Fireworks: Chemistry and Art on the Canvas of the Skies

From Madeira in Portugal to Sydney in Australia, and from the Chinese New Year to the Fourth of July in the United States, countless iconic places and celebrations around the world attract millions of tourists every New Year's Eve to witness what is technically known as pyrotechnics — a term derived from the Greek words pur (fire) and tekné (technique).

The fundamental component of fireworks is gunpowder, which historians believe was accidentally discovered by Chinese alchemists during the Tang dynasty in the 9th century. Over the centuries, gunpowder's pivotal role in military applications — including ammunition, rockets, cannons, and missiles — has led it to be regarded as one of history’s most significant inventions.

Although rudimentary forms of fireworks existed in ancient times, the vibrant and dazzling displays we know today only became possible with the emergence of chemistry as a scientific discipline. One key figure in this development was Antoine de Lavoisier, often considered the father of chemistry, who revolutionised gunpowder production during the French Revolution. His studies on combustion ignited further research into oxygen-rich compounds capable of producing more intense explosions and higher temperatures.

Technically, gunpowder is classified as an energetic material used to propel objects. Its basic composition combines potassium nitrate (or saltpetre), sulphur, and charcoal. Potassium nitrate, which makes up about 75 per cent of the mixture, serves as an oxidising agent, providing the oxygen necessary for the combustion of sulphur and charcoal. Charcoal accounts for approximately 15 per cent and acts as the primary fuel, while sulphur, comprising around 10 per cent, functions as a catalyst for combustion.

To produce the desired effect, ignition is the final crucial element. A spark triggers the combustion process by melting the sulphur when it reaches its melting point of 112.8ºC. The molten sulphur flows over the potassium nitrate and charcoal, which then ignite. This reaction rapidly generates a significant amount of energy and gas, culminating in an explosion. If the gas is channelled through a small opening, the resulting pressure launches the firework into the sky.

Shells and Stars

Modern fireworks are constructed using a "shell” — a cylindrical casing that holds gunpowder along with several small globes of explosive material known as "stars." Each of these stars contains four essential chemical components: a combustible material, an oxidising agent, a metallic compound responsible for the colour, and a binder to hold the components together. Finally, an ignition element is added to trigger the explosion.

The "stars" are where creativity comes to life, enabling the creation of a wide variety of visual effects. These effects depend not only on the composition of the stars but also on how they are arranged within the shell. The specific arrangement determines the patterns and shapes that will illuminate the sky.

A Spectacle of Colour

Although the fundamental principles behind fireworks remain the same as those used to launch traditional rockets centuries ago, advancements in chemistry and scientific knowledge have unlocked new possibilities. These developments have increased the brightness and expanded the palette of colours available, making displays even more spectacular.

The colours in fireworks are produced through a process known as luminescence. When an explosion occurs, the energy released is absorbed by the metal atoms present in the composition of the "stars." This energy causes the electrons in the atoms to move from their ground state to higher energy levels. As the electrons return to their fundamental state, they release energy in the form of visible radiation — coloured light.

The colours emitted are determined primarily by the metals used, showcasing the magic of chemistry. Strontium or lithium salts typically produce red, while calcium salts, such as calcium chloride, yield orange. Yellow is easily achieved with sodium salts, commonly sodium chloride, and green is produced using barium chloride. Blue, which is obtained with copper chloride, is the most challenging colour to reproduce due to the instability of copper compounds. Achieving the perfect blue requires precision: if the shade is too dark, it becomes difficult to see in the night sky; if too light, it appears white.

To ensure that this artisanal combination of chemistry and artistry delivers the desired effect, the stability of the compounds must be guaranteed, the explosion temperature strictly controlled, and contamination between colours prevented, making pyrotechnics a highly demanding chemical science. Sodium contamination is particularly problematic, as the intense yellow glow emitted by this element can easily overwhelm other colours.

Beyond colour, other pyrotechnic effects include variations in brightness, flashes, and spark intensities. Different metals are used to create these effects: aluminium, magnesium, and titanium produce white sparks, while iron generates golden sparks.

More Ecological and Technological

Like many other industries, the fireworks sector — generating approximately 2.6 billion euros annually worldwide — has been adopting more environmentally friendly practices. One significant advancement is the use of compressed gas for launching fireworks instead of traditional gunpowder, which helps reduce the emission of harmful gases such as nitrogen, carbon, and sulphur oxides. Chemistry has also provided alternative solutions, including nitrogen-rich pyrotechnic compounds that release energy without producing carbon oxidation reactions, unlike traditional mixtures.

These innovations not only reduce environmental impact but also improve the safety and efficiency of fireworks displays. Over the years, extensive regulations have been introduced, and much safer operating methods have been developed. One such advancement is the adoption of computerised remote triggering systems, which allow for precise and secure ignition of fireworks displays.