as unstable.
Potassium chlorate wasn't the only hazard fireworks makers used to face. Numerous chemicals were quite useful and had picturesque names but were also deadly poisons: Paris green (copper acetoarsenite), calomel (mercurous chloride), and realgar (arsenic sulfide).
Barium chlorate--which has its metal, chlorine donor, and oxidizer built into one compound--produces a brilliant deep green that many fireworks experts believe is unparalleled. But again, the compound suffers from chlorate's notorious instability. Now, fireworks makers rely on compounds such as barium nitrate, strontium carbonate or nitrate, sodium oxalate, and copper carbonate.
BLUE HAS ALWAYS presented a special problem for fireworks designers, because copper chloride doesn't survive well in a hot flame. But a big advance in fireworks colors has come in recent decades, with the use of a magnesium-aluminum alloy known as magnalium. Stars made with magnalium burn electric, almost fluorescent, green, red, yellow, and comparatively decent blue and purple. By themselves, magnesium and aluminum make silvers and sparkles and act as a fuel. The high heat generated by metal fuels can also increase the intensity of the colored molecular emissions. But the incandescence from the metal particles is usually so brilliant that it overwhelms the color.
Magnalium, however, still gets the flame hot without washing the color out. How this happens isn't exactly known, Conkling says. But one possibility is that the metal somehow forms vaporous species in the gas phase, and so doesn't incandesce.
Even with magnalium, though, a great blue continues to be fireworks makers' dream, one that they'll keep chasing. "Blue is still the weakest link in the whole fireworks picture," Conkling says.
Editor's Note: In addition to being C&EN's West Coast associate editor, Wilson is a pyrotechnician, licensed in the state of California.
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