Chemistry of Gunpowder v3

With the Fourth of July and American Independence Day on the horizon, a somehow topical post seemed in order. Having already examined the chemical compounds that give fireworks their colours in a previous graphic, I decided to examine another important firework component here: the first chemical explosive, gunpowder, also commonly referred to as black powder.

Until the mid-19th Century, gunpowder was the only known chemical explosive. Its use can be traced much earlier than this, however, and there are historical accounts of its use in fireworks in China as far back as 1200AD. In subsequent centuries it had military applications in rifles and cannons, but in these it has long since been replaced by modern, smokeless powders. The fireworks industry is one of the last major industries that still uses traditional black powder.

Rather than being one particular compound, gunpowder is actually a mix of three different components. It consists of potassium nitrate (75% by weight), charcoal (15% by weight), and sulfur (10% by weight). Each of these components plays an important role in the combustion of gunpowder.

Potassium nitrate, also known as ‘saltpetre’, or ‘saltpeter’, decomposes at high temperature to provide oxygen for the reaction. This means that gunpowder doesn’t need to be exposed to air to burn – and is why smothering fireworks won’t stop them burning! The charcoal is often represented simply as being a source of carbon, which acts as a fuel, though it’s actually a broken down form of cellulose, with the approximate empirical formula C7H4O. Finally, the sulfur can also act as a fuel, though its inclusion has more to do with the fact that it undergoes exothermic reactions (reactions that give off heat) at relatively low temperatures, providing more energy and lowering the ignition temperature of the charcoal.

It’s worth noting that just mixing these three constituents together isn’t enough to produce good quality gunpowder; they must be thoroughly mixed, moistened and ground to produce a reactive mixture. Deviations from the ideal ratio given above are sometimes utilised to alter the burning behaviour of the mixture, and the addition of small amounts of water to the mixture can also be used to extend the burning time.

The precise reactions of gunpowder are difficult to elucidate. Rather than being a simple single reaction, the combustion of gunpowder consists of many differing complex reactions. It’s possible, however, to provide simplified equation that provides an overall idea of the products of the various reactions, as shown in the graphic. A mixture of solid and gaseous products are produced by the reactions, along with a very small amount of water.

The obvious use for black powder in firework is as the ‘lift charge’, which propels the firework into the air. The fuse, which allows the delay before the bursting of the firework, and the bursting charge itself, will also utilise gunpowder. The burning of the charcoal in gunpowder is often the source of the sparkling tails of fireworks as they ascend. The gases produced by the combustion reaction are the causes of the propellant effect, and the eventual bursting of the firework.

In some cases, safer alternatives to gunpowder that are more stable and easier to handle are now used in modern fireworks. However, many still utilise gunpowder, continuing a centuries old custom.



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References & Further Reading