The scientific flash behind the fireworks

As you ooh and aah at the dazzling explosions of a fireworks display, there are three things going on that you probably wouldn’t guess: The chemists who made those pyrotechnics designed most of them so they wouldn’t explode, you’re actually seeing nature conserving energy, and most peculiar of all, when things are at their flashiest, you’re actually seeing the fireworks as they’re cooling down.

The rockets' red glare, and all those bombs bursting in air, are the product of pyrotechnic chemistry that’s been refined ever since the Chinese first started using black powder for noisy fireworks to scare away evil spirits.

The basic ingredients in black powder, and all fireworks, are the same as they’ve always been: a fuel source and an oxidizer. The fuel’s job, like the wax in a candle, is to provide heat. The oxidizer is there to provide more oxygen that the ambient air can supply, to accelerate the reaction - to speed up the burning.

Slower is better
But there’s more to making a basic firework than putting the ingredients together. 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.They want it to burn for a bit so it gives a good visual show. To achieve the desired effect, the size of the particles of each ingredient have to be just right, and the ingredients have to be blended together just right.

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 good example of the fuel/oxidizer/sparkle combination is - duh - the sparkler. It’s made of medium-sized grains of fuel and oxidizer to get the fire going, mixed with even bigger grains of aluminum.

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.

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