This year at the Memorial day weekend pyro get together we had a class on making 5 inch spherical shells. In my KaBoom post last year I described making canister shells which are cylindrical rather than spherical. Each of the participants in the class made a plastic shell and a paper shell. Much of the construction is similar except when it comes to sealing them up.
The basic idea is to build the shell in a way that it will burst symmetrically. There is a fuse consisting of several strands of black match that leads from the outside of the shell to the center. This is connected to a time fuse that is calculated to cause the shell to burst at the time it reaches the highest.
Above you can see a plastic shell with the fuse extending to the center. The stars, black balls in the picture are what burn to make the color when the shell bursts. They are arranged around the inside of the shell so that they will spread out in a nice pattern when the shell bursts.
A piece of tissue paper is then place in each shell half and filled with rice hulls coated with black powder.
Then comes the tricky part, getting the two halves together without disturbing the stars or powder.
Sometimes tapping the shell with a stick helps settle things in place so it will close properly.
After that the plastic shell halves are glued together. Here I am watching my shell getting glued.
It is interesting that there some modern tools in this rather ancient craft. The paper shells rather than being glued are taped with several layers of gummed paper tape. By hand this is a rather laborious task.
Here we have a computer controlled taping machine to do the hard part.
The paper shells are then set aside to dry overnight and the plastic shells are clamped to let the glue set.
Here we have the box of completed shells ready to go to the firing line. The lift charge is placed in a plastic cup and taped to the bottom of the shell with a quick match fuse. That fuse is amazing. It burns as a very high rate, something like 300 feet per second. At the firing line the end of the fuse is attached to an electric igniter so that it can be safely fired from a distance with the push of a button.
Above are a couple of the shells build in the class. You can also see a 3 minute video I made of the Saturday evenings shoot at http://youtu.be/7c-aLM5XZos
Credits: All photos by Gary Markley