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On the same night each year, millions of Americans gather to gaze at a spectacle of lights. Yet historically, this most American of events--Fourth of July fireworks--is oh so Chinese. And to science, it's simply the power of powder.
Some Fireworks History
Legend has it that more than a thousand years ago, a cook in China accidentally mixed saltpeter (used in ancient times as a food preservative) with sulfur and charcoal (probably fuel to cook a meal) to come up with "black powder."
The gastronomic applications were pretty limited, but the Chinese quickly learned that stuffing black powder into hollow bamboo and sealing both ends could produce an explosion. And leaving one end of the bamboo open created a bamboo rocket! Soon folks were using fireworks to celebrate holidays and great events throughout the East.
Fireworks arrived in Europe in the 13th century and quickly became the hallmark of kings and queens, who demonstrated royal grandeur with dazzling displays. During the Renaissance, fireworks became more available for the masses to view, and the Italians became undisputed masters of the art.
Still, early fireworks lacked a certain flair. Bright as they were, they didn't have any color. Italian pyrotechnicians solved this problem in the 19th century by adding potassium chlorate to black powder, allowing it to burn hotter. The hotter burn could ignite metal salts, which blazed in a brilliant array of colors.
Some Fireworks Science
Starting with the first flare of a lit fuse, all fireworks use a self-sustaining reaction of fuel in the presence of oxygen--a chemical process known as combustion. Today's fuel is essentially the same one discovered a thousand years ago: black powder. Composed of 75 percent potassium nitrate (saltpeter), 15 percent charcoal, and 10 percent sulfur, black powder has three chemical properties that give it maximum bang for the buck: high incandescence, low explosive power, and low heat of ignition.
The combustion of any material generally releases energy in one of two forms: heat or light. Different substances give off differing amounts of these energies when they burn. Some burn at much hotter temperatures than others; some generate almost no light. The Chinese initially liked black powder because it burns very brightly, a quality now known as high incandescence.
Yet despite the brilliant bang, black powder possesses low explosive force, which makes it more suited for entertainment than destruction. High explosives, like dynamite, produce much more force, but are nowhere near as spectacular to look at.
Black powder's low ignition point--about 620 degrees Fahrenheit--is double-edge sword. It makes fireworks easier to ignite than most other explosives, but it also makes fireworks less safe, and many laws regulating the construction and display of fireworks are now in place to prevent accidents.
Today's Big Bangs
The fireworks used in today's big public displays combine multiple explosions for maximum effect. Big fireworks are contained in a casing, or shell, which is generally made of paper, sometimes with string, that is rolled into tubes or spheres. Packed inside the shell are dozens, sometimes hundreds, of small black balls called stars. Made of black powder, metallic salts, and binding agents to keep them intact, these stars produce the brilliant flashes of light in a fireworks display.
All around the stars is a bursting charge composed of black powder. Attached underneath the shell is another chamber of black powder called the lift charge, which hurls the shell upward when it ignites. Fuses, generally made of string laced with more black powder, lead to the bursting charge in the shell and to the lift charge below.
The entire shell is fit tightly into a mortar--a steel tube placed upright and packed in sand. A fast-acting side fuse ignites the lift charge underneath the shell, exploding it and forcing the shell upward through the opening of the mortar. Simultaneously, a slower time-delay fuse begins to burn into the shell. Once the shell reaches its maximum height, the bursting charge explodes, hurling colorfully glowing stars into the sky. By arranging the stars in different ways, today's pyrotechnicians can wow with dazzling effects.
From: KnowledgeNews
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