|MadSci Network: Chemistry|
Hello, First of all, the major differences between the staged, large-scale fireworks and those little ones which those who live in the right state may buy and shoot are mainly in the size and sophistication of the devices used. I will try to answer your questions in order, hoever I doubt that I would be able to answer completely, since the subject is very large and there still are quite a few trade secrets. So... 1. Color-changing stars. It is, in theory, a very simple design. The stars are designed to have several concentric layers, each made of the mixture which burns differently. Historical practice of making such stars is as follows : one takes a small light object, such as a rice hull for example, and rolls it in a powdered mixture of a certain color-burning compound. While rolling, the mixture is sprayed with a fine mist of water or water/alcohol (or sometimes acetone). The mixture, naturally, should contain some kind of binder, which, when moisetned, makes the particles stick to the "seed". As one rolls, one gets a growing ball of compressed powder. Of course, usually one starts with many "seeds" , in order to make many stars. Next stage would be to use a different kind of powder, with the same very stars. As you can imagine, you can form several layers of different compositions, around a single seed. Sometimes, these layers are interspersed with thin layers of some fast-burning mixture, to allow for clearer separation of colors. At the end, the star is given a final layer of "primer" - a fast and hot burning mixture which ensures uniform ignition. This way of making multicolored stars is simple, and by now is automatized to a large extent. 2. No, this is not an illusion. There are two major ways to make a "twinkler" (technical name for a star which goes on and off) or a "strobe" (any kind of pulsing-flame torch, star or light). First way is not dissimilar to the abovementioned multilayered stars. Instead of using different colors, one uses a color interspersed with layers of relatively slow-burning composition, emitting very little light. The second way of making the strobing stars and fires takes advantage of the cyclic reactions. Generally, a composition is devised so that a certain product is accumulated on the surface of the burning paricle which changes the visual appearance of the flame. When there is too much of this compound, the star "overheats", sheds the outer layer and burns bright for a short while. Then the process repeats itself. The frequency of "twinkiling" depends on the composition. If you are to attempt making those, I'd recommend starting with a simple pink litium strobe, as it is the least tricky. This is only a very crude explaination, which does not go into the actual details of the process. Generally, these reactions are very complicated, and require good knowledge of chemical kinetics to calculate, so as far as I know, the recipes are mostly designed by experimentation (again this may have changed by now). 3. Timing and orchestration... these are probably the most technologically advanced areas of stage fireworks. There are many ways to orchestrate. Most used are : simultaneous electric ignition, ignition in sequence, controlled by a timer, ignition in accord with a music score (usually computer-controlled), ignition by altitude etc. I happen to have heard that Disney uses tiny microchips inside *each* shell to synchronize their burst. I assume that many other large-scale firework shows use the same. It is also possible to time things by statistics i.e. if your show is really large, you can hope that the devices made in the same fashion will behave in similar way, reporting or igniting on a (typically) gaussian distribution-type curve around the average time. But these days, I suspect, people time their displays exactly. There are several chemical fuses which burn at precisely controlled rates, and , of course, electronics is of great help. In other words, large scale firework displays may be orchestrated just like large-scale electric light shows, only with many more precautions due to the volatile nature of the elements. If you have further questions, please do not hesitate to contact me directly. There is a fair variety of firework-related sites on the net, such as "Tim Peregrin"'s site and many others, but I am sure that you have already seen those... Best regards, and hope it helps, A.G.E.
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