Colourful feasting

Metal chlorides for colourful sparks

Celebrating important events with fireworks goes back a long way: right up to Old China, more than a thousand years ago. Today, fireworks are popular on a variety of dates, from New Year's Eve to Olympic Games Closing Ceremonies.  

How does chemistry produce beautiful bursts of stars and cascading streams of colour in fireworks?  Well, the science and art of making fireworks - also called pyrotechniques - uses techniques that have been fundamentally unchanged for many years. The trick is to find substances that supply oxygen - we call them oxidisers - plus a fuel, such as charcoal. Add some other chemicals which serve as binders between both and which generate nice colours and sparks and the feast may begin.

In Europe, a long time ago, black powder (a mixture of potassium nitrate, charcoal and sulphur) was used for fireworks, before being adapted as gunpowder. But eventually, the chlorine-based compound potassium chlorate (KClO₃) took the place of potassium nitrate as the oxidiser. Today, this has been replaced by a more stable oxidiser - also based on chlorine - potassium perchlorate (KClO₄). Charcoal and sulphur continue to be used in the mix.

So we can already start oxidizing the fuel material. Before the 1800s, fireworks consisted only of loud explosions and a few sparkles, made by bits of metals such as iron, copper or zinc. Colours were mostly limited to shades of yellow and orange. But we want this reaction to be much more colourful.

Metal chlorides - of barium, strontium and copper - are responsible for the dazzling colours that flash against the night sky. The beautiful flashy green colour is provided by barium chloride (BaCl₂), red is traditionally generated by strontium chloride (SrCl₂) and copper chloride (CuCl₂) turns the sparks into little hell blue fires.

These chlorides have one disadvantage for firework manufacturers and users: they absorb moisture easily (what we call hygroscopic) which makes them become less effective as colour-producers. 

Recent advances in firework chemistry solve this problem. Instead of producing colour from the glow of very hot solid particles of metal chlorides, the new technique brings metal and chlorine together in a vapour during the burning process, providing energy to excite the molecules' electrons.  This produces flashes of colourful light. 

Let the party begin! But when using fireworks yourself, be sure to respect all safety measures described on the package. This beautiful chemistry also is dangerous chemistry!