Have you ever wondered how fireworks produce such varied shapes and brilliant colours every time they light up the sky? In fact, the shape and colour of each firework burst come from precisely engineered chemical reactions.

Let’s start with the most critical step: launching. A firework is a cylindrical tube. At its bottom lies a layer of black powder called lift charge. Once ignited, it burns rapidly and generates massive amounts of high temperature, high pressure gas. This hurls the whole firework into the sky just like a bullet, sharing the same principle as artillery — except its “projectile” is spectacular fireworks. When the firework reaches its preset altitude, the flame ignites the internal burst charge. The shell rips open instantly, flinging contents evenly in all directions to create the gorgeous bloom. This charge is also known as the bursting charge.

What makes fireworks detonate at just the right moment mid-air? The key is the time delay fuse. When you light the fuse for the lift charge at the bottom, it also ignites the fuse for the bursting charge. Fuses are calibrated at the factory to trigger detonation at a specific height. Fuse lengths vary for different altitudes, which explains why different fireworks explode at different heights. The bursting-charge assembly is generally round, filled with tiny pellets called stars. Each star contains combustible materials, oxidisers, binders and metal salts that produce colours. When heated by high temperature, electrons in metal atoms jump to higher energy states. As these electrons fall back to lower energy states, light of specific wavelengths is released — this is where firework colours originate.

Here are typical examples: sodium gives bright golden-yellow light; strontium produces vivid red; barium yields emerald green; and copper creates blue. How do fireworks form different shapes? It depends mainly on the 3-D arrangement of star pellets inside. Stars arranged in circles create circular bursts; heart-shaped arrangements produce heart-shaped fireworks. The explosion essentially “projects” this three-dimensional pattern onto the night sky. There are also multi-segment performance fireworks. Built with multi-layer structures and multiple delay fuses, they detonate in sequence: the first layer bursts, the second activates after a half-second delay, and then the third lights up.

One easily overlooked detail is explosion power. Too violent a blast shatters star pellets and muddles colours; too weak and the pattern cannot expand properly. Therefore, the mass of bursting charge must be accurate down to the gram. Every firework show you watch is an engineered art of explosion, carefully designed by pyrotechnicians.

A fleeting moment of sparks is romance written by mechanics and chemistry. By understanding the basic structure of fireworks, we can appreciate the beauty of pyrotechnics more sensibly and safely.