The colours in fireworks stem from a wide variety of metal compounds – particularly metal salts. ‘Salt’ as a word conjures up images of the normal table salt you probably use every day; whilst this is one type of salt (sodium chloride), in chemistry ‘salt’ refers to any compound that contains metal and non-metal atoms ionically bonded together. So, how do these compounds give the huge range of colours, and what else is needed to produce fireworks?
The most important component of a firework is, of course, the gunpowder, or ‘black powder’ as it is also known. It was discovered by chance by Chinese alchemists, who were in actuality more concerned with discovering the elixir of life than blowing things up; they found that a combination of honey, sulfur and saltpetre (potassium nitrate) would suddenly erupt into flame upon heating.
The combination of sulfur and potassium nitrate was later joined by charcoal in the place of honey – the sulfur and charcoal act as fuels in the reaction, whilst the potassium nitrate works as an oxidising agent. Modern black powder has a saltpetre to charcoal to sulfur weight ratio of 75:15:10; this ratio has remained unchanged since around 1781.
The combustion of black powder doesn’t take place as a single reaction and so the products can be rather complicated. The closest thing to a representative equation for the process is shown below, with charcoal referred to by its empirical formula:
6 KNO3 + C7H4O + 2 S → K2CO3 + K2SO4 + K2S + 4 CO2 + 2 CO + 2 H2O + 3 N2
Variation in pellet size of the gunpowder and the amount of moisture can be used to significantly increase the burning time for the purposes of pyrotechnics.
As well as gunpowder, fireworks will contain a ‘binder’ – used to hold the components together, and also to reduce the sensitivity to both shock and impact. Generally they will take the form of an organic compound, often dextrin, which can then act as a fuel after ignition. An oxidising agent is also necessary to produce the oxygen required to burn the mixture; these are usually nitrate, chlorates, or perchlorates.
The ‘stars’ contained within the rocket body contain the metal powders or salts that give the firework its colour. They will often be coated in gunpowder to aid in ignition. The heat given off by the combustion reaction causes electrons in the metal atoms to be excited to higher energy levels. These excited states are unstable, so the electron quickly returns to its original energy (or ground state), emitting excess energy as light. Different metals will have a different energy gap between their ground and excited states, leading to the emission of different colours. This is the exact same reason that different metals give different flame tests, allowing us to distinguish between them. The colours emitted by different metals are shown in the graphic at the top of the page.
It’s the metal atom present in the compound that’s important, then – but some compounds are better than others. Hygroscopic compounds (those that attract and hold water) aren’t much use in fireworks, as they can render the mixture damp and hard to burn. Some colours are also notoriously hard to produce. The copper containing compounds tend to be unstable at higher temperatures, and if it reaches these temperatures, it breaks apart, preventing the blue colouration from being exhibited. For this reason, it’s often said that you can judge the quality of a fireworks display on the quality of the blue fireworks! Purple is also quite hard to produce, as it involves the use of blue-causing compounds in combination with red-causing ones.