Modern fireworks are intricate pyrotechnic devices engineered to deliver a controlled sequence of light, sound, and motion. These spectacular aerial displays result from precise chemical mixtures and a carefully constructed physical design. The effectiveness and safety of a firework rely entirely on the interaction between its various component parts, from the outer casing to the chemical pellets that create the color.
Physical Structure and Ignition Systems
The external structure of an aerial firework shell, often called the casing, is typically constructed from strong, tightly wound paper or pasteboard. This shell is designed to be robust enough to contain the internal charges but weak enough to rupture cleanly at a specific point in the air. Professional displays utilize a launch tube, known as a mortar, which is a sturdy cylinder often made from High-Density Polyethylene (HDPE) or fiberglass, designed to withstand the immense pressure of launching the shell.
The process begins with the fuse, which is the ignition lifeline for the entire sequence. The most common type for consumer fireworks is the Visco fuse, a cord with a black powder core wrapped in textile and coated with a water-resistant lacquer. This fuse burns at a uniform, measured rate, providing a necessary delay for the operator to reach a safe distance before the firework ignites. In larger, choreographed professional displays, an electric match is often used, which allows for precise, instantaneous ignition controlled by a computer system.
The initial fuse carries the flame to the lift charge, positioned at the base of the shell. A second, internal time-delay fuse, known as a spolette in some designs, is also ignited at launch, designed to burn for a calculated duration. This internal fuse determines the altitude and timing of the final explosion, connecting the initial lift to the aerial burst.
Energetic Materials for Lift and Burst
Black powder is the primary energetic material responsible for both propulsion and the aerial explosion. This material is a simple mechanical mixture of three components: potassium nitrate (the oxidizer), charcoal (the fuel), and sulfur (the accelerant). A standard ratio for this mixture is approximately 75% potassium nitrate, 15% charcoal, and 10% sulfur by weight.
The lift charge is placed beneath the aerial shell within the mortar. When the fuse ignites this charge, the rapid combustion produces a large volume of hot, quickly expanding gases. Confined by the mortar tube, this sudden pressure buildup acts like a small cannon, propelling the shell high into the atmosphere. The lift powder is often granulated into specific grain sizes to ensure a high, fast-acting force in a short timeframe.
Once airborne, the burst charge is ignited at the shell’s peak altitude. The timed fuse, ignited during launch, burns until this point, igniting the central charge. This internal explosion forcibly ruptures the shell casing, dispersing the smaller components inside—known as pyrotechnic stars—outward in a symmetrical pattern. The primary role of the energetic material here is pure dispersion, not the production of light or color.
Pyrotechnic Stars and Color Chemistry
The aesthetic effects of a firework originate from the pyrotechnic stars, which are small, consolidated chemical pellets loaded inside the shell. A star is a complex chemical matrix containing five main ingredients: an oxidizer, a fuel, a color-producing metal salt, a chlorine donor, and a binder. The oxidizer, such as potassium perchlorate or potassium nitrate, supplies the oxygen needed for the composition to burn intensely and quickly in the air.
The fuel (e.g., charcoal, sulfur, or metal powders like aluminum or magnesium) rapidly reacts with the oxidizer to produce heat and light. A binder, such as dextrin or starch, is mixed with water to hold the composition together. This allows the stars to be formed into shapes that survive the launch and burst.
The color itself is produced by specific metal salts, which, when heated to high temperatures in the flame, emit light at characteristic wavelengths.
- Strontium salts (e.g., strontium carbonate or strontium nitrate) create deep red hues.
- Barium salts (e.g., barium nitrate and barium sulfate) are responsible for green coloration.
- Copper salts (e.g., copper acetoarsenite or copper carbonate) yield blue color.
- Sodium salts (e.g., sodium nitrate) produce yellow.
- A combination of strontium (red) and copper (blue) salts results in purple.
For a color to be truly vibrant, a chlorine donor is often necessary, as the metal atoms must combine with chlorine in the flame to form a short-lived metal monochloride molecule. Specialized effects are also achieved through chemical additives; for instance, aluminum powder creates bright white flashes and crackling effects, while iron filings produce a golden, glittering spark trail.