All fireworks rely on a combustion process that has three basic requirements: some sort of fuel to burn, a supply of oxygen, and a source of energy to initiate the reaction between the fuel and the oxygen. In the case of fireworks, the oxygen comes not from the air but is supplied by substances which release it by means of chemical reactions.
The earliest known oxygen supplier ("oxidizing agent") was potassium nitrate, KNO3, better known as saltpeter. The ancient Greeks were aware of deposits of this substance and discovered its value in warfare when mixed with a suitable fuel such as sulfur or oil. Ignition of this mixture resulted in a fire that was very difficult to extinguish due to the continuous internal production of oxygen. "Greek fire" struck terror into the hearts of the enemy!
But Greek fire was nothing compared to gunpowder, a substance that probably has had a greater impact on world history than any other chemical. An early formula was actually reported in an 8th century book by Marcus Graecus with the frightening title of "Book of Fires for Burning the Enemy." The recipe was probably based on the Chinese observation that a mixture of saltpeter, sulfur, and charcoal was highly flammable.
This "black powder" produced a large volume of gases when the two fuels, namely charcoal, and sulfur burned in the presence of oxygen released by the saltpeter. The Chinese actually used this mix as a propellant in primitive rockets made of bamboo poles. The forerunners of our modern missiles and fireworks!
The discovery of the explosive nature of gunpowder, however, is credited to the 13th century British alchemist Roger Bacon. He found that if the powder was ignited in an enclosed space, such as in a bottle, an explosion occurred. Bacon became so alarmed by his discovery that he concealed the formula for black powder in a code that was not unraveled for another hundred years. Once discovered, swords and arrows gave way to guns and cannons and the world would never be the same again.
Saltpeter, a decomposition product of manure, urine and garbage, became a much sought after commodity. "Nitre beds" were built outside villages where manure and garbage were piled and moistened with blood and urine. Napoleon actually issued an ordinance for citizens to urinate on nitre beds!
When a pile of black powder is ignited it noiselessly produces white smoke which actually consists of tiny suspended particles of carbon. If the same powder is sealed inside a cardboard tube and ignited, it produces a bang! The noise is created by the rapid generation of a gas at a high temperature which in turn causes the surrounding air to move quickly at the speed of sound. This is a firecracker! Light the same powder after it has been compressed into a tube with one end open and we have a rocket.
A typical firework is a rocket in combination with a firecracker. When the rocket is ignited, the hot gases generated by the burning black powder are jetted out the open end propelling the vehicle skyward. A timed fuse ignites the gunpowder in the "firecracker" part at an appropriate height and we hear noise and see smoke. But what about the sparks and colours?
In order to appreciate the color of fireworks, we have to understand a little about the nature of light and energy. When substances are heated, they absorb some of the heat and get hot. The absorbed energy can then be released in the form of light. That's why a burner on the stove glows red hot! If you have trouble relating to light as a form of energy just think about what happens to skin exposed to the ultraviolet light of the sun!
Different substances absorb and therefore emit different amounts of energy. We perceive these emissions as different colors. When sodium, for example, is heated, it gives off a yellow color. This is the characteristic yellow glow of the sodium vapor lamps that light our autoroutes.
Strontium produces a stunning red colour, the colour of emergency highway flares. Magnesium when heated glows brilliant white. It was used in old fashioned flashbulbs. Barium compounds are responsible for a striking green colour. So yellow, red, green, and white colours in fireworks are produced by adding the appropriate metals to the mix of fuel and oxidizer. But blue...that's a problem.
Blue is sort of the Holy Grail of fireworks. A deep blue colour is the ultimate challenge for the pyrotechnic designer. Copper monochloride will do it if the temperature is just right. But if it is not, then it is converted to other copper compounds that emit either red or green colours. The flame temperature can be controlled by using the right mix of oxidizing agents, a tricky business. Especially since potassium chlorate, the ideal reagent, is so dangerous that its use has been banned in England. Potassium perchlorate can do the job if the firework mix is properly prepared.
So the next time you are able to witness the wonderful spectacle of fireworks, you can appreciate the wizardry of the white sparkles and the ingenuity of the red, yellow, and green bursts. But if you see a deep blue explosion, you'll know that somebody really got the chemistry right!