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.
- These are small shells, about the size of a peach,
that you can buy at roadside stands in some states. The sphere is
the shell, and the small cylinder below is the lifting charge that
shoots it out of the launch tube. The green fuse lights the lifting
charge, which in turn lights the shell's fuse. Shells that you see
at a show are typically the size of a cantaloupe or even
larger.
The shell is launched from a mortar. The mortar might be a short,
steel pipe with a lifting charge of black powder that 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.
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.
--> From HowStuffWorks
