4th of July firework colours come from different elements of the periodic table — here's what fuels red, white, and blue stars in the sky

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America is celebrating its independence this week, but there’s nothing revolutionary about the way 4th of July fireworks are made.

Fireworks have been built from a mix of explosive powder, chemicals, and glue for hundreds of years; the earliest fireworks shows date back more than 1,000 years, well before the US made its debut as a country 243 years ago.

Not all fireworks are built the same – you can’t get a burst to look red by using the same ingredients as the ones inside a blue or white firework. That’s because the colour of a firework explosion depends on which kinds of elements are inside, from common metals to rarer minerals and even some salts.

Pyrotechnicians call these bursts of coloured light “stars,” and they’re made of a mixture of fuel, oxidizer (to help the fuel burn), colour-producing elements (like aluminium or copper), and a binder (glue), all packed inside a shell. That shell gets fired high into the air before a time-delayed fuse spits fire onto the stars and they take off.

California-based pyrotechnician and electrical engineer Mike Tockstein, who prepared the Los Angeles Coliseum for its 4th of July show last year, told Business Insider that it takes days of pounding, digging, wiring, and “well over 10,000 pounds of equipment” to set up for that kind of event.

So before you peer up into the sky this Independence Day, take a look at some of the common elements that are making your celebration possible.

Yellow fireworks are made from an element you might associate with the colour white: Sodium.

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You may think sodium belongs in your salt shaker, but burning-hot sodium produces a bright yellow explosion that’s perfect for lighting up the sky.

Red fireworks come from a common element called strontium.

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Strontium was used in the glass screens of a lot of old colour TV sets because it helped block x-rays from hitting us. The element has a yellowish colour, but it burns red hot.

Blue hues are still the biggest challenge for fireworks makers to produce. They’re made from copper.

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“Blue is still kind of the unicorn of fireworks manufacturing,” Tockstein said. The temperature of the flame has to be very precise, he added, otherwise you lose the colouring.

“There’s kind of a physics and chemistry limitation that prevents you from getting a good blue,” he said.

Green fireworks are a result of barium salts exploding in the sky.

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Most green fireworks are made from barium nitrate, which is toxic to inhale, so it’s not used for much else, though it can be an ingredient in grenades.

White light is made from aluminium or magnesium.

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These chemical elements have some of the highest burn temperatures. By adding them in to other colour creations, you can create lighter hues.

Firework makers also mix different elements together to create even more colours.

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Combinations of copper and strontium burn with purplish hues.

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Glittering golden chandeliers use one of the oldest fireworks ingredients around: carbon.

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A newer effect in fireworks is called “ghosting.” It’s basically a layering system that involves rolling different colours on top of each other inside each shell.

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Some fancy ghosting displays can make colours look like they’re dancing and moving.

“It’s more of the artistic side of pyro,” Tockstein said.

Eventually, what goes up must come down. The firework shell that brought lights into the sky falls to the ground as burnt cardboard.

Once the shell breaks, it falls to the ground as charred remains. Tockstein said that’s one of the main reasons you should enjoy fireworks shows at a distance.

“A shell itself is basically a sphere of cardboard,” he said.

It’s perhaps the least exciting part of the show, but for the people setting everything up, it’s a sign that the end of a long workday is near.

Update: This story was originally published on July 4, 2018.

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