They're made of paper mache andfired out of pipes, also called "guns."
Each shell undergoes two explosions. The first bag of gunpowder is attached to the exterior of the shell and, when ignited, provides the liftoff. The explosion also lights a delay fuse leading to the center of the shell, where a second bag of powder sits. That gunpowder bursts the shell 3 to 5 seconds later, when it's 300 feet to 1,200 feet in the air.
The falling colors that the audience sees are burning "stars" — dense nuggets of metal salts mixed with gunpowder, packed into the shell. The metals each burn a different color, Nitzan said. Strontium, for example, glows red, the color of a road flare.
Sometimes, fireworks change colors midair: The initial burst of blue turns red as the stars travel outward. Nitzan likens the structure of those stars to the candy Jawbreakers.
"You have one color of candy, and that's surrounding another color of candy. Same idea," he said. "So you have red material composition, and they put blue composition around that. When the star bursts, each star is blue and slowly as it burns down to the core, it becomes red."
To make the fireworks that explode into shapes such as Saturns, smiley faces and hearts, the stars are laid out in the shells in exactly those shapes.
"That cube shell actually has stars lined up in a cube orientation inside the shell, and the rest of the material is inert — rice hulls, typically," Nitzan said. "The simplest example might be a ring shell, where the northern and southern hemisphere of the shell are inert material, and the equator is live stars."
In addition to fireworks of varying shapes, the industry has seen the advent of "cake" fireworks. Basically a fireworks show in a box, they contain a hundred small cardboard tubes, preloaded with shells, Nitzan said.
The shower of pyrotechnics spouting from the deck of the Golden Gate Bridge for its 75th anniversary in May were cake products.
"You light one end, and it has a delay fuse that lights them one at a time," he said. "You can get some very nice effects."
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