The Chemistry of Fireworks: Bangs, Crackles & Whistles
With the 5th of November approaching, the distant reports of early fireworks displays can already be heard in the evenings here in the UK. Discussion on the chemistry of fireworks usually centres on the compounds used to generate their array of colours, but there’s a whole lot of chemistry behind the sounds they make too. Here we take a brief look at some of the ways in which pyrotechnic chemists give fireworks their characteristic bangs and screeches.
The obvious place to start is the bangs of fireworks when they explode into bursts of colour. These can be produced simply by compacted gunpowder, which if suitably confined can give a decent result. However, it’s more common for the ignition of a specific explosive mixture to be utilised. This mixture contains three main components: an oxidiser (such as potassium chlorate), sulfur (or antimony trisulfide) and metal pieces (commonly aluminium).
These compositions are known as ‘flash and sound’ compositions for rather obvious reasons. The oxidiser provide oxygen as it decomposes, and the metal acts as the fuel, producing a rapid, explosive reaction.
These compositions are notably dangerous; those made with potassium chlorate as the oxidiser have low ignition temperatures (as low as 200˚C), and even those made with potassium perchlorate are hazardous. As such, the mixture is usually created remotely.
Another common sound effect in fireworks displays is the fizzling, crackling sound. These fireworks use a slightly different composition to achieve the effect. Originally, lead tetroxide was one of the compounds used in this composition, but due to the toxicity of lead compounds, in the present day it’s more common for compounds like bismuth trioxide and bismuth subcarbonate to be utilised instead.
These compounds are mixed with magnalium (an alloy of aluminium and magnesium).
The composition is then formed into granules, and it is the rapid combustion of these granules that leads to the crackling effect. It has been suggested that the magnesium in the alloy first burns in the oxygen released by the decomposition of the lead or bismuth oxides, before the aluminium in the alloy then reacts with the remaining lead/bismuth oxide, reducing it to metallic lead or bismuth. The mixture is commonly referred to in the pyrotechnics industry as ‘dragon’s eggs’.
Finally, one of the most common sound effects that accompany fireworks displays are the screeching whistles as they rise into the sky.
These are the consequence of the use of some organic compounds. Various aromatic organic compounds can be used; salts of the very shock-sensitive picric acid were previously used, but now gallic acid, salicylic acid and benzoic acid salts are more commonly used.
These organic compounds are mixed with oxidisers, and then tightly packed into the firework tube. When they burn, the aromatic compounds produce small explosions which cause pressure changes in the gas being ejected by the burning mixture. This creates a standing wave in the tube, and as the distance between the end of the tube and the burning mixture increases, so does the wavelength, producing the characteristic descending whistle sound.