Where firework colours come from

1

Each colour is a metal in a flame#

A coloured star, the burning pellet that flies out of a shell, carries a salt of one metal. In the flame the metal forms small molecules (sodium and lithium stay as free atoms), and these give off light only at certain wavelengths: strontium makes red, barium green, copper blue, sodium yellow, calcium orange.

In 2003 the pyrotechnics researchers W. Meyerriecks and K. L. Kosanke sprayed each salt into a burner flame at about 1,900 °C and recorded the light. The coloured stars on this page are drawn from their tables, with sodium and lithium from the NIST atomic tables, and turned into colour with the standard model of human colour vision.

2

Through diffraction glasses, a spark becomes copies#

The glasses drawn here carry 500 lines to the millimetre, a density sold for watching fireworks. They bend each wavelength by its own angle: violet at 400 nm by 11.5°, deep red at 700 nm by 20.5°. Beside every spark you see a copy for each wavelength it gives off, on both sides, blue nearest.

Sodium gives off one colour: two lines at 589.0 and 589.6 nm, too close to tell apart here. So a sodium shell throws one sharp copy of itself to each side.

3

Two reds that look the same#

Above, strontium; below, lithium. To the eye they are almost the same red. Through the glasses they come apart. Almost all of lithium's light is in one line, at 670.8 nm, so it makes one clean copy.

Strontium makes a smeared ladder of copies from two molecules: strontium monochloride (SrCl), strongest at 674 and 661 nm, and strontium monohydroxide (SrOH), at 682, 671 and an orange 606 nm. This is how a spectroscope tells elements apart when the colour cannot.

4

Green is barium chloride#

The green of barium comes mostly from barium monochloride (BaCl), a molecule formed in the flame, with two strong bands at 514 and 524 nm. Through the glasses the copies are narrow and close together.

That is why a classic barium green needs chlorine in its mix. Without it, barium forms barium hydroxide and barium oxide instead; barium oxide glows across a broad range from green to red, and the green turns pale.

5

Why blue is the hardest colour#

Blue comes from copper chloride, which glows mostly between about 410 and 500 nm, strongest at 436 nm. Above a certain flame temperature the molecule breaks up. One estimate puts that ceiling near 1,500 kelvin (about 1,230 °C), thermodynamic modelling near 2,500 kelvin (about 2,230 °C); the 2003 measurements recorded strong copper chloride light in a flame of about 1,900 °C.

Above the ceiling, copper hydroxide, which glows green, takes over, and hot particles glow white. Below, a blue star drawn as burning too hot. Chemists call blue the most challenging colour of all: in a 2021 comparison of ten blue mixtures, the purest averaged a colour purity of 56%.

Put the glasses on: the good blue has narrow blue and blue-green copies close to the spark; the hot one adds strong green copies and a rainbow smear.

6

Gold and white are heat, not chemistry#

Some light has no wavelengths of its own. Burning charcoal and hot metal particles glow like the element of a stove: every colour at once, weighted by temperature. Through the glasses they make a full rainbow smear with no gaps.

The gold willow above is charcoal sparks; they cool and redden as they fall. Below, white stars of hot particles from burning metal. Magnesium came into fireworks about 1865 and aluminium in 1894. The temperatures drawn here are chosen to look right; they were not measured.

7

A pinch of sodium turns red into orange#

Sodium is in almost everything, and its yellow line is so strong that a trace of it changes a colour. The 2003 measurements found the 589 nm peak even when the team sprayed distilled water.

Above, a strontium star; below, the same star with a third of its light from sodium. To the eye it is orange. Through the glasses, a sharp yellow copy stands out of the red ladder.

8

Purple has no wavelength of its own#

The rainbow runs from red to violet, but the purple that mixes red with blue is not in it. A purple star burns a red metal and a blue one together, strontium and copper, and the eye mixes the two.

Through the glasses they separate again: blue copies close to the spark, red copies far out, and between them a faint blue-green from copper's weaker bands.

9

A heart turned sideways is a line#

A heart shell carries its stars laid flat in the shape of a heart. Seen face on, it is a heart (left). Turned a little, it narrows (middle). Seen edge on, it is a line (right).

The shell bursts at a random angle, and it cannot know where you are standing. That is why hearts and smiling faces in the sky so often come out wrong.

10

First the flash, then the bang#

Light arrives almost at once. Sound travels through air at about 343 m/s at 20 °C, so the bang comes later the further away you stand.

Turn the sound on with the button on the picture, then move the launch site. The bang is synthesised; the delay is the real one.