Most of us will have a basic knowledge of what pyrotechnics are, and are sure to have seen some displays in recent weeks heralding in the New Year. The term itself comes from the Greek words pyro (‘fire’) and tekhnikos (‘made by art’) – literally, fire art. This of course refers largely to fireworks, but can also include items such as safety matches, oxygen candles, explosive bolts and fasteners, gunpowder, ejection charges, flares and sparklers.
The substance, or mixture of substances, used to produce pyrotechnics are known as a pyrotechnic composition. The effect is achieved by a combination of heat, light, sound and gas/smoke, as a result of non-detonative self-sustaining exothermic chemical reactions. Pyrotechnic compositions are mainly homogenised mixtures of oxidizers and fuels with typical fuels based on metal or metalloid powders. Common fuels include aluminium, magnesium, iron, steel, titanium, zinc, copper and tungsten. We also use metalloids such as silicon, boron and antimony.
One of the main features of fireworks are the bright colours, which are produced by heating metal salts that then emit characteristic colours. This happens when the atoms of each element absorb energy and release it as light of specific colours. Energy, when absorbed by an atom, rearranges its electrons from their lowest-energy state, or ground state, to a higher energy state known as an excited state. The excess energy is then released as light. Each element has a characteristic amount of energy to release which will determine the colour of the light emitted. For example, red light can be produced by lithium carbonate and strontium carbonate, but blue light will be produced by copper compounds and chlorine producer.
When making fireworks the metal salts are put into pyrotechnic stars, which have 5 main components – a fuel, an oxidiser, the colour producing chemicals, a binder to hold the pellet together and a chlorine donor which strengthens the colour of the flame. The appearance of a firework is determined by the stars which are made by hand and very carefully packed in cardboard compartments within the shell.