A firecracker contains black powder made up of potassium nitrate, charcoal or sugar, and sulfur, which burn rapidly when ignited by a fuse. The pressure from the gases created becomes too great for the tube, causing a loud popping noise and a flash of light. Fireworks laws control the amount of black powder allowed, and firecrackers should only be used under controlled conditions.
A firecracker is perhaps one of the first fireworks that a young aspiring 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 step back. Within a few seconds there should be a flash of light and a loud bang. If you light a whole row of firecrackers, 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 start burning. A typical firecracker contains a small amount of black powder or gunpowder, which burns rapidly when in contact with an open flame, such as chemically impregnated paper wick.
Black powder is made up 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 of these chemicals plays an important role in the reaction that begins when a flame reaches coal and starts a fire.
Inside a cardboard tube, the compacted black powder generally remains stable until the fuse is fired. When the fuse blows in the tube, the charcoal ignites first. The sulfur interacts with the carbon in the burning coal and starts creating gas. The potassium nitrate acts as an oxidizer, essentially adding more oxygen to the mix and making the fire hotter. The gases continue to expand, but they have nowhere to vent into the firecracker tube.
Eventually, the pressure from the gases becomes too great for the tube and it ruptures, creating the loud popping noise 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 blown away by the wind, which is why there is often no sign of the firework after ignition.
International fireworks laws control how much black powder a fireworks manufacturer can put into a single firecracker. Small ones used during Chinese New Year or Fourth of July celebrations generally don’t contain much gunpowder, but some illegal firecrackers can contain levels approaching those of commercial dynamite. Different firework manufacturers sell larger versions that purportedly 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.
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