It’s Independence Day, and that means it’s time for controlled explosions in the sky. No, not Texas post-rock — the great scientific display that is a fireworks show.
"Fireworks are an application of chemistry and engineering: you need good chemistry to get the effects up in the sky and good engineering to make sure they get to the right altitude and burst at the right time," John Conkling, the former director of the American Pyrotechnics Association, tells The Verge.
Firework shows last between 15 to 20 minutes on average, but the amount of planning and preparation that goes into producing these displays can take up to two years. Designers need ample time to determine the right colors and shapes they want to use, and to time the explosions to the soundtrack.
To produce the colorful patterns and shapes you see, fireworks utilize a precise chemical mixture that’s going to burn at the right temperature, at the right time, and with the right colors. That requires four main chemical ingredients — an oxidizer, a fuel, a colorant, and a binder.
Fireworks need plenty of oxygen to facilitate the burn, which is where oxidizers come in. An oxidizer is pretty much what it sounds like — a chemical rich in O2. Oxidizers release excess oxygen to make a better explosion. The most commonly used oxidizers are nitrates, chlorates, and perchlorates.
Of course, any fire needs fuel. In fireworks, that’s typically charcoal or sulfur. The fuel combines with the oxygen released by the oxidizer, setting the stage for an explosion when fire’s added. These chemicals used for the fuel and the oxidizer are some of the same ones found in standard gunpowder, which is an essential element in fireworks.
Fireworks utilize a precise chemical mixture
The fuel and oxidizer chemicals are used in three different ways. The fuse used to light the tiny rocket is made of very fine gunpowder, which allows the wick to burn at a very controlled rate. This gives whoever lit the firework some time to get out of the way. The burning fuse then lights much larger granulations of gunpowder at the bottom of the firework. That explosion carries the rocket into the sky.
Finally, pellets of gunpowder are packed in the firework’s body. These capsules are what ultimately force the firework to burst apart. Also contained within these pellets is the colorant and the binder. The colorant chemicals help determine the different colors we see, and the binder — often a type of starch called dextrin — binds the fuel, oxidizer, and colorant together within the pellets. When the packets explode, the chemical elements emit light, and the colorant produces very specific wavelengths that can be seen by the naked eye.
The colors you see all depend on the different chemicals you use. For red, you want a strontium compound, and for green, you need a barium compound. Copper compounds are needed to make blue, while sodium compounds will give you a yellow-orange.
There are limits on the types of chemicals you can use, however. For one, they can’t be agents that collect moisture, or else they won’t burn properly when lit. Sodium is great for making yellow fireworks, but you can't use sodium chloride — or common table salt — since it holds water. You also want to make sure your explosion isn’t propelling any toxic chemicals into the environment — so any compounds used in fireworks must be benign.
"Years and years ago, blue was made with an arsenic compound; it made a beautiful blue, but clearly arsenic is not something we want to be putting in the atmosphere," says Conkling. "So you want to make sure it’s green — not the color green — but environmentally green."
So from its initial lighting to its final spectacular explosion, a firework’s life begins with a lit gunpowder fuse, followed by a gunpowder-boost into the sky, and finishes with an explosion of a chemical medley of fuels, oxidizers, colorants, and binders. As you enjoy these fiery tributes this weekend, remember how much science is involved behind the rockets’ red glare.