SciTech

How Things Work - Chemical and physical science explains types of fireworks

Credit: Courtesy of Andrews Aguiluz Rios via Wikimedia Commons Credit: Courtesy of Andrews Aguiluz Rios via Wikimedia Commons

With the end of summer comes the end of one of summer’s greatest traditions — man-made explosions in the sky.

Whether it’s for baseball games, the Fourth of July, amusement parks, or personal pleasure, fireworks are a prevalent part of summer. But how exactly do pyrotechnists create all those colors and shapes in the middle of the night sky?

The discovery of fireworks can be traced back to the discovery of gunpowder, which occurred around 2,000 years ago in China. It is hypothesized that a cook accidentally mixed together and heated potassium nitrate, charcoal, and sulfur to produce a black, flakey powder. When this powder was inserted into a bamboo shoot and tossed in a fire, the pressure inside the shoot from the gases of combustion caused the shoot to blast apart.

Aside from replacing the bamboo shoot with a paper tube and adding a fuse, our modern definition of a firecracker hasn’t really changed. Sometimes, aluminum is added to the powder to create a brighter explosion.

Over time, another fun, handheld pyrotechnic soon emerged — the sparkler. Sparklers consist of a thin metal wire dipped in sparkler composition and left to dry. Sparkler composition typically contains a metallic fuel, an oxidizer, iron or steel powder, and a combustion binder to hold it all together. Adding certain pyrotechnic colorants also produces different colored sparks — strontium for red, calcium for orange, sodium for yellow, barium for green, and copper for blue.

The sparkler is lit from one end and burns for several minutes, depending on the length. Since all the chemicals are proportioned in a sparkler, the sparkler burns much more slowly than a firecracker.

However, we can all agree that the ultimate pyrotechnic is the firework. Aerial fireworks are found in the form of a shell consisting of a cylindrical paper container, various shapes of a sparkler-like composition known collectively as stars, a bursting charge at the center of the shell, and a fuse.

A mortar — usually a short, steel pipe — launches the shell into the air using the resulting explosion of a lifting charge inside the pipe. The lifting charge also lights the shell’s fuse, and depending on the length of the fuse, the bursting charge is ignited at a specific altitude and the shell explodes.

As the shell explodes, the outsides of the stars in the shell are also ignited, causing a bright shower of sparks. Since the stars are thrown in multiple directions via the shell’s explosion, the shower of sparks forms a huge sphere of sparkling light — the most basic firework shape.

Shells can also ignite in multiple phases, known as multibreak shells. These can be shells within shells, or shells with multiple sections that are broken apart using break charges. These shells can contain stars with differing colors or compositions to create different lighting and sparkle effects.

But if all fireworks were spherical, the displays wouldn’t have as much pizzazz as they currently do. The shape of the firework largely depends on the shape of the shell and the arrangement of the stars.

To create any two-dimensional figure, the stars must be placed in pellets and arranged in an outline of the figure in the shell. The pellets are surrounded by break charges to separate them simultaneously from the shell and contain explosive charges to propel the stars outward in a blown-up form of the figure.

Pyrotechnists have standardized many of the popular shaped fireworks. To name a few, palm fireworks contain charges in the shape of a solid cylinder that travel outward before exploding, then curl downwards after exploding to form the limbs of a palm tree. Willow fireworks (my personal favorite) contain stars with a higher composition of charcoal to make the sparkles burn longer so when the stars fall they form the shape of a willow tree.

Pyrotechnists are pushing the limit of what shapes and colors can be achieved with fireworks with every show, leading to an increasingly enjoyable viewing experience.