How Do Fireworks Work? Four Jobs in One Cardboard Shell

An aerial shell sits in a mortar tube above a lifting charge of black powder. Black powder is a mixture rather than a compound, combining potassium nitrate as the oxidiser, which supplies oxygen internally so combustion does not depend on air, with charcoal and sulphur as fuels. Ignited, it burns fast enough to generate a large volume of gas very quickly and slow enough not to detonate, which is exactly what is wanted: it pushes the shell out of the tube rather than shattering it. The same ignition lights a time fuse running into the shell, so the delay before bursting is set by the fuse length and is calculated to reach the top of the trajectory, where the shell is momentarily near stationary and the burst is symmetrical. Inside, a bursting charge scatters the contents outward. Getting that timing wrong by a fraction of a second produces a burst that is too low, which is dangerous, or too high, which wastes the effect.

Colour comes from two distinct physical mechanisms and the difference explains why some colours are easy and others are not:

• •Incandescence, where a substance glows because it is hot, giving the white and gold of burning metal powders, with aluminium and magnesium producing brilliant white and iron and charcoal producing warm golden sparks
• •Atomic emission, where heat excites electrons in a metal salt and they emit light at wavelengths characteristic of that element, which is the same physics that lets astronomers identify elements in stars
• •Strontium compounds for red, barium for green, sodium for yellow, copper for blue and a copper and strontium mixture for purple
• •Sodium is a nuisance as well as a colour, because its emission is so strong that traces of it contaminate other colours, which is why formulations avoid sodium-containing ingredients where possible
• •Blue is notoriously the hardest, because the copper compounds that produce it decompose at the temperatures needed for brightness, so a blue firework is always dimmer than the others and getting a good one is a mark of a skilled manufacturer
• •Chlorine donors are added deliberately because metal chlorides emit more strongly than the bare metals, which is why perchlorates appear in most colour formulations

The pattern a shell produces is determined by how the burning pellets, called stars, are physically arranged inside it before the burst. A ring of stars packed in a single plane inside a spherical shell produces a ring in the sky, and shapes including hearts, smiles and planets are made the same way, by placing stars in that outline and accepting that the shape only reads correctly from one direction. A shell packed with stars distributed evenly over the inner surface of a sphere produces the classic symmetrical break, and a Japanese-style shell with concentric layers of different colours produces a burst that changes colour as it expands, with each layer visible in turn. Multi-break shells contain several sub-shells with their own fuses, producing a sequence. Willow and chrysanthemum effects come from star compositions that burn slowly and leave long trailing sparks, while strobe effects use compositions that burn intermittently.

Fireworks are explosives and the professional industry treats them accordingly, with mortars secured, firing done electrically from a distance and increasingly controlled by computer so that a display can be synchronised to music with the delays of each shell accounted for. Injury statistics from consumer fireworks remain substantial in countries that permit them, concentrated in hands, eyes and faces, with sparklers responsible for a surprising share of injuries to small children because they burn hot enough to melt some metals. The environmental questions have grown. Perchlorate oxidisers are water-soluble and have been detected in water supplies after displays and at manufacturing sites, where they interfere with iodine uptake by the thyroid. Displays produce a short sharp spike in fine particulate air pollution, measurable for hours. Metal salts including barium are toxic. The responses include perchlorate-free formulations, nitrogen-rich fuels producing less smoke, and in a growing number of cities replacement of fireworks with drone light shows, which produce images fireworks cannot and lack the one thing an audience reliably responds to, which is the concussion you feel in your chest.

A lifting charge of black powder throws the shell from a mortar and simultaneously lights a time fuse cut to burst it at the top of its flight. Colour comes either from hot metal glowing white or gold, or from metal salts emitting at characteristic wavelengths, with strontium for red, barium for green and copper for blue, which is the hardest because its compounds break down when hot. Shapes are made by physically arranging the burning stars inside the shell before it bursts.