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How it Works: Fireworks

How it Works: Fireworks

By B. James McCallum & Stuart Smith

We're all familiar with the high-pitched whish, the bright flash of light, the ear splitting retort. They can be festive, frightening, beautiful and dangerous, but we’re all forced to agree that there is something primal about them that means we just can’t get enough. Yet few are aware of the amazing history and science that fireworks represent.

The history of fireworks is inseparable from the history of gunpowder, which by most accounts was invented in China about 2,000 years ago. It is said that a Chinese cook mixed together several ingredients common to Chinese kitchens at the time – saltpeter, sulphur, and charcoal – and noted how fast they burned together.

Presumably the charcoal was a fuel source for fires, but why on earth were saltpeter and sulphur in the kitchen? Saltpeter (potassium nitrate) is used as a flavour enhancer, in a similar way to salt, in some regions of China, so would have been readily available. The sulphur is a bit harder to explain, but it could have been there to help start fires.

A cracking good party
It didn’t take long to figure out that if the powder was packed into a bamboo tube and placed in a fire it produced a satisfying loud noise and bright light. The fire cracker was spawned. Soon fire crackers were placed on the ends of arrows and used as military weapons, but it’s likely some of them exploded while still in the air - the first aerial fireworks.

Some claim fireworks were brought to Europe by that most apocryphal of explorers, Marco Polo. Others claim the Mongol hoards or even returning crusaders were the source. The Italians swiftly realised their potential in pageantry and started creating more artistic displays. The Germans, meanwhile, are credited with scientific advancement in pyrotechnics.

Fireworks became extremely popular in Great Britain during the reigns of Queen Elizabeth I and King James I. After Guy Fawkes attempted to blow up Parliament, however, politicians got understandably twitchy and restrictions were placed on the manufacture of fireworks.

Whizz bang chemistry
Though the earliest fireworks were literally gunpowder encased in a bamboo tube, modern fireworks are quite complex. A minimal amount of energy added to a simple saltpeter, charcoal, and sulphur firework creates a highly reactive mixture that produces heat, as well as oxygen, which speeds the burning of the other fuels and hot gases. Today, saltpeter is frequently replaced by other more reactive chemicals, such as chlorates or perchlorates. Modern fireworks also contain binders (usually starch dextrin) which hold the fireworks together, regulators to speed up or slow down the process of combustion, and, perhaps most interestingly, colouring agents.

When elements are heated, their electrons are excited and jump to a higher energy level. As these electrons return to their normal state, they give off energy in the form of light. Different elements produce different wavelengths of light and so can be used to create the multi-coloured displays we see on bonfire night. A chemist can use different elements like powders in a paint palette to mix colours. For example, copper (blue) and strontium (red) together produce a purple color. Various other elements can be added to produce special effects – zinc creates smoke and antimony, glittering effects. The mixtures are made into small packages called stars, which are then packed into cardboard tubes.

The mixture of charges used depends on the kind of fireworks required. A se
lf-propelling rocket, for instance, is made by placing a mixture that releases a lot of hot gases in a cylinder, with one end crimped into a nozzle and the other closed off. The same sort of charge placed in a stationary cylinder results in the charge and any attached star being propelled out of the cylinder like a bullet. A delayed fuse leading to a star packed with chemicals, including copper, makes a firework that propels itself into the air and then explodes in a brilliant blue.

Multiple stars and delayed fused burst charges can be added to both scatter the stars and ignite them, as well as whistles through which hot gases are
channeled for obvious effect. The result is a brilliant display of light and sound. It is then a simple matter of varying the composition of the stars, the delay fuses, and number of devices that are linked together to achieve almost any firework.

More from B. James on his home page.

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Title image: Nintaro
Other image: Patrick Nijhuis

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04 Jan 2011
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