Fireworks were a part of America's very first birthday party on July 4, 1776. After the Continental Congress proclaimed the colonies' independence from England, John Adams wrote: "The day will be the most memorable in the history of America... it will be celebrated by succeeding generations with... bonfires and illuminations from one end of the continent to the other, from this day forward forevermore."

Early settlers set off black powder explosives to celebrate holidays. As impressive as they were in their time, those early fireworks looked nothing like modern ones. They weren't very bright, didn't go very far, and were all the same color. Eventually firework makers discovered that they could create dull orange and yellow hues by adding iron filings to the mix. But fireworks didn't become truly spectacular until the nineteenth century.

Colorful Chemistry
In the 1830s, the Italians made a big breakthrough in pyrotechnic chemistry. They began adding the compound potassium chlorate to the traditional black powder mixture. The compound helped oxidize the reaction, enabling it to burn hotter and brighter. The strongly exothermic (heat-giving) reaction made additional reactions involving other compounds possible — reactions that would soon add dazzling colors and special effects to fireworks.

To understand how the brilliant hues of fireworks are created, we need to zoom down to the atomic world. It all starts with excited electrons. The electrons in the cloud surrounding an atom's nucleus are found in different levels called orbitals. Electrons can be raised to higher-energy orbitals when an atom absorbs energy. The electrons then return to their original orbitals as the atom emits the absorbed energy as light.

The atoms of different chemical elements release very specific wavelengths of light corresponding to the different colors that we see. Colors at the warm (red and orange) end of the spectrum correspond to longer wavelengths of light. Colors at the cool (purple and blue) end of the spectrum have shorter wavelengths.

Throughout the nineteenth century, the fireworks masters put excited electrons to good use. They added brilliant new hues to their pyrotechnic palettes as they mixed different compounds with the black powder. The most impressive colors came from the explosion of salts of certain metals, such as barium, strontium, and copper.

Barium salts emit wavelengths of bright green light, while strontium salts glow a fiery red. Copper salts create different shades of blue and purple. The addition of sodium ions produces luminous golds and yellows.

Unfortunately, copper salts tend to form a highly explosive and hard-to-control compound called copper chlorate. So making blue was risky business at first. The riddle wasn't solved until chemistry advanced further in the twentieth century.

Look at the following table comparing colors and their wavelengths. The wavelengths are given in nanometers, or billionths of a meter.

Color	Element	Wavelengths (in nanometers)
	Copper (in copper salts)	420 - 460 nm
	Barium (in barium salts)	505 - 535 nm
	Sodium ions	589 nm
	Strontium (in strontium salts)	636 - 688 nm

• Which element on the table emits the shortest wavelengths of light in fireworks? What color do we see?
• Which element emits the longest wavelengths? What color do we see?

These days, other metals are added for brightness or special effects. For example, aluminum and magnesium are added to make electrifying white light. Titanium adds sparks and a mighty kaboom. Zinc helps create dreamy smoke clouds.

• The shell is the main container for modern fireworks.
• Stars are the pea- or marble-sized units packed inside the shell. They hold the metal salts and other chemicals and materials that create the colors and patterns.
• Breaks are the separate cardboard sections that separate the multiple stages of a shell. Each break has its own supply of black powder: the same potassium nitrate (75%), charcoal (15%), and sulfur (10%) mixture used for centuries.
• Spegettes are small, time-release fuses loaded into each of the breaks. Changing the amounts of explosives in the fuses will determine the exact second during flight that each break will ignite.
• A pouch of black powder is loaded into the bottom of the shell.
• The shell fits snugly inside a launch tube called a mortar, made from plastic or metal, which is usually buried in the ground. A mortar can vary in size from 2 inches to up to 3 feet in diameter, and can weigh up to 700 pounds. If the shell is not packed tightly enough in the mortar, the pressure created upon ignition will escape. That's called a dud.

In modern fireworks displays, the launch of the shell from the mortar is controlled by a computer. The computer triggers the lighting of the fuse, which ignites the black powder at the bottom of the shell, and the shell zooms into the sky.

For Pros Only! Remember that the science and technology described in this story are what professional fireworks masters use in putting on large public fireworks displays only.

To calculate the distance, just remember that old trick you learned to figure out how far away lightning is.

Light travels at the speed of 186,000 miles/second. Unless you're watching fireworks on Earth from the space shuttle, that's virtually instantly. By contrast, sound moves at the snail-like pace of about 1,100 feet/second. Simply count how many seconds pass between the sight of an explosion and the sound of its "kaboom."

• If you've counted five seconds between the flash and the bang, approximately how many miles are your from the explosion? (Remember that 1 mile = 5,280 feet.)
• If 17 seconds elapse, approximately how many miles away are you?