The Chemistry Behind Firework Colors

Creating firework colors is a complex endeavor, requiring considerable art and application of physical science. Excluding propellants or special effects, the points of light ejected from fireworks, termed stars, generally require an oxygen-producer, fuel, binder (to keep everything where it needs to be), and color producer. There are two main mechanisms of color production in fireworks, incandescence, and luminescence. Incandescence Incandescence is light produced from heat. Heat causes a substance to become hot and glow, initially emitting infrared, then red, orange, yellow, and white light as it becomes increasingly hotter. When the temperature of a firework is controlled, the glow of components, such as charcoal, can be manipulated to be the desired color (temperature) at the proper time. Metals, such as aluminum, magnesium, and titanium, burn very brightly and are useful for increasing the temperature of the firework. Luminescence Luminescence is light produced using energy sources other than heat. Sometimes luminescence is called cold light because it can occur at room temperature and cooler temperatures. To produce luminescence, energy is absorbed by an electron of an atom or molecule, causing it to become excited, but unstable. The energy is supplied by the heat of the burning firework. When the electron returns to a lower energy state the energy is released in the form of a photon (light). The energy of the photon determines its wavelength or color.​ In some cases, the salts needed to produce the desired color are unstable. Barium chloride (green) is unstable at room temperatures, so barium must be combined with a more stable compound (e.g., chlorinated rubber). In this case, the chlorine is released in the heat of the burning of the pyrotechnic composition, to then form barium chloride and produce the green color. Copper chloride (blue), on the other hand, is unstable at high temperatures, so the firework cannot get too hot, yet must be bright enough to be seen. Quality of Firework Ingredients Pure colors require pure ingredients. Even trace amounts of sodium impurities (yellow-orange) are sufficient to overpower or alter other colors. A careful formulation is required so that too much smoke or residue doesnt mask the color. With fireworks, as with other things, cost often relates to quality. The skill of the manufacturer and date the firework was produced greatly affect the final display (or lack thereof). Table of  Firework Colorants Color Compound Red strontium salts, lithium saltslithium carbonate, Li2CO3 = redstrontium carbonate, SrCO3 = bright red Orange calcium saltscalcium chloride, CaCl2calcium sulfate, CaSO4xH2O, where x = 0,2,3,5 Gold incandescence of iron (with carbon), charcoal, or lampblack Yellow sodium compoundssodium nitrate, NaNO3cryolite, Na3AlF6 Electric White white-hot metal, such as magnesium or aluminumbarium oxide, BaO Green barium compounds + chlorine producerbarium chloride, BaCl+ = bright green Blue copper compounds + chlorine producercopper acetoarsenite (Paris Green), Cu3As2O3Cu(C2H3O2)2 = bluecopper (I) chloride, CuCl = turquoise blue Purple mixture of strontium (red) and copper (blue) compounds Silver burning aluminum, titanium, or magnesium powder or flakes Sequence of Events Just packing colorant chemicals into an explosive charge would produce an unsatisfying firework! Theres a sequence of events leading to a beautiful, colorful display. Lighting the fuse ignites the lift charge, which propels the firework into the sky. The lift charge can be black powder or one of the modern propellants. This charge burns in a confined space, pushing itself upward as hot gas is forced through a narrow opening. The fuse continues to burn on a time delay to reach the interior of the shell. The shell is packed with stars that contain packets of metal salts and combustible material. When the fuse reaches the star, the firework is high above the crowd. The star blows apart, forming glowing colors through a combination of incandescent heat and emission luminescence.