A firecracker is perhaps one of the first fireworks a young would-be pyrotechnician learns to handle. It consists of a small cardboard tube wrapped in decorative paper, with a wick-like fuse extending from one end. The idea is to put it on the ground, light the wick, and then stand back. Within a few seconds, there should be a flash of light and a loud bang. If an entire string of firecrackers is lit, the result is a series of small explosions and a generous supply of smoke and fire.
So how do these things actually work? The answer lies in the nature of the chemicals packed inside the tube and the pressure they create once they begin to burn. A typical firecracker contains a small amount of black powder or gun powder, which burns rapidly when it comes in contact with an open flame, such as the chemically impregnated paper wick.
Black powder consists of three ingredients: potassium nitrate, charcoal (or sugar), and sulfur. A typical ratio of these three ingredients would be 75% potassium nitrate, 15% charcoal or sugar, and 10% sulfur. Each one of these chemicals plays an important role in the reaction started when a flame reaches the charcoal and starts a fire.
Inside a cardboard tube, the tightly packed black powder generally remains stable until the fuse is lit. When the fuse burns down into the tube, the charcoal ignites first. The sulfur interacts with the carbon in the burning charcoal and starts to create gases. The potassium nitrate acts as an oxidizer, essentially adding more oxygen to the mix and making the fire even hotter. The gases continue to expand, but they have nowhere to vent in the firecracker tube.
Finally, the pressure from the gases becomes too great for the tube and it bursts apart, creating the loud popping sound associated with fireworks. The burning black powder also becomes visible, causing a brief flash of light and a plume of black smoke. The firecracker itself becomes light enough to be carried off by the wind, which is why there is often no trace of the firework after ignition.
International laws concerning fireworks control how much black powder a fireworks manufacturer can put in an individual firecracker. The small ones used during Chinese New Year or July 4th celebrations do not generally contain much black powder, but certain illegal firecrackers may contain levels approaching that of commercial dynamite. Several fireworks manufacturers do sell larger versions that allegedly contain the highest amount of black powder allowed by law, but actual results may vary.
As with any other pyrotechnic device, a firecracker should only be lit under controlled conditions and should never be modified before use. Even a small one can cause serious damage to a user's body or property if not used as intended. It should also never be lit during drought conditions or in cities where Class C fireworks are considered illegal.