Aerial shells are the most well known types of fireworks. These are what rise into the air and create the magnificent colorful bursts of light seen at public fireworks displays. A shell consists of a spherical ball or cylinder and a lift charge. These are placed in large called mortars, which can be buried in the ground or built into racks. When ignited by the pyrotechnician, the lift charge on the bottom of the shell – no more than a bag or cup full of black powder – explodes, shooting the shell into the air like a bullet from a gun. As the shell is rising upwards, a time fuse inside of it is burning towards the burst charge. At the precise altitude, the burst charge – consisting of black powder impregnated on rice hulls – blasts it apart and ignites the color pellets (stars) on the inside. Display shells (for professional use only) can range anywhere from 3 to 24 inches in diameter! Shells generally go 100 feet up for every inch in diameter they are. Thus, an 8-inch shell will go approximately 800 feet into the air before bursting.
|Sphereical shells used by professionals are very similar to the smaller versions sold on the consumer market. The most obvious difference is the larger size, which lets the shell hold more stars.In the diagram on the left, the internal time fuse goes out through the bottom of the shell and is ignited by the lifting charge as is the case with consumer shells. On large shells, however, the blast of the lifting charge could literally blow the fuse away without igniting it, so the time fuse is made to go out through the top of the shell. If this method is used, the time fuse is coated with a priming compostion and is ignited by the hot gases of the lift charge.
Smaller display shells can usually be held by the quick match fuse when being loaded into the mortar, but larger ones have a string attached to them which is used to lower it down – otherwise, the weight of the shell would rip the quick match out of the lift charge.
|Cylindrical shells were invented in Italy and are more common in Europe and America than spherical shells. Although they don’t give perfectly sphereical bursts of stars like round shells, they are easier to make and store. The time fuse is ignited by the quickmatch as the fire goes down to the lift charge. The time fuse is nearly impossible to ignite from the end, so holes are punctured in the sides and strips of black match fuse are inserted. This is known as a “cross match” assembly. Black match takes fire very easily, and transfers it into the powder core of the time fuse. The time fuse gives off no “side spit”, to the burst charge doesn’t ignite until the black match on the opposite end is ignited by the time fuse. The cutaway diagram below shows a more detailed look at cross-matched fuse.