Thursday, July 03, 2008

The Science Behind The Fireworks

While we sit and watch the fireworks this fourth of July we often over look the amount of science involved in getting those brilliant colors and perfect effects. Fireworks have been around for thousands of years, and while little has changed in the basic formula we have added some modern ingenuity to those time tested recipes.

A basic firework is comprised of two things, a fuel source and an oxidizer. The fuel’s job is to provide the heat. The oxidizer's job is to provide more oxygen to accelerate the reaction - to speed up the burning. However good visual effects come from a slower reaction. Pyrotechnic chemists, who are trying to create bedazzle instead of bang, don’t want their work to explode an explosion provides little visual effect.

Think of it this was, a firecracker only gives of a boom, you might see a small flash but no actual show. Pyrotechnics, such as fountains, or the larger mortars that are shot into the sky, are designed to slow down that reaction providing a controlled burn of sorts. Controlling the burn rate creates the dazzling effects you see in the sky.

To slow down the burning, chemists use big grains of chemicals, in the range of 250 to 300 microns (the size of a small grain of sand), and they don’t blend the ingredients together very well. That makes it harder for the fuel and oxidizer to combine and burn, and produces a longer and brighter effect.

For the really sparkly parts of fireworks, they use even bigger grains, roughly 1,000 microns in size, which are ignited by the black powder fire around them and combine with the air to burn with a spark effect.

A perfect example of the fuel/oxidizer/sparkle combination would be the sparkler. Sparklers are made of medium-sized grains of fuel and oxidizer to get the fire going, mixed with even bigger grains of aluminum (the aluminum is what gives the great spark).

When ignited, those grains burn in combination with the oxygen in the air, giving off the sparks. Aluminum burning at 2,700 degrees Fahrenheit (1,500 degrees Celsius) produces golden sparklers. At hotter temperatures, up to 5,400 degrees F (3,000 degrees C), the aluminum produces white sparks.

Producing the right effect

Its the signature chemicals in fireworks that create the difference in colors. Without these specialized chemicals we'd be looking at gold or white fireworks every time. Different chemicals emit light at a specific wavelength, producing a specific color: strontium equals red ... copper equals blue ... barium equals green ... sodium equals yellow/orange. Mixing those basic chemicals, much like mixing paint can give you additional colors. Strontium (red) plus copper (blue) equals purple.

The chemists produce little pellets of colorant chemicals, the size of sugar cubes, with a mixture of colorant and basic fuel blended to the right degree, and with the right-size particles so the pellet will burn at the desired rate.

When producing mortars design artists have to figure out how to get the fireworks to explode in shapes, and with sounds. Those screaming, whistling (annoying fireworks) are easily produced. They pack some basic fuel into a cardboard tube, open on one end. As the fuel burns down inside the tube, the carbon dioxide it gives off rushes out the open end, making a whistling sound. Essentially its the same effect as blowing through a whistle.

The shapes of the exploding lights depend on how the basic fuel and colorant pellets have been packed. If the explosive charge in the shell is in the middle, surrounded by a ring of pellets of sodium, when the timer fuse sets off the explosion, it ignites the sodium pellets and shoots them out into that familiar nice round yellow/orange circle. Two rows of colorant pellets around a central “bomb” gives you a double ring.

If the inside of the shell is a mix of basic fuel and colorant all interspersed, the explosion ignites the colorant pellets that then spread out and fall down in a shower, producing a glowing willow tree pattern.

To get the really tricky shapes, like stars or hearts, the colorant pellets are pasted on a piece of paper in the desired pattern. That paper is put in the middle of the shell with explosive charges above it, and below. When those charges go off, they burn up the paper, and send the ignited colorant pellets out in the same pattern they were in on the sheet of paper, spreading wider apart as they fly.

Source: - The scientific flash behind the fireworks

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