8. Commonly Used Chemicals in Pyrotechnics
Ignitibility and Reactivity
The secret of making a good pyrotechnic mixture is _homogeneity_. The better the contact with the oxidiser and the fuel is, the fiercer the composition. Finely ground fuels and oxidisers are essential for good stars and propellants. The required intimacy also implies that mixing can never be thorough enough. For consistent results, use the same sieves and same mixing methods. Wet mixing is sometimes more efficient than stirring the dry composition; moreover, it is almost always safer. Star compositions and granulated powders can almost always be mixed with water or some other solvent. Good, homogenous compositions also ignite more easily. Large amounts of loose, fine powder of almost any pyrotechnic composition represent a large fire and explosion hazard. But when such a powder is kneaded and cut into stars or carefully pressed in a tube, it will take fire easily and burn smoothly. This is the pyrotechnist's dilemma: the best compositions are often the most dangerous ones, too. But not always. There are chemicals and compositions with much worse safety records than today's compositions have. In the list of pyrotechnic chemicals below, the most notorious ones have been indicated. Aluminium, Al -- Fuel This is used in many compositions to produce bright white sparks or a a bright white flame. There are many grades of aluminium available for different spark effects. Most pyrotechnic compositions that involve sparks use aluminium, e.g. sparklers, waterfalls etc. Ammonium Nitrate, NH4NO3 -- Oxidiser This is used very infrequently in pyrotechnics due to its hygroscopic nature and the fact that it decomposes even at relatively low temperatures. Even when dry, it reacts with Al, Zn, Pb, Sb, Bi, Ni, Cu, Ag and Cd. In the presence of moisture it reacts with Fe. It reacts with Cu to form a brissant and sensitive compound. It is best not to use any bronze or brass tools when working with ammonium nitrate. Ammonium perchlorate, NH4ClO4 -- Oxidiser Used as an oxidiser in solid rocket fuels, most notably the solid booster rockets for the Space Shuttle. Using it in a composition improves the production of rich blues and reds in the flames. As with any ammonium salt, it should not be mixed with chlorates due to the possible formation of ammonium chlorate, a powerful and unstable explosive. Anthracene, C14H10 -- Smoke Ingredient Used in combination with potassium perchlorate to produce black smokes. Antimony, Sb -- Fuel The metal is commonly used in the trade as 200-300 mesh powder. It is mainly used with potassium nitrate and sulphur, to produce white fires. It is also responsible in part for the glitter effect seen in some fireworks. Antimony trisulphide, SbS3 -- Fuel This is used to sharpen the reports of pyrotechnic noisemakers, e.g. salutes. It is toxic and quite messy. Barium salts -- Colouring Agents Used to colour fires green. several are used: Barium carbonate, BaCO3 -- Colouring Agent, Stabilizer As well as being a green flame-colourer, barium carbonate acts as a neutralizer to keep potentially dangerous acid levels down in pyrotechnic compositions. Barium chlorate, Ba(ClO3)2.H2O -- Colouring Agent, Oxidiser Used when deep green colours are needed. It is one of the more sensitive chemicals which are still used, best to avoid if possible, but if used it should be in combination with chemicals which will reduce its sensitivity. Barium nitrate, Ba(NO3)2 -- Colouring Agent/Enhancer, Oxidiser Not very strong green effect. Used with aluminium powder to produce silver effects. Below 1000C aluminium burns silvery-gold, characteristic of aluminium-gunpowder compositions. Above 1000C it burns silver, and may be achieved using barium nitrate. Boric acid should always be used in compositions containing barium nitrate and aluminium. Barium oxalate, BaC2O4 -- Colouring Agent Sometimes used, generally in specialised items with magnesium. Boric acid, H3BO3 -- Stabilizer This is a weak acid, often included in mixtures that are sensitive to basic conditions, notably those containing aluminium. Calcium carbonate, CaCO3 -- Stabilizer Used as a neutralizer in mixtures that are sensitive to both acids and bases, for example chlorate/aluminium flashpowder. Calcium oxalate, CaC2O4 -- Colour Enhancer Used to add depth to colours produced by other metal salts. Carbon black/Lampblack, C -- Fuel A very fine form of carbon made by incompletely burning hydrocarbon fuels. Commonly used in gerbs to produce bright orange sparks. Charcoal, C -- Fuel Probably the most common fuel in firework manufacture, it is not pure carbon and may contain in excess of 10% hydrocarbons. Indeed, the purer carbon charcoals (e.g. activated charcoal) do not necessarily give better results, and are very often worse than less pure grades. It is included in the vast majority of pyrotechnic compositions in various mesh sizes and grades, or as a component of black gunpowder. Clay This is an important material for making fireworks, not as a reagent but to perform various practical applications such as blocking or constricting the ends of tubes for crackers or rocket nozzles, or coating lead shot prior to the application of star composition when making rolled stars. Copper and copper compounds -- Colouring Agents Used to add both green and blue colours to flames: Copper metal, Cu -- Colouring Agent Both the bronze and electrolytic forms are occasionally used, but easier methods are available for the same effect. Copper acetoarsenate, C4H6As6Cu4O16 -- Colouring Agent Commonly called Paris Green, this chemical is toxic but used to produce some of the best blue colours in combination with potassium perchlorate. Copper carbonate, CuCO3 -- Colouring Agent This is the best copper compound for use with ammonium perchlorate for production of blue colours. Also used in other blue compositions. Copper (I) chloride, CuCl -- Colouring Agent Cuprous chloride is probably the best copper compound for creating blue and turquoise flames, and it can be used with a variety of oxidizers. It is non-hygroscopic and insoluble in water, but it is oxidised slowly in air. Copper oxides, CuO/Cu2O -- Colouring Agent Used for many years for blues, but needed mercury chloride to intensify colours. Seldom used. Copper oxychloride -- Colouring Agent Occasionally used in cheap blue compositions. Cryolite, Na3AlF6 -- Colouring Agent Also known as Greenland spar, this is an insoluble sodium salt. Sodium salts are used to produce yellow colours, but as sodium salts generally absorb water this tends to be a problem. By using cryolite this problem is surmounted. Dextrin -- Binder Dextrin is a type of starch that is added to many firework mixtures to hold the composition together. It is the most commonly used binder in pyrotechnics. Gallic acid (3,4,5-trihydroxybenzoic acid) This is used in some formulas for whistling fireworks. Whistle mixes containing gallic acid are generally the most sensitive of the whistling fireworks, with high sensitivity to both friction and impact when used with chlorates, but cannot be used with perchlorates either. There are safer alternatives for whistle compositions. Gum arabic (Gum Acacia) -- Binder An example of the various wood-resin-based adhesives used to bind firework compositions. Others used include Red Gum and Gum Copal. Gunpowder Black powder is the mainstay of pyrotechnics. At a basic level it is a mixture of potassium nitrate, charcoal and sulphur. However, simply mixing these ingredients together will not produce proper black powder. It merely produces a much milder version, which itself is used extensively in pyrotechnics, and is commonly called meal powder. True black powder takes advantage of the extreme solubility of potassium nitrate by mixing the very fine milled ingredients into a dough with water, then using strong compression to force the water out of the mixture, so that tiny crystals of potassium nitrate form in and around the particles of the other ingredients. This produces a product that is far fiercer than the simple meal powder. Hexachlorobenzene, C6Cl6 -- Colour Enhancer Used as a chlorine donor in coloured compositions that require one. Rarely used, with PVC, Saran and Parlon being preferred. Hexachloroethane, C2Cl6 -- Smoke Ingredient The basic ingredient in many military smoke formulas. Not often used with inorganic smoke mixtures, except those containing zinc. Iron, Fe -- Fuel The metal filings are used mainly in gerbs to produce sparks. Iron will not keep well in firework compositions, and so it is generally pre-coated with an oil/grease. One simple method is to add 1 gram of linseed oil to 16 grams of iron filings, mix, and boil off the excess oil. Linseed oil -- Stabilizer Used to coat metal powders in order to prevent them from oxidation, both prior to use and in the firework composition. Polyesters are used in commercial fireworks, but linseed oil remains an accessible option to the amateur. Lithium carbonate, Li2CO3 -- Colouring Agent Used to colour fires red. It has no advantage over strontium salts for the same purpose. Magnesium, Mg -- Fuel Used to produce brilliant white fires. Should be coated with linseed oil/ polyester resin if contained in a composition which is not to be used immediately, as it may react with other components of the mixture. The coarser magnesium turnings are sometimes used in fountains to produce crackling sparks. Magnesium-aluminium alloys give similar effects, and are rather more stable in compositions. Parlon -- Colour Enhancer, Binder Parlon is a chlorine donor, and a key ingredient in many coloured stars. It is a chlorinated isoprene rubber, chlorine content 66%. It interferes with burning less than PVC or saran, and can be used as a binder. It is soluble in methyl ethyl ketone (MEK) and partially in acetone. Compositions made with parlon and acetone or MEK are nearly waterproof. Phosphorus, P -- Fuel Phosphorus is rarely used in pyrotechnics today, except for a few specialized applications. It was used commonly many years ago, but as the hazards associated with its use became known it dropped out of use. Phosphorus comes in several forms, of which the red and the white/yellow varieties were used. Red phosphorus (used in the strikers on the side of matchboxes) is the more stable form, while white phosphorus (used by the military in incendiary devices) ignites spontaneously in air, and must therefore be stored under water or otherwise protected from the atmosphere. Both forms are toxic. Polyvinylchloride (PVC) -- Colour Enhancer, Binder PVC is a commonly used chlorine donor. It is not as good as Parlon for this purpose, but is cheaper and more readily available. PVC is soluble in tetrahydrofuran (THF) but almost all other solvents are useless. Methyl ethyl ketone (MEK) will plasticise PVC to some extent, however. Potassium benzoate, C6H5CO2K -- Fuel Used in whistling fireworks, in combination with potassium perchlorate. It must be very dry for this purpose, and should be less than 120 mesh. Potassium chlorate, KClO3 -- Oxidiser Originally used very commonly in pyrotechnics, potassium chlorate has gradually been phased out due to its sensitivity, in favor of potassium perchlorate. Mixtures containing potassium chlorate and ammonium salts, phosphorus or anything acidic are particularly dangerous. For this reason mixtures containing potassium chlorate and sulphur are to be avoided, as sulphur (especially the common "flowers" of sulphur) may contain residual amounts of acid that can sensitize the mixture. In general, potassium chlorate should be avoided unless absolutely necessary. Chlorates have probably caused more accidents in the industry than all other classes of oxidisers together. The reason lies in their sensitivity to acids and their low decomposition temperature. When mixed with an easily ignitable fuel, such as sugar or sulfur, chlorates will ignite from a fingernail striking a wire screen. Moreover, sulfur is often acidic, a fact that has lead to spontaneous ignition of sulfur-chlorate compositions. If you intend to use chlorates, pay extra attention to safety. Potassium nitrate, KNO3 -- Oxidiser A very common oxidising agent in pyrotechnics, potassium nitrate is one of the chemicals you should never be without. From its essential use in gunpowder to general applications in most fireworks, you will find potassium nitrate used wherever a relatively mild oxidiser is required. In fireworks it should pass 120 mesh, but can be used at 60 mesh. The fine powder should be used as soon as possible after grinding or milling as it will soon cake and have to be re-ground. Potassium perchlorate, KClO4 -- Oxidiser More expensive than potassium chlorate, but a better oxidising agent and far safer. In almost all mixtures that previously required the chlorate, safety factors have led to its replacement with potassium perchlorate. It should be used in place of the chlorate wherever possible. Potassium picrate This is a shock sensitive compound that is used in some whistle formulas. While safer than gallic acid formulas in this respect, care should be taken to keep it away from other metals such as lead, because some other metallic picrates are extremely sensitive. Saran -- Colour Enhancer, Binder Saran is another plastic chlorine donor. It is most commonly encountered in the form of the cling wrap used to protect foodstuffs. It is slightly soluble in tetrahydrofuran (THF) and will be plasticised by methyl ethyl ketone (MEK). Shellac -- Binder Shellac is an organic rosin commonly used as a binder where a water- soluble binder would be inappropriate. It can be bought at hardware stores in the form of lustrous orange flakes, which can be dissolved in boiling ethanol. Sodium salts -- Colouring Agents Sodium salts are sometimes used in place of the corresponding potassium salts, but this is uncommon due to their hygroscopic nature. They rapidly absorb water from the air, which can ruin a pyrotechnic composition. In particularly dry environments they can be used without too much trouble, and are therefore used in places like Egypt due to the relative cheapness of some of the salts with respect to the potassium ones. Sodium salts are also used as colourising agents, producing a characteristic orange flame. Strontium salts -- Colouring Agents Used to colour flames a brilliant red: Strontium carbonate, SrCO3 -- Colouring Agent, Retardant Used often for producing red colours, and as a fire retardant in gunpowder mixtures. Strontium oxalate, SrC2O4 -- Colouring Agent, Retardant, Stabilizer As for strontium carbonate, generally, but suffers from greater water content. Strontium nitrate, Sr(NO3)2 -- Colouring Agent, Oxidiser This is the most commonly used strontium salt, because it provides the most superb red colour available. Best results will be acheived if the strontium nitrate is anhydrous. Sulphur, S -- Fuel Another basic fuel in pyrotechnics, sulphur is used in many pyrotechnic formulas across the range of fireworks, most obviously in black powder. It is recommended to avoid the common "flowers" of sulphur, as they contain residual acid. If they cannot be avoided, a small amount of a neutralizer such as calcium carbonate should be added if acid is likely to present a problem. Titanium, Ti -- Fuel The coarse powder is safer than aluminium or magnesium for producing sparks, and gives rise to beautiful, long, forked blue/white sparks. Fantastic for use in any spark composition, especially gerbs. Petroleum jelly (Vaseline) -- Stabilizer Very occasionally used to protect metal powders e.g. iron by coating them with a thin film of petroleum jelly. Zinc, Zn -- Fuel, Smoke Ingredient Zinc metal is used in what are known as zinc spreader stars, which produce a very nice effect that looks like a green glowing cloud. Also used in several smoke formulas, due to the thick clouds of zinc oxide that can be produced. SPECIAL CAVEATS --------------- AVOID: Mixing chlorates with: acidic ingredients sulphur or sulphides ammonium salts phosphorus pitch or asphalt gum arabic solution. Mixing picric acid with: lead or lead compounds almost any other metal. Mixing ammonium nitrate with metals especially copper. Mixing nitrates with aluminium WITHOUT boric acid. Further Information ------------------- Further information about these chemicals, for example chemical, physical and toxicity data, can be obtained from the following books: The Merck Index The CRC Handbook of Physics and Chemistry Ullmann's Encyclopaedia of Industrial Chemistry Kirk-Othmer's Encyclopaedia of Chemical Technology The information may be found elsewhere, but these are the most comprehensive and readily available. --*** Many thanks to Dave Pierson, Christian Brechbuehler, Ken Shirriff, --*** Petri Pihko, Bill Nelson, Robert Herndon, Mike Moroney, Geoffrey Davis --*** and others for their helpful comments, corrections, additions and advice.
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