We have all seen some amazing fireworks displays and marvelled at their artistry and color. People everywhere of every culture enjoy the fantastic explosions and brilliant light displays. However, these spectacles are much more than just a form of entertainment. Each firework launched into the sky “is a precisely formed assembly of chemicals and fuel, carefully calibrated to produce a particular effect “, for example, a red chrysanthemum spray accompanied by a powerful explosion, or a blue strobe for variety. But how did such vibrant color come about?
To understand its development we have to have a quick look at the history of fireworks. They were first used 2,000 years ago by the Chinese in spiritual ceremonies to frighten away evil spirits and have changed tremendously since then. 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, causing them to burst forth in all their colorful glory. 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 color spectrum that we see in Britain on Guy Fawks night or the Americans enjoy on the Fourth of July. The old alchemists could also create fireworks of sparks, gold and silver, but had no idea how to get all the other vibrant colors we enjoy today. So what was to be done about improving the colors? This question baffled Europeans for centuries.
Then in the 1830s, Italian pyrotechnicians (the people who make fireworks), who had been developing the basic fireworks over the years, made a breakthrough. They changed the gunpowder recipe, replacing potassium nitrate with potassium chlorate. Pyrotechnicians rely on their knowledge of chemicals to create fireworks of different colors. They know that 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. 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 ‘excited’ energy levels. When the electrons fall back to their stable states, they release energy in the form of light and/or heat, or emissions. The level of emissions from each type of atom corresponds to a particular wavelength of light.
By burning potassium chlorate, they were able to speed up the rate of oxygen delivered to the chemical reaction and increased the combustion temperature from 1700 degrees celsius to 2000 degrees celsius. Now they were able to work with a new set of chemicals that burned at higher temperatures and produced more vivid colours. The colors were actually produced by heating metal salts, such as calcium chloride or sodium nitrate. For example, if you put copper in fire, it will glow with a blue flame and a good strong blue flame is the best background to a great fireworks, enabled by copper oxychloride. Adding strontium to that blue mix gives a red flame and adding barium to the mix makes a green flame. Those are the three main mixes, but by adding other metal salts, one has a spectrum of colors that can be used.
The color spectrum is made up of wavelengths of visible light. The metal salt with the longest wavelength is strontium that appears red when it burns. Copper salts have shorter wavelengths and appear blue when they burn. In short, low-energy emissions have relatively long wavelengths and give off light nearer the red end of the light spectrum while high-energy, short-wavelength emissions correspond to colors nearer the violet end of the spectrum. By carefully mixing various chemical compounds, many colour combinations can be created. For example, purple is made by a mixture of strontium (red) and copper (blue) compounds. Such chemical knowledge has enabled different compounds to be combined into the elaborate patterns that lure audiences to fireworks displays each year. If no salts are added, all of the spectrum colors combine in the fireworks to make their light appear bright white.
Sources
http://sciencesowhat.direct.gov.uk/articles/space-and-technology/high-tech-world/fireworksthesciencebehindbigbangs
http://scifun.chem.wisc.edu/CHEMWEEK/fireworks/fireworks.htm
http://sciencedude.freedomblogging.com/2009/07/02/the-colors-of-fireworks-what-is-the-secret/39861
