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OUT OF THIS WORLD | Earth, Space And The Stuff In Between - a daily journey through weather, space and science with meteorologist/science writer Scott Sutherland

What makes one firework red, and another green?

Scott Sutherland
Meteorologist/Science Writer

Thursday, June 29, 2017, 1:07 PM - Going into the Canada Day weekend, we can expect to see a lot of fireworks going off across the country, from small backyard displays for the family to huge public shows to delight the crowds attending celebrations. So how do these delightful, yet potentially dangerous explosives work?

Remember waiting for the fireworks to begin, and filling the time by running around, waving a sparkler through the air and possibly making your parents worried about you potentially hurting yourself or setting something on fire? Well, you may not have realized it, but you were holding in your hand the very basis of the colourful displays of fireworks that you were waiting for.

Sparklers don't explode, of course. They're a mixture of a fuel source (charcoal and sulfur), an oxidizer (likely potassium nitrate), metal powder (for colour) and a binder (like starch), all stuck to a thin metal rod. Apply a flame to the sparkler anywhere along the length of the mixture and the gunpowder ignites, the oxidizer releases oxygen, and the entire reaction becomes self-sustaining, spreading to the full length of the mixture. As the gunpowder burns and the oxidizer super-charges the reaction, the heat from this causes the grains of metal powder to glow brightly and throw off the characteristic sparks.

Take away the metal powder and binder, pack the remaining gunpowder mixture into a paper cylinder or sphere, slip a fuze in to deliver the ignition to the powder, and you have what could be called the 'next step up' from sparklers - the firecracker. These are simply designed to explode, with the only flash of light coming from the explosion itself.

It's when you combine these two - the sparkler and the firecracker - that you get the main event: the huge, explosive fireworks that generate colourful blossoms of light high in the sky.

A standard firework 'shell' has a firecracker at its core, called the 'burst charge', surrounded by gunpowder, with pea-to-dime-sized spheres made of sparkler mixtures, called stars, packed into the gunpowder. Sometimes the stars are packed randomly (for just spherical blossoms), but specific patterns of the stars in the gunpowder will produce those same patterns in the explosion. A standard firework shell either has no propellant, thus relying on a launch cylinder, or they can have a gunpowder-filled "lift charge" at the bottom.

When the fuse is lit, this ignites the gunpowder to launch the shell into the air, while also burning the timing fuse that will set off the burst charge(s). Since the fuse to the burst charge burns longer than the fuse to launch the shell, the shell can reach a height where it will be visible for kilometres around when it goes off.

When the burst charge explodes, it causes everything in the shell to be thrown outward in all directions, while simultaneously igniting the gunpowder. The burning gunpowder brings the heat to the sparkler spheres, which ignite and burn down to their core. These glow brightly as they burn, throwing off sparks and leaving bright trails of light through the sky to amaze and delight the audience.

The colours of the fireworks are from very specific mixture of metals or chemicals that go into the sparkler stars.

Meanwhile, special effects are achieved by organizing different layers of gunpowder and stars that go off at different times, by putting different shells together as separate compartments of a larger shell, or by changing the composition of the stars so that they burn for a long time, crackle and whistle as they burn, or burn down and then expose a core that explodes, possibly producing even more stars as a result.

And the really great thing is that this knowledge of the science that goes into making these amazing displays takes nothing away from the awe they inspire. Enjoy a safe and fun weekend, and click the Play button below for the big finale!

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