What could be more exciting that the finale of a fireworks display? The endless bursts of color, the detonations, the feeling that the fireworks are going to envelop you – well, it just doesn’t get much better than the combination of terror and delight that drives people into awestruck wonder.
How do fireworks deliver that punch to our holidays and celebrations? Their secret is the topic of a specialized field of chemistry known as pyrotechnics. Pyrotechnics not only fuel our parties, but also deliver thrills as special effects for movies and television. Musical groups use fireworks to enhance their live shows as well as their music videos. They commemorate the birth of our nation, and follow professional wrestlers into the ring. Initially developed by the Chinese over a thousand years ago, fireworks are an inextricable part of our culture.
But why do they stir our hearts and minds so? To some extent the impact is simply due to the dramatic shattering of the night sky, the sound of the explosions, the anticipation between the firing of the mortars and the blooming of the fireworks in the sky. But an essential part of the impact made by fireworks is their delivery of intense light in multitudinous colors. In some ways a display of all-yellow fireworks would be as impressive, but the introduction of the spectrum of colors takes our reaction to the next level. The excitement of a disco mirror ball or laser show pales in comparison to true fireworks!
What changes fireworks from simple fire to radiant colors of the rainbow? An applied knowledge of physics and chemistry. A simple rule is that hot objects radiate light. As the temperature increases, the visible light emitted by a hot object changes from a dull red to a bright white. But temperature differences are not the cause of the magnificent colors which appear in the best fireworks. What then is the secret?
The brilliant colored light in fireworks is generated by the burning of pellets, called stars, which are composed of special pyrotechnic compounds. Stars contain a colorant, which is a metal or metal compound that produces colored light and a chlorine donor which has the effect of increasing the light output. In some cases it is possible to combine these functions and use fewer materials. The surface of the star is often coated with black powder to insure ignition.
The color of the burning star is determined by the atomic emission spectra of the metal salts in the pyrotechnic compound. The metal atoms are excited to a higher quantum state by the heat of the burning fuel, and then emit light characteristic of the metal as they relax back to their ground state. Addition of different metals or combinations of metals will produce brilliant light of controllable colors.
A chlorine donor is commonly added to colored fireworks to form metal chlorides, which are volatile at the burning temperatures found in fireworks. The chlorine from a chlorine donor will compete with oxygen from the oxidizer, forming some quantity of the metal chloride. The metal chloride easily volatilizes in the environment of an exploding firework, with the result that the characteristic color emitted by the metal is far brighter than in a firework without a chloride donor.
Also, the ionic species that emits a desired color of light is not always a simple metal ion – it may rather be a molecular ion or radical. For example, barium oxide gives yellow light, while the barium monochloride ion produces green light. Many desirable colors are obtained using metal species which include chlorine in one manner or another. As a result, a chlorine donor is often required in the pyrotechnic mixture.
Now, how do pyrotechnicians get the wonderful range of colors that we see in firework displays? Most colors are not difficult to produce, although there is a great deal of art in increasing the purity and intensity of the emitted color. However, in the words of the professor who teaches the only for-credit university course on pyrotechnics: “This is probably where the art of it is, to get the right temperature for your reaction. Some colors are pretty easy, and those colors would be red and green, but you can tell how good a firework manufacturer is by the quality of their blues.”
Blue fireworks use copper chloride to produce that difficult color. It requires a lower flame temperature than do most other colors, and hitting just the balance between sufficiently low temperature for survival of the copper chloride and sufficiently high temperature for the production of bright light requires very delicate design balances.
A short listing of firework colors and the additives which produce them follows:
Red Lithium carbonate
Bright Red Strontium carbonate
Orange Calcium chloride, calcium sulfate
Gold Incandescent iron
Yellow Sodium nitrate, cryolite
Electric White Aluminum, magnalium, barium oxide
Green Barium compound with chlorine donor
Purple Mixed strontium and copper compounds
Silver Aluminium, titanium, or magnesium powder
Blue Copper acetoarsenite with chlorine donor
Turquoise Blue Copper chloride
The fireworks industry is a mixture of art and science that brings endless delight to audiences throughout the world. The human heart is profoundly touched by brilliant and choreographed firework displays which, at their core, are the result of simple chemical reactions between metals and oxidizers. Fireworks supply a dramatic symbolic demonstration of the constant truth of our Universe, that beauty emerges from simplicity.