When we think of fireworks, we think of explosions. But surprisingly, much of the science and skill behind great fireworks are slowing chemical reactions down to controlled burn rates, so you see the beautiful shapes, and colors, we've come to love.
David Ropeik, of MSNBC Online writes an explosive expose of the Surpringly Scientific Flash Behind Fireworks: How pyrotechnicians use physics and chemistry with flair.
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 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.
Image: Fireworks Doug Kanter /AFP file
The slower the chemical reactions the better the visual effect. So pyrotechnic chemists use larger particle sizes, and exact blends of fuel and accelerants to control burn rates and brightness.
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.
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.
One new fact I learned from this article is that the colors we see come from the electrons returning to their original orbits and energy states, not from the excitation of the explosion.
There are other chemicals used to produce colors, but they all do their dazzling thanks to the first law of thermodynamics: Nature conserves energy. Energy from the fire in the basic fuel is transferred to the atoms of the colorant chemicals. That excites the electrons in those chemicals into a higher energy state. The electrons literally orbit further away from the atom’s nucleus.
Then, as they cool down, they move back to a lower state of energy. But remember, nature conserves energy. Energy is never lost, it’s just transferred somewhere else. As the electrons “calm down,” the energy they give up is converted into radiation. Light. That’s where the light of fireworks comes from. You actually see the colors in fireworks as they’re cooling down.
The signature chemicals in fireworks each emit light at a specific wavelength, producing a specific color: strontium equals red ... copper equals blue ... barium equals green ... sodium equals yellow/orange. Just as you could combine crayon colors when you were a kid, combining the colorant chemicals can give you additional colors. Strontium (red) plus copper (blue) equals purple.
This article is too long to do justice to within fair use guidelines. Ropiek explains more about how different colors are produced, how the shapes are produced in the packing arrangement, and dispersion explosions. If you enjoy fireworks, you will enjoy reading this great report on how they are made.
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Have a Happy and Safe Fourth of July.