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A candle building the cup that holds its own melted wax Three candles side by side, drawn as solid blocks standing on a shelf. The first is unlit and its top is flat. The second is lit and a shallow dish has been hollowed out of its top. The third has burned longer: the dish is now a deep cup with a wall of solid wax standing all the way round it, and a flat pool of melted wax lying in the bottom.

A Candle Makes Its Own Cup

About 15 minutes

A candle looks like the simplest thing in the world. A stick of wax, a bit of string, a small yellow flame. There is nothing to it.

There is a great deal to it. A candle is a machine with no moving parts, and it is running the whole time it is lit. Over the next six parts we are going to take one apart — not with a knife, but by watching it carefully and asking awkward questions. By the end you will know what the flame is made of, what it is eating, what it leaves behind, and why you are doing very nearly the same thing right now.

Start with something you can see in the first minute.

The cup that was not there when you started

Light a candle and leave it alone for two or three minutes. Then look at the top.

The wax around the outside is still solid. In the middle there is a small pool of clear liquid. Between them is a thin wall of wax, standing up like the rim of a bowl.

Nobody built that cup. The candle made it, and it made it for a reason.

The heat of a flame goes almost entirely upwards. Hot air rises, and it takes the heat with it. So the wax directly under the flame gets hot enough to melt, and the wax a centimetre further out does not. The rim stays cool because the flame is not heating it — the air is carrying the heat past it and away.

That cup is doing a job. It holds the melted wax in a pool instead of letting it run down the side. A candle in a draught cannot keep its cup — the flame leans, the heat lands on one side of the rim, the wall breaks, and the wax pours out. That is what a "guttering" candle is doing. It has lost its cup and it is spilling its fuel.

A cut-through view of the top of a burning candle The top of a candle drawn as if sliced down the middle, with each part named beside it. A wall of solid wax stands up on both sides. A pool of melted wax lies between the walls. A wick runs up out of the pool. Above the wick the flame is drawn as an outline with a dark middle, which is the wax that has turned to gas and has not burned yet. the flame the dark middle the wick the pool the wall fuel meeting air wax as a gas melted wax solid wax, still standing
The three states of one substance, all at once. Solid wax in the wall, liquid wax in the pool, and wax as an invisible gas in the dark middle of the flame.

A wick is not a fuse

Here is a fair question. If the flame is at the top of the wick, and the fuel is in a pool at the bottom, how does the fuel get up there?

The wick does not burn down like a fuse. Watch one for ten minutes and you will see it barely shortens at all. The wick is not fuel. It is a pump.

Cotton thread is a bundle of tiny channels, and a liquid will climb a narrow channel all by itself, with nothing pushing it. This is called capillary action, and you can see it in about four seconds.

The journey one piece of wax takes from the side of the candle to the flame A candle drawn from the side with its five working parts marked in turn: the solid wax standing in the wall, the pool of melted wax lying in the cup, the wick carrying that liquid upwards, the space just above the wick where the liquid has boiled into a gas, and the flame where the gas finally meets the air and burns. solid wax standing in the wall melted wax a pool in the cup up the wick it climbs on its own wax as a gas too hot to stay liquid it burns where it meets the air

Step through what one bit of wax actually does on its way to the flame.

  1. It starts as solid wax, standing in the wall of the candle.
  2. Heat from the flame melts it into a pool of liquid in the cup.
  3. The wick draws the liquid upwards through its tiny channels.
  4. Near the flame the liquid gets so hot it turns into a gas you cannot see.
  5. That gas meets the air and burns. The flame is the gas burning.

Dip the corner of a paper towel in a saucer of water and hold the rest of it in the air. The water climbs. Nothing is pulling it and nothing is pushing it. Water sticks to the paper more strongly than it sticks to itself, so it creeps along every fibre until gravity finally wins.

Liquid wax does exactly this in the wick, and the flame keeps taking wax away at the top, so the climb never stops.

The thing that actually burns is not the wax

Now for the part that catches almost everybody out.

The flame is not burning the liquid wax. Liquid wax does not burn — you can dip your finger in the pool of a candle you have just blown out and it will be hot, but it will not set your finger alight.

Near the flame, the liquid gets so hot that it turns into a vapour: wax as an invisible gas. That is what burns. The candle's whole job is to turn a solid into a gas, a little at a time, at exactly the rate the flame can use.

You can prove it in one go, and it is the best trick in this book.

  1. Light the candle and let it settle for a minute, until the flame stops wavering and the cup has formed.
  2. Blow it out — one short puff, not a big one — and look straight away at the thin white trail rising from the wick. That trail is not smoke from something burnt. It is wax vapour that got hot enough to boil and never got to burn.
  3. Hold a lit match in the trail, five or six centimetres above the wick. Not touching the wick. In the trail.
  4. Watch the flame run back down the trail and relight the candle from a distance.

The flame travelled down a column of invisible gas. That is what a candle flame sits on: a fountain of wax vapour, coming up off the wick as fast as the heat can make it.

Why a flame is a teardrop

One last question before we go inside the flame itself. Why is it that shape?

A flame is very hot gas, and hot gas is lighter than the cool air around it, so it rises. As it goes up it pulls cool air in underneath from every side. That inrush squeezes the bottom of the flame narrow and drags the top out into a point. The teardrop shape is not the shape of burning. It is the shape of air moving.

You can tell, because if you take the moving air away, the shape goes too. On the International Space Station, where nothing falls and so nothing rises, a candle flame is a small blue sphere. No point, no yellow, no flicker. Astronauts have lit them on purpose to watch exactly this.

You blow out a candle and hold a lit match a few centimetres above the wick, in the rising trail. The candle relights. What was the flame travelling down?

So we have a machine that melts its own fuel, pumps it upward, boils it into a gas, and burns the gas. That is the outside of a candle sorted out.

Now look hard at the flame itself — right into the middle of it. It is darker in there than it is at the edge, and that is a very strange thing for a flame to be.