One of the oldest forms of chemistry — fireworks — today burns more spectacularly than ever, brightening the sky on the Fourth of July in a kind of pyrotechnical ballet, orchestrated by modern computer programming.
Why are fireworks burning brighter?
In the past, the colors were produced by igniting charcoal, starches and gums, while today these have been replaced by metal fuels, explains chemist John A. Conkling, Ph.D., of Chestertown, Md., former technical director of the American Pyrotechnics Association. "These metals produce hotter flames, which produce more photons of emitted light and a variety of more vivid colors," he said in a recent interview. For example, barium monochloride in a flame produces green; strontium monochloride produces red; and copper produces blue emission, according to Conkling.
Today, he adds, the colors are even brighter because of fine-tuning of the percentages of the ingredients used in the fireworks. Besides the fuel and oxidizer, a typical fireworks burning mixture consists of a compound containing one of the metals and a chlorine-giving compound, he explains.
The first fireworks had fewer ingredients, reports Chemical & Engineering News, the newsmagazine published by the American Chemical Society, the world's largest scientific society. To access a C&EN article tracing the beginning of this form of entertainment to the Far East go to http://www.cen-online.org and click on "What's That Stuff?" The site is a winner of a Scientific American Sci Tech Web Award for one of the best science Web sites for the public.
More than a thousand years ago, probably in China, someone discovered that a mixture of sulfur, charcoal and saltpeter (potassium nitrate) burned quickly and with a flash, the newsmagazine reports. The mixture, which later became known as gunpowder, was used in China for centuries to scare off evil spirits and, later, to fuel military rockets.
Gunpowder eventually made its way to Europe, probably during the early 1200s, C&EN says. During the Middle Ages, gunpowder-based creations — the precursor to modern fireworks — were limited to booms and a few sparkles helped along by iron filings or some copper and zinc. The colors were yellows, oranges and the occasional white-hot. It wasn't until the 1800s that chemists began to use synthesized compounds that burned in reds, greens, blues and purples.
The colors changed little until the late 1900s, when chemists began using today's metals, Conklin says. He also notes that besides the brighter colors "the synchronization of the displays are getting better and better as they are being tied in with music, using computer technology."
While fireworks have always entertained children and adults, because of the explosive nature of rockets and firecrackers there are, of course, accidents when fireworks are misused. And the possibility of injuries isn't the only potential hazard associated with fireworks, Conkling says. Perchlorates, the salts of strong oxidizers used to create vivid colors in the fireworks, may be contributing to environmental problems at manufacturing and disposal sites. However, he adds, when ignited during normal use in backyards or at large fireworks displays, fireworks compositions efficiently convert perchlorates to harmless chlorides. The industry has been concerned over the past few years with the persistence of perchlorates in the environment and it is now looking for alternatives like nitrates to produce the brighter colors, according to Conkling. Perchlorates, which also are found in rocket fuels and fertilizers, are "showing up in measurable levels in drinking water supplies, rather than reacting with oxidizable materials in the soil," he notes.
Some clinical trials have linked perchlorates to thyroid problems, although the level at which a health risk exists has yet to be firmly established," Conkling says. "While fireworks may be a very minor source of the perchlorate found in ground water, the industry is seeking alternatives as well as modifying manufacturing and disposal methods to minimize any contamination."
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