And why it’s so hard to make blue ones.

Image Credit: Matthew Bellemare, Wikimedia Commons

Fireworks are part of countless celebrations around the world—including United States Independence Day festivities dating back to the very first bash in 1777. But how do those glittering displays actually work? It turns out that the firework shows that dazzle crowds across the country each year are backed up by millennia of engineering—and science that continues to evolve.

The basics of fireworks have remained the same for a thousand years, starting with a recipe believed to have been devised by Chinese alchemists around 800 CE. The “black powder” they dreamed up is three quarters potassium nitrate and the rest charcoal and sulfur. Stuff that in a paper container, and you’ve got the beginning of a firework. 

When you light that baby, the sulfur melts first, flowing over the potassium nitrate and charcoal and setting them alight. This flame releases gas and energy: an explosion. If there is a conveniently placed hole in the container, the gas escapes and launches the firework skyward before it blasts apart. 

In modern aerial fireworks, black powder is packed into a container connected to a fuse. When the fuse is lit, it burns for a set amount of time to allow the firework to reach a desired altitude, at which point it reaches the “bursting charge.” Inside, all that black powder has been packed with “stars,” small spheres or cubes made of metal salts, in a particular pattern. The exploding powder throws those stars outward, creating intricate spheres or flowers of sparkling light with names like “palm,” “willow,” and “chrysanthemum.” More complicated “multibreak” shells can even burst in two or three phases, sometimes including shells-within-shells, fuses that ignite each other, or explosives that break each section into pieces.

We’ve come a long way from the simple Chinese launch-and-boom fireworks of a thousand years ago. These days, modern firework designers can use computer programs to choreograph their shows—sometimes along with music—down to a fraction of a second. They use a mix of virtual modeling, precisely engineered launching brackets, and circuits equipped with computer chips to map out and execute the timing of each firework, its launch angle, and how high it should sail in the air before exploding.

Plus, 21st century fireworks have something the original Chinese inventors could only dream of: color. In the original versions, which used potassium nitrate as a base of the black powder, the sulfur’s electrons would get excited, jumping into a further orbital away from each atom. When they fell back to their original state, that extra energy would be released as yellow light. 

But in the 1830s, Italian firework makers discovered that using potassium chlorate instead of nitrate opened up a huge realm of sparkling possibilities. The new material delivered oxygen more quickly to the powder, increasing combustion temperature from 1,700 to 2,000 C. And that increased temperature created an opportunity to add a new set of chemicals with higher burn temperatures that could create brighter light—and new colors.

More specifically, Italian firework innovators began adding metal salts to their works of explosive art. Each of these ionic compounds emits light from a different part of the spectrum when its excited electrons bounce from their outer orbital neighborhood back to home base. Strontium and lithium both make deep reds; calcium blooms orange; sodium glitters yellow; barium radiates green.

All that is good for creating a visual masterpiece—but the new chemicals adding to our oohs and ahhs have some drawbacks. The strontium chloride that creates beautiful red blooms in the night sky can also create carcinogenic fallout. In 2015, researchers looking for more environmentally friendly, chlorine-free options found possibilities in materials like hexamine, a preservative, and 5-amino-1H-tetrazole, an airbag propellant, Chemical & Engineering News reported. That could be helpful in military contexts, since red flares used in training can be a regular source of harmful fallout.

And in 2011, a U.S. Army team of pyrotechnics experts found that boron carbide, “a compound long dismissed as inert” could help replace the toxic, barium-based chemicals currently used in green fireworks, Nature reported.

Environmental and safety concerns aren’t the only challenge left for pyrotechnic artists. There’s still the firework holy grail: the perfect blue. Blue fireworks have always been among the most difficult to develop. They must stand out against the dark night sky, but their signature color is created using a copper gas that burns at a very high temperature. The firework maker's task is to help the color come out without overheating it and causing the color to wash out, turning from blue to white. That makes a really vivid blue firework an aerial feat of chemistry that marks a true master.

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