How do Fireworks Work?

Fireworks can be characterised as self-contained explosive devices which contain a mixture of many components such as oxidising agents, fuels and metal salts. We often associate fireworks with celebration, but the first fireworks were used to warn off evil spirits. Simple firecrackers are thought to have been developed around 1000 years ago by Chinese monk Li Tian, who compressed gunpowder inside a piece of bamboo before throwing the explosive into fire and allowing it to explode.1

In 1613, the ruler of Japan Tokugawa Ieyasu was introduced to the invention that was the firework,2 but it wasn’t until 120 years later that Japanese leader Tokugawa Yoshimune held the first ever firework festival. This festival is now known as Sumida River Fireworks Festival and is still held every year on the last Saturday in July.

Aerial fireworks usually have five different components that work together to deliver an aesthetically pleasing display (fig. 1). From the bottom to the top of a firework:

A; a wooden stick that is embedded in the ground before ignition.

B; the fuse of the firework which can be ignited to start the reaction.

C; the charge where the explosive chemicals are held.

D; this section contains ‘stars’, separate layers of chemicals responsible for the colourful explosions.

E; the head of the firework, usually pointed to make the firework more aerodynamic.

Figure 1. Labelled diagram of an aerial firework, where BE are known collectively as the ‘shell’.

Section C of a firework contains chemicals known as oxidisers which are a source of oxygen. Common examples of oxidisers found in fireworks include potassium nitrate and potassium perchlorate (Fig 2).

Figure 2. Reaction of oxidisers with heat to generate reactive oxygen atoms.

Once the temperature is elevated, these compounds release oxygen atoms which then react with fuel present in the firework to produce energy.

There are three main types of fuels used in fireworks:

  • Metal powders such as aluminium, magnesium, iron and titanium
  • Elemental fuels such as charcoal, sulfur and phosphorus
  • Organic compounds such as starch, sugars and plastics

Reaction of oxygen with these fuels releases heat and causes a pressure build up at the bottom of the firework which propels the shell into the air. Whilst the shell is still in the air, a second explosion occurs which causes the shell to burst open, releasing the stars into the sky.3 Metal fuels can also influence the colour of fireworks as at high temperatures, metals burn brightly and release incandescent light.4

The stars in section D are often composed of metal salts which produce luminescent light of different colours once heated. Depending on the salts incorporated, different colours can be achieved (fig. 3).

Figure 3. Metal salts responsible for the different colours displayed in firework explosions.

As well as being colourful, fireworks are also known for their crackling and whistling sounds. The crackling noises are caused by chemicals, usually metal salts mixed with an alloy of aluminium and magnesium called magnalium (fig 4. Left hand side).5 Many fireworks also contain whistle mixes, a pyrotechnic mixture of fuels that make a whistling noise when ignited. These fuels commonly contain potassium perchlorate alongside organic salts (fig 4. Right hand side).6

Figure 4. Left; metal salts responsible for crackling noises. From top to bottom: lead dioxide, lead tetroxide, bismuth oxide, bismuth subcarbonate. Right; organic salts responsible for whistling noises. From left to right: potassium benzoate, potassium picrate, sodium salicylate.

References

  1. K. Robinson, Patrick Moore ’ s Practical Astronomy Series – Starlight, 2009.
  2. S. Kamiya, Summer: The Season of ‘Fire Flowers’, https://www.japantimes.co.jp/news/2010/07/14/reference/summer-the-season-of-fire-flowers/#.XTCpDpNKiqA.
  3. C. Brownlee, ChemMatters, 2010, 8–10.
  4. A. M. Helmenstine, The Chemistry of Firework Colors.
  5. F. Wood-Black, The Boom in Fireworks, https://inchemistry.acs.org/content/inchemistry/en/atomic-news/the-boom-in-fireworks.html.
  6. A. R. Workman, Geothermics, 1985, 14, 595–599.

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