How do fireworks work?

19. December 2024

For many people, the night that heralds the new year is also the most spectacular. And in two weeks it will be that time again: While our dogs and cats cower under the table, we’ll be gazing up at the sky in wonder. But how do fireworks get up into the air in the first place, and once there, why do they explode in a wide variety of colours and shapes?

The flame crackles its way along the fuse before it reaches the base of the rocket; this is then launched with a loud hiss up into the sky, exploding way above the rooftops in an extravaganza of red, yellow and green spheres or bursting outwards in long trails of light. All the while, mouths on the ground are agape and eyes wide.

Fireworks fascinate us – and have done so for centuries. Their first recorded use was back in the days of the Chinese Empire. Gunpowder was probably invented there in the middle of the eleventh century and used in incendiary devices. At religious festivals, however, the Chinese would also decant the highly flammable mixture into bamboo canes and throw them into the fire – creating the first Chinese firecrackers. Tubes filled with gunpowder in this way would also be tied to arrows, ignited and shot into the air: sometimes to deter enemies, sometimes to drive away evil spirits.

It was probably Dutch sailors who first brought gunpowder to Europe, where it was initially only used for military purposes. The first “pleasure fireworks” were documented in Italy in the year 1475. In Germany, the first approved fireworks were set off in 1506 – in honour of Emperor Maximilian I. Later, in the Baroque era, fireworks would become a must-have for every princely celebration. Frenzied spectacles of light in the sky were an excellent way of demonstrating princely power and majesty in a manner that was visible from afar.

Even though the rockets used at that time would only explode into a single colour, the propellant that launched them into the sky – gunpowder – has remained broadly unchanged to this day. This mixture of charcoal, sulphur and saltpetre burns incredibly fast and releases gases such as CO₂ and nitrogen.

If you were to sprinkle a heap of gunpowder on a table and ignite it, it would simply burn down. Once locked in the paper envelope of a firecracker, however, the expanding gases generate enormous pressure: The firecracker explodes with a small flash of light and a loud bang.

In a rocket, on the other hand, the gases are given a small means of escape: the hole that the fuse has burned into the base of the rocket. The gas mixture flows out through this jet and generates a recoil effect – the rocket takes off and rises up to 100 metres in the sky.

The long wooden stick attached to a rocket’s side ensures that it doesn’t get into a tailspin. It keeps the centre of gravity of the rocket beneath the propellant charge, thus ensuring a stable flight. Rockets with a broken stick should therefore not be ignited under any circumstances, given the risk that they might streak off uncontrollably at ground level and injure people.

Rockets often rise into the air with an accompanying howl or whistle. These sounds are caused by small explosions that follow each other three to four thousand times per second. Like in a flute, these explosions cause a column of air inside the firework to vibrate rapidly, making it whistle. And just as you can produce different sounds with a longer or thicker flute, you can also achieve other acoustic effects depending on the length and diameter of the rocket shell and the structure of the fuel.

The gunpowder fuel for the ascent is calculated in such a way that it is used up at exactly the point when the fuse, which continues to burn slowly inside the rocket, reaches the heart of the explosive projectile: a chamber with another gunpowder charge and the ingredients required for the brightly coloured effects. These “pyrotechnic stars” usually consist of a blend of metal salts.

Why these then start to glow is a matter of atomic physics: The metal salts burn at temperatures of over 1,000 degrees. This heat excites the electrons around their atomic nuclei, which jump to a higher energy level, before falling back again at lightning speed. In the process, energy is released in the form of light. The colour of the light depends on the chemical element in question: Potassium salts glow purple, strontium salts red, barium salts green, copper salts blue, while sodium salts – effectively table salt – produce yellow hues.

Whether the rocket explodes in a ball of light or spreads its flaming cargo across the night sky in a weeping willow of crackling sparks will depend on its construction and the packaging of the metal salts: If the effect you are looking for is that of lots of little flaring stars, you have to enclose the chemicals in lots of small but robust spheres. These hold the metals together for longer during the explosion, thus generating compact light effects. If the effect-producing chemicals are arranged in a circle around the explosive device, large and round explosion patterns are created. In pyrotechnics, one speaks of a bouquet, like a bouquet of flowers, in the firmament.

Cylindrical explosion chambers, on the other hand, can be used to create forms like the laburnum, whose far-flung trails of light seem to reach right down to the earth like the twigs and flowers of the shrub of the same name. And if the rockets are intended to drag a trail of sparks behind them as they rise, they are equipped with an additional load of pyrotechnic stars which are ignited at the same time as the propellant charge.

The effects and shapes used have become more and more sophisticated and impressive over the years and decades. But like the gunpowder used as propellant, fireworks have remained true to their origins in one further respect: More than 90 percent of the rockets and firecrackers used worldwide are manufactured in China. The largest importer is the US. The annual fireworks budget for the Walt Disney Company’s theme parks alone is estimated at around 50 million US dollars. That's pretty much one-third of the total cost of every firework display in Germany. However, the most expensive fireworks display ever recorded was set off in Abu Dhabi, the capital of the United Arab Emirates, in 2009. To mark its national holiday, the desert state spent an impressive 17.5 million euros on its multicoloured explosive fiesta.