7e. Basic Pyrotechnic Devices
Stars ----- A star is an amount of pyrotechnic composition that has by some means been fashioned into a solid object. These are the bright burning objects you see ejected from Roman candles, shells, mines etc. Usually the pyrotechnic composition is mixed with a binder and a small amount of solvent to make a doughy mass which is then fashioned into stars, although some use has been made of so-called pressed stars, which involve the composition being pressed extremely hard into a mold with a hydraulic press or similar, thus doing without the solvent. The usual methods are to make the composition into a flat pancake or sausage and cut it up into stars ("cut stars"), pushing it through a tube with a dowel, cutting it off at regular intervals ("pumped stars") or rolling cores of lead shot coated in fire clay in a bowl of the composition ("rolled stars"). Cutting and pumping produce cubic or cylindrical stars, while rolling produces spherical stars. Pumped stars are the most suitable for Roman candles, because it is easy to get the correct width. The stars are often dusted with a primer, usually meal black powder, to ensure ignition. Shell ----- The shell is a sphere or cylinder of papier mache or plastic which contains stars and a bursting charge, together with a fuse. It is fired into the air from a tube using a lift charge, usually black powder. The time the fuse takes determines the height above the ground at which the shell will burst, igniting and spreading the stars. Rocket ------ A rocket consists of a tube of rocket fuel, sealed at one end, with a constriction, or nozzle, at the other end. The burning fuel produces exhaust gases, which, when forced out the nozzle, produce thrust, moving the rocket in the other direction. Solid fuel rockets can be one of two types - end-burning, where the fuel is solidly packed into the tube, so the fuel can only burn at one end - and core-burning, where there is a central core longitudinally through the fuel, so the fuel can burn down its full length. At the top of the rocket can be a smoke composition, so it is possible to determine the maximum height ("apogee") of the rocket, or a burst charge and stars. Lance ----- A lance is a thin paper tube containing a pyrotechnic composition. These are most commonly used in large numbers to make writing and pictures at fireworks shows - this is referred to as lancework. The tube is thin so burns completely away as the lance burns, so as not to restrict light emission from the burning section. Gerb ---- These are pyrotechnic sprays, often referred to as fountains or flower- pots. They consist of a tube full of composition, sealed at one end and with a nozzle at the other, similar to a rocket. Unlike a rocket, they are not designed to move anywhere, so all the emphasis is on making the nozzle exhaust as long as pretty as possible, with large amounts of sparks, nice colours etc. The sparks are produced by metal powders or coarse charcoal in the gerb composition, with coarse titanium powder being the chemical of choice. Gerb compositions in a thin tube set up in a spiral arrangement are used as wheel drivers, for spinning fireworks e.g. Catherine wheels. Waterfall --------- These are similar to gerbs, but usually do not spray as far. They are usually mounted horizontally in banks of several tubes, placed some distance above the ground. When ignited, the effect is like a brilliant waterfall of sparks. Mine ---- These have a mortar arrangement similar to that for a shell, but are not designed to send out a shell. The lift charge sends up a bag full of stars and a bursting charge, with a short fuse set to spread the stars relatively close to the ground. Because the bag has much less strength than a shell, the stars are not spread as far, and the final effect is that of a shower of stars moving upward in an inverted cone formation.
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