One of the most beautiful and entertaining uses of fire occurs in firework displays. Fireworks need a source of combustible material for energy such as black powder, a mixture of charcoal, sulfur, and saltpeter (an old name for potassium nitrate), or smokeless powder such as cellulose nitrate. In addition, fireworks contain substances that give off bright, colorful light when heated. A common example of such material is sodium in table salt. If salt is sprinkled into a flame, an orange color appears. The colored flame is a result of electrons in sodium ions absorbing energy and moving up to higher energy levels and then falling back to their ground state, emitting specific amounts of energy that correspond to colors of light. For centuries, this phenomenon has been the basis of flame tests in chemistry laboratories.
Chemical ingredients of fireworks are chosen to produce specific colors. Barium compounds produce green colors when heated, copper salts produce green and blue flames, sodium salts are yellow in flame, lithium compounds produce red colors, magnesium metal produces brilliant white light when burned, and strontium compounds produce brilliant red colors. Salts used contain both metallic cations and nonmetallic anions . Anions such as chlorates, perchlorates, and nitrates also contribute oxidizing power to the chemical mixture.
|Chemical Element||Color Produced|
While the metallic element dictates the color produced, the compound that contains the element has a profound effect on the type of flame. Calcium does not produce an exciting color by itself, but it enhances colors of other substances. Chlorine does not produce colored flames by itself, but the presence of chlorine greatly enhances the development of color from metallic elements. Chlorine-containing substances such as chlorate or perchlorate oxidizers or organic chlorine compounds such as polyvinyl chloride or hexachlorobenzene provide chlorine atoms to enhance volatility and light emission. Certain substances are included for specific effects. Iron filings sparkle and flash when mixed with other burning materials; the metallic iron oxidizes to produce Fe 2 O 3 , a process that produces a large amount of energy sufficient to cause the reacting iron particles to glow. Titanium metal is also used for production of sparks. Zinc is used in some smoke formulas and to produce star effects.
Fireworks consist of a source of energy such as a mixture of a fuel and an oxidizing agent that react to produce high temperatures and some substance that will emit brightly colored light. One of the simplest firework devices is a sparkler. Sparklers typically consist of a metal wire coated with a mixture of fuel and an oxidizer (mixed in proportions to allow burning), iron filings, and a glue to hold the components together. When the sparkler is ignited, the fuel and oxidizer burn, heating the iron filings so that they sparkle. Other substances such as zinc or magnesium alter the character of the sparks.
Firecrackers contain flash powder (a mixture of an oxidizer such as potassium chlorate or perchlorate and powdered aluminum or magnesium) or black gunpowder in a paper tube. An attached fuse ignites the flammable mixture, which burns explosively, producing gases that rapidly build up pressure and burst the container. Aluminum and magnesium components produce brighter flashes.
Aerial fireworks usually are of two types, aerial shells fired from tubes and the traditional skyrocket. Rockets are made of cardboard tubes filled with a mixture of fuel and oxidizer in proportions that allow continuous burning rather than explosion. Expulsion of gases from the tube propels it skyward. Rockets often contain explosive charges to explode after the propellant charge burns out; the composition of the explosive charge determines the colors produced.
Aerial shells are small balls of explosive material fired from a steel or cardboard tube or stand. A lifting charge throws the ball skyward, and the explosive charge fires when the embedded fuse burns down after a time period appropriate for the shell to reach the desired altitude. The shell usually contains a bursting charge and stars made up of cubes or spheres of material that will burn, sparkle, or explode. Multibreak shells are made up of combinations of shells designed so that the explosion of one shell ignites the next.
Shells designed to explode with a bang are called reports or salutes. The whistling effect of some devices is produced by packing techniques that cause intermittent burning. Specialized shells designed to burst forming patterns such as hearts or circles are made by surrounding the break charge with pellets containing explosive charges. When the break charge explodes, the pellets are blown outward, producing a pattern.
In addition to their value as entertainment, pyrotechnics have military applications as signaling, training, and combat devices. Burning naphthalene and anthracene produce black smoke that can be used to screen off an
area, but may be dangerous in populated areas. White smoke produced by vaporizing zinc chloride or oil or burning phosphorus is sometimes used to provide cover during combat; the hydrolysis of silicon chloride (SiCl 4 ) produces a white smoke as well.
SiCl 4 + H 2 O → SiO + HCl
The moisture in the air is usually sufficient for producing the desired reaction. Colored smokes for signaling are usually produced by volatilization of organic dyes. Burning mixtures that provide enough heat to vaporize the dye, but not enough to decompose it, are chosen. Dyes chosen must be volatile, but nontoxic.
A simple and safe home experiment can be carried out by squeezing an orange peel near a candle flame. The oils of the peel produce tiny flashes of light as they burn. Bananas contain large amounts of potassium; a banana peel in a bonfire shows the characteristic violet color of potassium flames.
Conkling, John A. (1985). Chemistry of Pyrotechnics. New York: Marcel Dekker Inc.
Donner, John (1997). Professional's Guide to Pyrotechnics: Understanding and Making Exploding Fireworks. Boulder, CO: Paladin Press.
Dotz, Warren; Mingo, Jack; and Moyer George (2000). Firecrackers: The Art and History. Berkeley, CA: Ten Speed Press.
National Council on Fireworks Safety. Available from http://www.fireworksafety.com .