Fireworks have changed tremendously since they were first used nearly 2,000 years ago by the Chinese in spiritual ceremonies to frighten away evil spirits. As forms of entertainment, fireworks have become louder, brighter, more complex, and more colorful.
Black powder is the oldest and most important chemical component of fireworks. This mixture of 15 percent charcoal (carbon), 10 percent sulfur, and 75 percent saltpeter (potassium nitrate) provides the fuel and the explosive force that carries fireworks high into the sky and causes them to burst forth in all their fiery brilliance. However, like many chemical compounds, black powder produces only one color when it is heated. It gives off a bright yellow flame that is nothing like the brilliant reds, oranges, greens, blues, and violets that fill the night sky each Fourth of July in the United States.
To create fireworks of different colors, pyrotechnicians, the people who make fireworks, rely on their knowledge of chemicals. Just as different elements and compounds have specific densities and melting points, combustible elements and compounds also produce light of a specific wavelength, or color. For example, strontium chloride, a metal salt commonly used in fireworks, produces a characteristic red flame, while barium chloride produces a burst of green light.
These characteristic colors are the result of energy transfer. When a substance is heated or burned, electrons surrounding the nuclei of the atoms that make up the substance are raised temporarily to higher energy levels. When the electrons fall back to their stable states, they release energy in the form of light and/or heat. The level of energy released from each type of atom corresponds to a particular wavelength of light. Low-energy emissions have relatively long wavelengths and give off light nearer the red end of the light spectrum. High-energy, short-wavelength emissions correspond to colors nearer the violet end of the spectrum. Pyrotechnicians use their chemical knowledge to combine different compounds into the elaborate patterns that lure audiences to their displays each year.