How do fireworks explode?
Despite all their different colours, shapes, and sounds, all fireworks have the same basic components.
Aerial fireworks consist of a shell made of heavy paper that holds the ‘lift charge’, the ‘bursting charge’, and the ‘stars’. All of these glittery spectacles come from good old- fashioned combustion. Combustion is a chemical reaction between two substances (a fuel and an oxidant) that produces light and heat. The heat causes gasses to expand rapidly, building pressure. The shells are tightly wrapped cylinders, which provide good resistance to this pressure, giving it a short time to build in intensity. Then, when the reaction overpowers the shell, you get the explosive firework effect.
It all starts when the shell is placed into a mortar (a cylinder the same size as the shell, which holds the firework in place while the fuse burns). The lift charge, at the bottom of the shell, is basically concentrated black powder (charcoal, sulphur, and potassium nitrate). When lit by the dangling fuse, the lift charge sends the shell into the air. Basic firecrackers are just paper-covered black powder: you light the fuse and listen to the popping sound.
The bursting charge is another round of black powder with its own time-delayed fuse higher up in the shell. The bursting charge creates the heat to activate the stars that surround it and explode them outward from the shell. The stars are where the magic happens.
Stars are balls made up of fuels, oxidisers, colour- creating combinations of different kinds of metals, and a binder to hold everything together. The stars can be arranged within the firework shell to create shapes. The shapes can be things like hearts, stars, and circles. Hundreds of stars can be used in a single firework shell.
More complex fireworks – for example, ones that produce a shape like a smiley face, have multiple phases of different colours, or make extra sounds like whistles – have shells with a more intricate infrastructure. In these types of fireworks, there are more time-delayed fuses linked to various bursting charges with their own surrounding stars. Each of these may sit in its own individual interior shell. These are called ‘multi-break shells’.
While a sight to behold, fireworks are individually wrapped chemistry experiments. Tapping one too hard or creating a static electricity shock with your synthetic-material clothing could be deadly and one exploding near to your face could result in horrific burns and even blindness. They don’t have the word ‘fire’ in them for nothing.
What makes the bang?
You start off with an element, such as magnesium, that is made up of a large number of molecules that contain electrons, protons and neutrons. The spark of fire will heat up the electrons and excite them. As they get more excited, they are released from their molecular bonds. This results in both a physical movement of the electrons away from the nucleus and a release of the energy that has built up during the heating stage. The energy released can trigger further interactions, in turn creating a chain reaction. The higher the initial energy of the spark, the quicker the electrons get excited, resulting in a more violent explosion.
What makes the colours?
Colours involve different measurements and combinations of oxygen producers, fuels, binders, and colour producers. You can make colour through incandescence – light created through heat (orange, red, white), or luminescence – light created from a chemical reaction without extreme heat (blue, green). It’s all about temperature control and balance.
The history of fireworks
Fireworks were first invented by the Chinese, pretty much by accident. Around 200 BCE, they noticed that bamboo sticks in their bonfires exploded with a bang. This happened because the fast-growing bamboo traps pockets of air inside its stalks, so when the oxygen comes into contact with the fire, a huge reaction takes place, resulting in light and sound.
The Chinese took this a step further and created the first form of gunpowder, mixing sulphur, saltpetre (potassium nitrate), arsenic disulphide and honey. When set on fire, this produced an explosion caused by the sulphur and honey acting as the energy source for the reaction and the potassium nitrate providing the oxygen. Honey was replaced with energy-dense charcoal and the mixture was put in tubes and angled at enemies who were terrified by the loud bangs and bright sparks. At this point, fireworks divided into weaponry and spectacular crowd-pleasing displays.
In the 1830s, scientists in Italy realised that by adding a metallic salt like strontium or barium and chlorine powder, the light that was created took on the colour of the salt. Slow-burning gunpowder was used as a fuse to fire the rocket into the air, while inside, potassium-rich gunpowder allowed for a bigger reaction, faster and hotter with a more impressive explosion.
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