The fuse is lit and the capsule takes flight. Tension builds as the shell soars higher and higher, tearing through the air with a sharp, piercing whistle. As the wick burns down to the black powder the shell explodes, lighting up the night sky and raining flaming metal and chemicals down on the landscape below. In most places in the world this would be cause for concern, in the western world we call it a fireworks show.
When you get down to it, fireworks are nothing more than a bomb that is designed to be all sizzle and no substance. All the flash with none of the carnage. Aerial fireworks are the most iconic, so this discussion will mostly focus on them, but the mechanics involved are transferable in part to sparklers, firecrackers, and those screaming pinwheel things you nail to a tree.
A good aerial firework has four main parts: The container (or shell if you prefer), stars, a bursting charge, and a fuse. The container, which is usually made from paper and string is launched from a mortar (the pipe or wire you stick into the ground and aim away from your face) using an initial charge of black powder. This initial, elevating blast lights a fuse that burns its way into the container towards the bursting charge at the centre of the shell. The fuse is designed to burn out when the shell reaches its maximum height, setting off the bursting charge which overwhelms the binding agent and launches burning “stars” in every direction.
Stars are what people come to see. They are small clumps of explosive metals and chemicals that are engineered to burn in any one of the myriad of colours you see at a typical fireworks show. The metals used are most often aluminum, magnesium, and titanium because they burn with an intense, hot light that can be seen from miles away. Colours are the tricky part, and designers of fireworks have been tinkering with recipes since the air-borne displays were first invented in China over 1000 years ago.
Reds, oranges, and yellows are the easiest colours to produce, as any campfire will evince. These are produced using salts comprised of strontium, calcium, lithium and a few other basic chemicals that are both reliable and easy to work with. The real challenge of fireworks comes in trying to produce the colour blue. Next time you are at a fireworks show, keep your eye out for a real, vivid blue (not pale blue or purple). If you see it, odds are you are being entertained by a true professional. The chemicals used to create blue light (copper and chlorine compounds) are notoriously finicky. Give them too little heat and they won’t ignite. Give them too much heat and the light washes out to pale blue or white.
Nothing says summer like a cool night breeze, a glass of lemonade, and a few fireworks to celebrate an extra day off of work. Next time you find yourself in awe of such a spectacle, take a second to think about the complex mechanics and chemistry that go into producing it. Who knows? You might even appreciate the show on a whole new, intensely nerdy level.