The bits that did not get to burn
Why Does a Fire Make Smoke?
Smoke is tiny solid specks of fuel that never found enough oxygen, carried upward by hot air.
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Step 1 of 5
Smoke is not gas. It is dust.
Smoke looks like a cloud but it is made of solid specks so small they float. That is why it settles as a black film on a cold surface.
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Step 2 of 5
It appears when there is not enough air.
Burning needs oxygen for every scrap of fuel. Crowd the fuel or starve it of air and some carbon is left over, drifting off unburnt.
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Step 3 of 5
Hot air carries it upward.
The air above a fire is hot, so it is light, so it rises — and it takes the specks with it. In still air, smoke goes straight up.
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Step 4 of 5
A wick makes smoke when it is too long.
A long wick pulls up more wax than the flame can burn. The extra becomes soot, which is why trimming a wick stops a candle smoking.
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Step 5 of 5
Smoke is why fires are dangerous indoors.
The specks and gases in smoke choke you long before flames reach you, and they rise — which is why you crawl under smoke rather than walk through it.
Soot specks are about 0.05 to 1 micrometre across — small enough to slip past the hairs in your nose and reach the deepest part of your lungs.
The short version
Smoke is tiny solid specks of fuel that never met enough oxygen to burn, lifted away by the hot air above the fire.
Try it yourself
Hold a cold metal spoon in a candle flame for a few seconds. The black film on it is the smoke you could not otherwise catch.
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