They flood the sky with brilliant bursts

of scarlet, emerald, and ivory,

but never blue.

Pyrotechnicians have tried to produce

blue fireworks for centuries,

and they have yet to succeed.

Why is blue so elusive?

John Conkling: The blue has been

very, very difficult to achieve

at a level comparable to the greens and reds and whites,

just because it's a stability issue at high temperatures.

[Narrator] That's John Conkling.

He's one of the world's leading experts in pyrotechnics,

and he says the problem comes down to chemistry.

You see, to make fireworks,

you need four basic components:

fuel (usually gunpowder),

a compound that produces color,

a fuse,

and glue to hold it all together.

You mix this stuff up into what's called a pellet,

and then shoot it into the air.

When the fuse burns up,

it sets off the gunpowder, which explodes.

That explosion heats up those color-producing compounds,

causing them to glow.

And it turns out...

Conkling: The hotter you can get the molecules

in your flame, the more emission you're gonna get,

so the brighter and more intense the flame color's gonna be.

[Narrator] But there's a limit,

because temperatures that are too hot

will break down those molecules and wash out the color.

But some molecules are hardier than others.

Strontium chloride, the compound used to make red fireworks,

can withstand at least 1,500 degrees Fahrenheit.

That's hotter than some lava.

But to make a blue firework, you need copper chloride,

which is much more fragile.

As soon as it gets hot enough to blaze blue,

at least 1,000 degrees Fahrenheit,

it starts to break down.

So pyrotechnicians need to find a mixture

that gets hot enough to set off the blue compound

but not so hot that it destroys the color.

Conkling: It's a delicate balance.

[Narrator] And even after centuries of searching,

we still haven't discovered the right one,

nor have we found a more stable replacement

for copper chloride.

And even if we do, we'd better hope that it's cheap

and nontoxic.

Conkling: Arsenic, for example, has been used

in some old fireworks formulations,

but obviously an arsenic compound

is not something you wanna put up in the smoke

where people are watching the fireworks.

[Narrator] To be fair, we've gotten close-ish.

Conkling: There's some respectable pale blues

that are used more in special effects,

where the audience is a little closer to the action,

and usually the color is more visible.

It's been a long search, and we're not there yet.

[Narrator] But there's still hope for bright blue.

Conkling: Certainly it's possible,

as there are people working on it,

so there could be a breakthrough one of these days.

[Narrator] And even if we never find that brilliant blue,

there's still plenty to get excited about on the horizon,

like fireworks that burst into different shapes

and patterns, even letters.

So maybe one day we can have an American-flag firework

for the Fourth of July.

We just need to get that blue.