One of the key ingredients for firecrackers, ground fireworks, and aerial fireworks (ones which explode in the sky) is black powder. It was invented by the Chinese about 1000 years ago. It’s a blend of potassium nitrate (saltpeter), charcoal, and sulfur in a 75:15:10 ratio. Black powder is used to launch aerials and also causes the explosions necessary for special effects like noise or colored light.
In sparklers, black powder is mixed with metal powders and other chemical compounds in a form that will burn slowly, top to bottom. In simple firework rockets, black powder is confined in a tube around a fuse. When lit, the powder creates a force that results in an equal and opposite reaction. This pushes the firework off the ground and causes the compounds inside it to explode in the air. More complex fireworks shells are launched from a mortar, a tube with black powder that causes a lift-off reaction when lit. The firework shell’s fuse is then lit as it goes up into the air, and at the right time an explosion inside the shell causes its special effects charges to burst.
The bright, colorful part of the fireworks display is caused by “excited” electrons in the atoms of different metal and salt compounds. These compounds are in little balls called stars, made of a similar compound to what makes a sparkler work. Different metals burn in different colors. For example, if a copper compound is lit, its flame will be a blue-green color. Calcium burns red-colored and potassium burns purple. In fireworks, metals are combined to create different colors.
When the star compounds inside a firework are heated, the excited atoms give off light energy. This light falls into two categories: incandescence and luminescence. Incandescence is light produced from heat. In fireworks, reactive metals (aluminum or magnesium) cause a burst of very bright light — sometimes at temperatures over 5000° F! Compounds that are less reactive don’t get as hot, resulting in dimmer sparks. Luminescence, on the other hand, is produced from other sources and can occur even at cold temperatures.
The electrons in the compound absorb energy, making them “excited.” The electrons can’t maintain this high level, though, so they jump back to a lower level. They release light energy (photons) in the process. Barium chloride gives fireworks a luminescent green color, and copper chloride makes a blue color. For either kind of light, it’s important to use pure ingredients since traces of other compounds will obscure the color.