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A candle flame drawn as a shell with an unburnt core inside it One large candle flame standing on a solid candle. The flame is drawn as three outlines inside one another, and the space right in the middle is left dark, because that is where the wax has turned to gas and has not burned yet. The burning only happens out at the edge, where the fuel can reach the air.

The Dark Part of the Flame Is the Fuel

About 13 minutes

You know now that a candle boils its wax into an invisible gas, and that the gas is what burns. That leaves something odd to explain.

Get a candle burning steadily, out of any draught, and look straight into the flame. Not at it — into it. Just above the wick there is a space that is noticeably darker than the yellow around it. It is not a shadow. There is nothing in front of it. The flame is dimmer in the middle than it is at the edge.

That is a peculiar thing for a fire to be, and it is the clue to the whole business.

A flame is a shell, not a blob

The dark space is the wax vapour that has not burned yet.

It has boiled off the wick, it is hot, it is rising, and it is completely surrounded by more of itself. It cannot burn, because burning needs air, and there is no air in the middle of a flame. The vapour has to travel to the edge before anything can happen to it.

So a candle flame is not a lump of fire. It is a thin shell of burning, wrapped around a core of fuel that is waiting its turn. The bright part you see is only the surface where the gas finally meets the air.

The parts of a candle flame, each one a separate place A large candle flame with three nested regions inside it. The middle region is the dark core of unburnt wax gas. Around that is the bright yellow region full of glowing specks of carbon. The thin outer edge is where burning is complete. Below the flame is the wick. Each of the four parts can be chosen on its own. the dark core the bright yellow the outer edge the wick fuel, not yet burning where burning finishes soaked in liquid wax

Choose a part of the flame to find out what is happening there.

  • The dark core — wax vapour rising off the wick. Hot enough to glow if it could, but there is no air in here, so nothing is burning at all.
  • The bright yellow — the vapour has begun to break apart in the heat, leaving tiny specks of solid carbon. They are white-hot, and glowing specks are what you are actually looking at.
  • The pale outer edge — where fuel and air meet properly and burning is complete. It is the hottest part, and it gives out very little light.
  • The wick — not burning. Pumping. It is soaked in liquid wax right up to where the heat boils it away.

Faraday showed this to a hall full of children by putting one end of a bent glass tube into the dark middle of a flame and letting the other end hang out. Something came down the tube. It came out as a white smoke, it sank because it was heavy, and — this is the part that settles it — you could light it. He was piping raw fuel out of the middle of a fire.

The heat is in a ring

If the burning happens at the surface, then the heat should be at the surface too. It is, and you can see the pattern it leaves.

Take a strip of ordinary paper, hold it flat, and lay it right across the middle of a steady flame for one second — an adult holds the paper, and takes it straight back out. When you look at the paper afterwards it is scorched in two brown bands, one on each side, with paler paper between them.

The paper has drawn a slice through the flame. Where it crossed the burning shell, it burned. Where it crossed the cool fuel in the middle, it hardly marked at all.

A strip of paper laid across a flame comes out scorched in two bands Above, a candle flame drawn in outline with a dashed line running straight through the middle of it to show where a strip of paper was held. Below, the same strip of paper afterwards, lying flat. It is dark in two bands, one on each side, where it crossed the burning shell of the flame. The middle of the strip is pale, because there it only crossed the unburnt fuel inside. the paper goes straight through here barely marked scorched scorched
A strip of paper laid flat across a flame comes out scorched in two bands with a pale gap between them. The gap is where the paper crossed the unburnt fuel in the middle.

Where the yellow comes from

Now the light itself. Why is a candle flame yellow, when a gas hob is blue?

In the middle of the flame the wax vapour is being cooked without enough air. It breaks apart, and one of the things it leaves behind is carbon — solid, sooty specks, far too small to see one on its own. Those specks get dragged out through the hottest part of the flame and they get white-hot on the way.

Anything hot enough glows. A horseshoe in a forge glows, a toaster wire glows, and a speck of carbon a thousandth of a millimetre across glows too. A candle flame is yellow because it is full of tiny glowing solids, not because the gas itself is yellow.

There is a neat proof. Hold a cold metal spoon in the yellow part of a flame for a moment, and the back of it comes out black. You have caught the specks before they had a chance to finish burning. A candle is a soot factory that normally burns its own soot on the way out — and when you put something cold in the way, you get to collect some.

A gas hob mixes air into the gas before it burns, so the carbon never gets a chance to gather into specks. Complete burning, no glowing solids, no yellow. A blue flame is hotter than a yellow one and gives out much less light. Candles are yellow because they are slightly bad at burning, and that is exactly why they were worth having for four thousand years.

You hold a cold spoon in the yellow part of a candle flame and it comes out with black soot on it. What is the soot?

So the flame is a shell, the dark middle is fuel, the yellow is glowing carbon, and the burning only happens where the fuel meets the air.

That last word has been doing a lot of quiet work. Air. The candle plainly needs it, and it needs it at the surface, and it cannot burn without it. So what is the air actually giving up?