The primary chemical component in nearly all fireworks is "black powder". The recipe for black powder, a mixture of 75 percent saltpeter (potassium nitrate), 15 percent charcoal, and 10 percent sulfur, originated in China about 1,000 years ago with a slightly different proportion of ingredients. Black powder has been used in loud and fiery displays ever since -- first to ward off evil spirits, and later to entertain and celebrate. Today's fireworks rely on black powder for two critical functions. Gas released when the powder combusts first propels the firework skyward and later blasts its contents outward into the elaborate patterns that spectators come to see. However, other chemical ingredients are also needed; without them fireworks would produce little more than yellowish-orange sparks and white smoke. Due to differences in atomic structure, different atoms give off different wavelengths of light after being heated in a flame. Strontium chloride, for example, burns with a red flame, while barium chloride produces a green flame. To produce multicolored displays, pyrotechnicians, the people who create fireworks, combine several different chemical ingredients into a single firework. The lifting charge, made of black powder, propels the firework skyward. As it travels higher and higher, a time-delay fuse burns. When the firework reaches its peak, the fuse ignites one or more charges that blast the firework's "stars" -- individual pieces made up of these various chemicals -- into the air to form the beautiful patterns that audiences love. In general, the components of a firework expand evenly when the firework explodes. Therefore, the placement of stars inside a firework determines their arrangement when the explosion occurs. For example, a shell loaded with stars made of strontium chloride in the center and barium chloride around the perimeter will produce a flower-like display of red surrounded by green. By varying the type of chemical, as well as the size and configuration of stars, nearly infinite color and pattern combinations are possible.