"Baby, you’re a firework. Let your colors burst:” Science exploding 10,000 feet in the air

Credit to Katy Perry "Firework" Capitol Records

Credit to Katy Perry "Firework" Capitol Records

Musician Katy Perry sang it right when her song “Firework” became popular (1). Fireworks are mesmerizing, colorful, and light up the night sky. As the summer season officially started here in the states with America’s Independence Day, known for shooting fireworks in the sky, I don’t think we or Katy Perry think about the science behind those oohs and ahhs. 

Firecrackers and sparklers: what makes them spark?

Before we spark the light onto what an aerial firework is, let’s talk about firecrackers and sparklers.

Firecrackers originated in China and have been around since ~200BC. Used to ward away evil spirits, firecrackers were popular during the Lunar New Year. They were also used in special occasions like births and weddings. The use of firecrackers for these traditions is still practiced today.

AncientChinaLife.com

AncientChinaLife.com

The tube shape that a firecracker is known for resembles the shape of a bamboo, which, back then, was used as a firecracker. It is thought that the Chinese would roast the bamboo in which it would then explode when heated due to its hollow air pockets. Soon after the invention of gunpowder, stuffing gunpowder into bamboo and lighting them came into practice. Thus the first fireworks were born. Nowadays, flash powder is commonly used and packed into a tight paper tube with a fuse (2). 

Sparklers were invented in Germany in the 1850s. They work in similar fashion to the firecracker except a piece of wire was dipped into a combustible chemical paste which when burned, makes that iconic spark (2).

A lot of chemistry goes into the magic of firecrackers and sparklers. A composition of charcoal, sulfur, and potassium nitrate are commonly used in firecrackers and sparklers. Sometimes a firecracker will have aluminum to brighten the explosion. Sparklers also have a binder to bind the chemical compositions and then coat onto a wire.  When lit, a sparkler burns much slower than an exploding firecracker because the fuel and the potassium nitrate are proportioned along with the other chemicals (2, 3).

Make sure to also check out Seth's post on the History and Chemistry of Explosives.

A firework reaction lit into action

The composition of an aerial firework is derived from a combination of the chemical compositions and reactions of a firecracker and a sparkler, except packaged in a different fashion.

Credit to Business Insider

Credit to Business Insider

An aerial firework is normally formed as a shell that consists of four parts:

·Container - Usually pasted paper and string formed into a cylinder
·Stars - Spheres, cubes, or cylinders of a sparkler-like composition
·Bursting charge - Firecracker-like charge at the center of the shell
·Fuse - Provides a time delay so the shell explodes at the right altitude

Here’s how it works:

What makes a firework different from a firecracker and a sparkler is the addition of “stars.” Stars are like the sparklers that we like to wave around. However, they come in all shapes and sizes. In fireworks, they are commonly small spheres. These stars are poured into a tube of the shell and then surrounded by explosive powder like gunpowder.

When the fuse is lit and burns into the shell, which contains the stars and explosive powder, it ignites the bursting charge, causing the shell to first explode. The explosion then ignites the outside of the stars, which begin to burn with sparks. Since the explosion throws the stars in all directions, you get the huge sphere of sparkling light.

Fireworks are too dangerous to discharge on the ground. Below the shell, there is a lifting charge that shoots the firework into the sky. To launch the firework, the shell is launched from a mortar - this is basically a short steel pipe with a lifting charge of explosive powder that explodes in the pipe to launch the shell. When the lifting charge fires to launch the shell, it lights the shell's fuse. This starts an explosive chain reaction. The shell's fuse burns while the shell rises to the appropriate height in the sky, and then ignites the bursting charge so it explodes (4).

Exploding art

How do pyrotechnicians get those cool patterns in the sky and awesome variety of colors?

A cool infographic credit to CompoundChem.com

A cool infographic credit to CompoundChem.com

It all starts with how the stars are arranged. To create a specific pattern in the sky, you create an outline of the figure in star pellets, surround them as a group with a layer of break charge to separate them simultaneously from the rest of the contents of the shell, and place explosive charges inside those pellets to blow them outward into a large figure. For example, if the star pellets are equally spaced in a circle with black powder inside the circle, you will see a circle pattern in the sky. Each charge has to be ignited at exactly the right time or the whole thing is spoiled (3).

Along with the patterns, there are also a variety of colors that can be ignited within the stars. The stars can explode into different colors depending on the chemical elements they contain.

The following are some of the different chemical elements that can be placed into a star for color and/or the effect that it can produce (5):

Aluminum - Aluminum is used to produce silver and white flames and sparks. It is a common component of sparklers. 
Barium - Barium is used to create green colors in fireworks.
Calcium - Calcium is used to deepen firework colors. Calcium salts produce orange fireworks. 
Copper - Copper compounds produce blue colors in fireworks. 
Iron - Iron is used to produce sparks. The heat of the metal determines the color of the sparks. 
Lithium - Lithium is a metal that is used to impart a red color to fireworks.
Magnesium - Magnesium burns a very bright white, so it is used to add white sparks or improve the overall brilliance of a firework. 
Sodium - Sodium imparts a gold or yellow color to fireworks.
Strontium - Strontium salts impart a red color to fireworks. 
Titanium - Titanium metal can be burned as powder or flakes to produce silver sparks. 
Zinc - Zinc is used to create smoke effects for fireworks and other pyrotechnic devices.

Fireworks are a quintessential part of summer scenes. They ward off spirits during celebrations in many cultures and they are the symbol of summer, particularly in American cultures. As their “colors burst,” leaving people “in awe, awe, awe”, it takes a careful professional hand to work with fireworks mixed in with a chemistry. Please obey your state laws and be safe. They are explosives after all! 

Citations
(1) http://www.billboard.com/articles/news/949557/katy-perrys-firework-shines-over-hot-100
(2) http://www.pyrouniverse.com/history.htm
(3) http://science.howstuffworks.com/innovation/everyday-innovations/fireworks.htm
(4) http://www.theverge.com/2013/7/4/4489980/how-do-fireworks-work
(5) http://www.compoundchem.com/2013/12/30/the-chemistry-of-fireworks/