Shells and Mortars

Fireworks use black powder, also known as gunpowder. They are classed as low explosives (1.3G) in contrast to the infinitely more powerful high explosives such as TNT and dynamite. In pyrotechnics an oxidizer and a fuel react together to produce light.

Shells and Mortars
The central tool in fireworks is the shell. A shell is usually round or cylindrical in shape and is covered with a casing of cardboard and/or plastic and twine - with a lift-charge beneath it made from black powder.The shell is inserted into a mortar (a gun-like cylinder with a bottom plug) made of cardboard, HDPE plastic or steel for the larger shells. The mortar serves as the shells' launching pad.
The mortars and shells are wired to a central firing panel, from which the shells' lift-charges are ignited electrically throughout the show.When the lift-charge explodes, the shell shoots out of the mortar up into the air. In the core of the shell is another exploding charge with a delay fuse. When the shell is fired from the mortar, it in turn ignites the delay fuse, which is cut to a length that will set off the explosive charge inside the shell at the right height and the right time.
The explosive charge at the center of the shell is made of black powder or flash powder (composed of potassium perchlorate, aluminum powder, and/or magnesium which, upon ignition, creates a violent explosion combined with a flash). The explosion sends the burning stars out in a particular pattern determined by the way the shell was packed by the shell maker - creating the colorful effect the crowd sees.

An aerial firework is normally formed as a shell that consists of four parts:
Container - Usually pasted Kraft paper and string formed into a cylinder or sphere. Generally six or seven layers thick, glued and tied at the top and bottom (as in cylinder shell.)
Stars - Spheres, cubes or cylinders of a sparkler-like composition. Tightly packed balls of chemicals that burn as they fly, making bright colors and throwing sparks in all directions.
Bursting charge - Firecracker-like charge at the center of the shell.
Fuse - A string of woven threads containing gunpowder ignites gunpowder mixture, lifts charge at base of shell and launches it. Additional fuses provide a time delay so the shell explodes at the right altitudes.
The sphere is the shell, and located just below the shell is a small cylinder that contains the lifting charge, which shoots it out of the launch tube. The fuse lights the lifting charge, which in turn lights the shell's fuse.

Pictured (above left) are Chinese-made shells in various sizes. Cylindrical shells (above right) are usually domestic-made shells.

The shell is launched from tubes called mortars. Mortars can be constructed from heavy cardboard, plastic (HDPE=High Density PolyEthylene), GRP ( Glass Reinforced Plastic) or metal sunk into the ground or mounted in racks. The shell must fit snugly inside the mortar to allow proper thrust. Shells measure from two inches to three feet in diameter and can weigh up to 700 pounds. Dozens of mortars of various sizes are used at large professional fireworks displays.
The lifting charge of black powder explodes in the pipe to launch the shell. When the lifting charge fires to launch the shell, it lights the shell's fuse. The shell's fuse burns while the shell rises to its correct altitude, and then ignites the bursting charge so it explodes.

A simple shell used in an aerial fireworks display. The gray and the red balls are the stars, and the black is black powder. The powder is packed into the center tube, which is the bursting charge. It is also sprinkled between the stars to help ignite them.

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.