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By Ryan Hopkins
Across the world, fireworks lit up the sky to celebrate the New Year. The dazzling arrays of colors, patterns, and sounds of fireworks displays have been mesmerizing us for millennia (they were first used in the 7th century in China), but few watching a fireworks show are aware of how they work. Let's illuminate their mechanisms. Fireworks can be broken down into three primary components: colored light, noise, and smoke. Designing a successful display requires arranging all three effects in ways that complement each other and leave the audience awed. Although planning a show like this is a creative endeavor, designing the pyrotechnics involved is a fascinating area firmly grounded in science.
Firework shows have come a long way in the millennia since their invention. The earliest pyrotechnic displays were comprised of hand-lit fireworks which exploded at ground level. Today, shows are executed with precision by computers able to coordinate launches with a score. This change would not have been possible without advances in all areas of technology. Better materials for mortars and launch sites have made shows safer for operators. Research on salts has given fireworks better more vivid color. Progress in the world of computers changed shows forever, allowing operators to time launches well in advance and stand a safe distance away. In many tech-savvy shows, microchips allow both the launch and the midair explosion to be carefully controlled.
Fireworks emit such vivid color because of how elements behave when they are heated. Choosing the right elements for a desired color is a difficult task because of the how photon emission works. Check out one of our previous articles at www.nature.com/scitable/blog/scibytes/making_a_bang_the_science to learn about the mechanism in detail.
Once you've picked the right elements to produce your favorite colors, you need to think about how you will use them in an actual firework. In some cases, the element itself can be packed inside a firework. Titanium powder can be used like this, but it is a rare example. Sodium produces an intense yellow, but if it comes in contact with water it reacts violently. Strontium produces a vivid red, but will burn in air. These and compounds like them are kept as salts so they can be used safely. When picking a salt to use, it is important to avoid ones that will react with other compounds. For example, if you wanted to design a firework that produced yellow and white light, you would use aluminum power to produce the white and a sodium salt to produce the yellow. Sodium carbonate (water softener) and sodium bicarbonate (baking soda) are easily acquired salts of sodium that would seem to work, but they corrode aluminum. A more hazardous and expensive compound, sodium oxalate, would have to be used.
After choosing what colors and salts your firework needs, you need to give it a way to get into the sky. Black powder (a mixture of sulfur, potassium nitrate, and charcoal) is used to launch fireworks because it burns slowly enough that its explosion can be guided by a mortar or tube. Coarse-grained black powder is usually used for fireworks as it has larger grains, reducing the surface area of each piece, which further slows down the explosion. While the firework is flying into the air, the firework's fuse burns up to the compartment containing the compounds that will produce the light, color and sound. It is common for flash powder to accompany the color producing compound. Flash powder refers to a family of compounds, but when it is used it fireworks it is most commonly a mixture of potassium perchlorate and aluminum powder. This mixture produces a loud sound and quick, bright explosion when lit, so it is a great way to ignite the color of the firework.
One of the most popular types of fireworks is called the willow. The firework explodes and sends out pieces of salt and/or powder called stars, which are ignited and pushed out by the explosion. In a willow firework, the stars contain a large amount of charcoal that allows the trails of the stars to burn on their way down to the ground. The stars must contain a large amount of salt so that the color will last for the extended period of time.
What's next for fireworks? Chemists are constantly looking for new compounds to produce better colors. Currently, getting blue into the sky is the biggest challenge. The copper ion used for blue has its emission washed out by the intense heat needed to explode the shell. So next time you watch a fireworks show, take a moment to consider the amazing chemistry occurring above you, and keep on the lookout for rare blue fireworks!
Temple, Robert K. G., and Joseph Needham. The Genius of China: 3,000 Years of Science, Discovery, and Invention. New York: Simon and Schuster, 1986.
Kosanke, K. Pyrotechnic Chemistry. Whitewater, CO: Journal of Pyrotechnics, 2004. Print.
"American Chemical Society." American Chemical Society. N.p., n.d. Web. 10 Jan. 2015.
