Heat in, light out
A firework’s colors come mainly from metal-containing compounds that glow when heated in a flame. The underlying process is the same one that makes a gas-stove flame flash yellow when salty water boils over. Heat gives energy to electrons in the metal atoms and pushes them into higher energy levels. The electrons do not stay there. When they fall back, they release the extra energy as light.
Each element has its own set of possible energy levels, so each releases light of particular colors. That is the reason different metals give different flames, and why chemists have long used flame tests to help identify elements. The link between energy levels and the color we see is part of the broader science of light.
The usual suspects
Fireworks makers use metal salts, compounds that include a metal and another element. Textbook-level pairings include:
- Red: strontium compounds, and sometimes lithium.
- Orange: calcium compounds.
- Yellow: sodium compounds. Sodium is so bright that it can overwhelm other colors, so makers manage it carefully.
- Green: barium compounds.
- Blue: copper compounds.
- Silver, white and gold sparks: burning metal powders such as aluminum, magnesium or titanium, and sometimes iron or charcoal for gold effects.
Purple is usually made by combining a red-producing and a blue-producing compound in the same star, which is a good reminder that mixing is also part of the craft.
Why blue is hard
Blue is often described as the most difficult color. The copper compounds that give blue light tend to break down if the flame is too hot, and the color washes out if it is too cool, so the temperature has to sit in a fairly narrow window. Other substances in the mix can also pull the color toward white. Designers often work to keep the flame within that range, though the details are specialized and vary between makers.
Chlorine-containing compounds also play a role in many bright colors, because the molecules they help form in the flame are good light emitters. This is why a recipe is more than just “add the metal.”
What is inside a shell
An aerial shell is a carefully built package. A common design includes:
- Stars: small pellets that hold the color-producing chemistry. Each star burns on its own once lit.
- A bursting charge: a small amount of explosive that breaks the shell open at the right height and pushes the stars outward.
- A lifting charge: a charge at the bottom of the launch tube that fires the shell into the sky.
- Fuses: which time the burst after launch.
The pattern you see, whether a sphere, ring or heart, depends on how the stars are arranged inside the shell. A ring, for example, comes from stars laid out around a flat layer. The pattern is made, not accidental.
Oxidizers, fuels and binders
A firework star needs more than a color source. Burning needs a fuel and an oxidizer, a compound that supplies oxygen so the mixture burns even high in the air and in a compact pellet. Traditional black powder is a mix of potassium nitrate, charcoal and sulfur, and it is still used in lift and burst charges. Binders hold everything together in the pellet.
All of this is chemistry that releases a lot of energy quickly, which is why fireworks are dangerous outside of professional hands. Many places regulate who may make, store and fire them. The right approach for most people is to watch a licensed public display from a safe distance.
Sound and sparkle
The bang comes from rapidly expanding hot gases, and sometimes from special mixtures designed to produce a loud report. Glitter and crackle effects often use metal powders or other compounds that burn in stages. Because light travels far faster than sound, you see a burst a moment before you hear it, a gap you can use to estimate your distance if you know that the speed of light is nearly instant on this scale.
Pollution and cleaner designs
Burning metal salts releases fine particles and metal-containing smoke, which can affect air quality briefly near large displays. Some manufacturers are exploring alternatives to certain compounds, though the effectiveness and the details vary and are still developing. The topic is a good example of the way chemistry trades off brilliance, safety and environmental effects.
A tiny periodic table in the sky
If you know the periodic table, a firework display is an invitation to guess which elements are burning. The alkali and alkaline earth metals in the first two columns, such as sodium, calcium, strontium and barium, supply many of the warm colors. A display is, in a sense, a piece of atomic physics put on at a festival.