Nozzle
From PyroGuide
A nozzle is a plug in the exhaust end of the firework, with a small opening. The nozzle does a number of things. Firstly, it allows the gasses produced by the composition to build up more pressure. This increases the exhaust velocity, and therefore thrust. The higher pressure, however, also increases the burn rate of the composition, producing gas at an increased rate, which leads to an even higher pressure, which increases the burn rate, which increases the pressure further.
If this process amplifies itself too much for the casing to withstand the pressure it will result in 'catastrophic failure' or 'CATO'. The dimensions of this opening are critical. Ideally, we would like the opening to be small enough to allow the internal pressure to rise, yet not so small as to allow the pressure to rise to a point that the casing explodes.
The proper dimensions are the result of many factors that are most often not accurately known for simple rockets and are therefore most easily found by trial and error. A proper nozzle design can also greatly improve the efficiency of a rocket engine or firework. A nozzle can be designed to make the most out of the potential energy stored in the pressurised gas in the 'internal combustion chamber' as it is released into the atmosphere.
Nozzles for fireworks rockets are usually made with clay. Bentonite and kaolin clay work well. If necessary, a small amount of fine sand may be added to improve the nozzles grip on the casing. The dry clay powder is rammed into the casing, producing a solid plug. Cheap kitty litter is often made of bentonite clay and may be used instead. Grind up the kitty litter into a fine powder, which is most easily done with a ball mill. This powder can be used to produce rock-hard nozzles that erode very little.
Another nozzle material that is quite resistant to erosion is quick-hardening cement. It is wetted with water and pressed into the casing. After a few minutes it has solidified enough to continue with the construction of the motor. Disadvantages are that it is very sticky when wet (making working with it very messy), and the short hardening time, which makes it necessary to work quickly.
'Durhams rock hard water putty' is often mentioned as a nozzle material.