An aerial firework is normally formed as a shell that consists of four parts:
Container - Usually pasted paper and string formed into a cylinder
Stars - Spheres, cubes or cylinders of a sparkler-like composition
Bursting charge - Firecracker-like charge at the center of the shell
Fuse - Provides a time delay so the shell explodes at the right altitude
Located just below the shell is a small cylinder that contains the lifting charge.
Simple shells consist of a paper tube filled with stars and black powder. Stars come in all shapes and sizes, but you can imagine a simple star as something like sparkler compound formed into a ball the size of a pea or a dime. The stars are poured into the tube and then surrounded by black powder. When the fuse burns into the shell, it ignites the bursting charge, causing the shell to explode. The explosion ignites the outside of the stars, which begin to burn with bright showers of sparks. Since the explosion throws the stars in all directions, you get the huge sphere of sparkling light that is so familiar at fireworks displays.
Multibreak ShellsMore complicated shells burst in two or three phases. Shells like this are called multibreak shells. They may contain stars of different colors and compositions to create softer or brighter light, more or less sparks, etc. Some shells contain explosives designed to crackle in the sky, or whistles that explode outward with the stars.
Multibreak shells may consist of a shell filled with other shells, or they may have multiple sections without using additional shells. The sections of a multibreak shell are ignited by different fuses. The bursting of one section ignites the next. The shells must be assembled in such a way that each section explodes in sequence to produce a distinct separate effect. The explosives that break the sections apart are called break charges.
Next, we'll look at how fireworks explode in various patterns during a display.