The Workings of Fireworks
My friend is a licensed pyrotechnician. Every July 4th, I help her setup and fire off a local city’s fireworks show. When you experience a fireworks show from below, you see, hear, feel and almost taste the fireworks. You are also privy to the behind the scenes drama as the fireworks is hand lit, like watching a play from back stage. It is hard to ever go back to watching the pretty lights up in the sky from far away. And it has made me appreciate fireworks more, even from a scientist’s perspective.
There is more to fireworks than pretty lights up in the sky. How are the bursts of colored light created? How do they make the different effects? How do they get up so high in the sky? There is plenty of science behind a fireworks show.
Black powder is used to lift each fireworks shell into the air. It has been around for many centuries. It is a combination of potassium nitrate, charcoal and sulfur. If you burn black powder in the open, the heat and gas from the explosion quickly dissipates. So you put the black powder inside the bottom of the fireworks shell and place the fireworks shell inside a mortar (launch gun). This allows you to trap the heat and gas from the burning black powder, causing the gas pressure to build up until an explosion launches the fireworks shell high into the air. The fireworks shell must fit snugly in the mortar, or pressure will escape and cause a misfire. A variety of different sized shells and corresponding mortars are used to create an interesting fireworks show.
The heart of a fireworks shell is the multiple compartments of combustible materials, called stars. Each kind of shell consists of different kinds of stars, in order to get the different colors and effects that we all enjoy. Great care goes into a shell design. Each star is made from a combination of binders, oxidizers and coloring agents. Binders (typically dextrin) are used to hold everything together. Oxidizers are used to produce the oxygen needed for the mixture inside the star to burn. The most common oxidizer is potassium perchlorate. Perchlorate ions have a chlorine atom bonded to 4 oxygen atoms, so perchlorates are relatively stable compounds that release a lot of oxygen. The fireworks colors are imparted by different metal compounds, such as: magnesium or aluminum for silver; strontium carbonate for red; calcium salts for orange; sodium oxalate for yellow; barium nitrate for green; and copper carbonate for blue. As a star burns, the perchlorate releases oxygen and its chlorine combines with the metal compounds to form metal chlorides. These metal chlorides release energy in the form of visible light when they reach high temperatures. The color (and wavelength) of the emitted light varies with the temperature and metal compound. Blue is the hardest color to produce, because it requires a higher temperature.
A fireworks shell is ignited by lighting the main fuse. This simultaneously lights both a fast-acting side fuse and a slow time-delayed fuse. The side fuse quickly ignites the black powder to launch the shell high into the air. The time-delayed fuse burns slowly into the center of the shell as it hurls into the sky, causing the aerial fireworks display when it reaches and ignites the stars. The amount of black powder in each shell is precisely determined so that the time-delayed fuse ignites the correct star compartment when the shell is reaching the apex of its upward flight.
A shell can also contain sound charges, creating the exciting crackling, bangs and booms to accompany the light show. When you watch fireworks, you see them much sooner than you hear them because light travels much faster than sound.
So when you watch fireworks on Sunday, you may want to think about all the science that goes on to produce the show. Or you may just want to “ooh” and “ah” in appreciation of the beautiful aerial display. Just make sure that you verbalize your pleasure, because the crew working the show will love hearing your encouragement.