Fireworks for the Fourth

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 a thousand years ago, a Chinese cook accidentally mixed saltpeter (used as a food preservative) with sulfur and charcoal (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 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. 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

All fireworks use a self-sustaining reaction of fuel in the presence of oxygen--a chemical process known as combustion. The 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. Black powder burns brightly, yet for all the brilliance, possesses low explosive force, which makes it ideal for entertainment (as opposed to serious destruction). Its low ignition point is a double-edged sword, making fireworks easier to ignite than most other explosives, but also making them less safe.

Today's Big Bangs

The fireworks used in today's big public displays combine multiple explosions for maximum effect. Generally, they're contained in a shell of paper rolled into a tube or sphere. 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 of more black powder. Underneath the shell is another chamber of black powder called the lift charge, which hurls the shell upward when it ignites. The entire thing fits snugly into a mortar--a steel or plastic tube placed upright and packed in sand.

A fast-acting fuse ignites the lift charge underneath the shell, exploding it and forcing the shell up through the top of the mortar. Simultaneously, a slower time-delay fuse begins to burn toward the bursting charge. 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 make shapes and dazzling effects, and earn our oohs and aahs.