The smoke is little bits of candle that started to burn but didn’t burn completely. When the candle is lit it’s hot enough to completely burn the wax. When you put it out the temperature goes down but not enough to completely stop it from burning so it continues to poorly burn the wax and give off smoke until it is cool enough that no burning at all happens.
(Solid) candle wax doesn’t burn. It has to be torn up into little molecules – then, if it’s hot enough, it will react with oxygen in the air to burn.
A candle flame is a tiny cloud of vaporized candle wax that’s super hot (that’s why it glows). When the hot molecules reach the edge of the cloud they hit oxygen and burn. This reaction heats up the cloud more and keeps the flame burning.
When a candle goes out, that little cloud gets too cold and no longer burns when it touches oxygen. You can see the vaporized wax drifting away as ‘smoke’.
This is why you need a wick, to start burning and melt the wax which is then drawn up and vaporized. The wick is protected from burning because the wax cloud keeps oxygen away from it. In the old days candlewicks would need to be trimmed every now and then, as they’d get super long and start producing too much wax vapor to burn cleanly. Candles would be super smoky after a while unless you cut the wick down. Modern candles have flattened wicks that actually curve to the side as they come free from the wax so that they poke out of the flame and burn down to a consistent length. Pretty cool technology! There are lots more cool candle facts but I’ll stop there.
The flame vaporizes the melting wax and the mixture of vaporized hydrocarbons and oxygen from the air is what causes the flame. When you put out the flame, the residual heat continues to vaporize the wax for a short while and these particles are carried up into the air by the heat. It is basically unburned fuel. You can even reignite it above the wick and if that smoke column is cohesive and dense enough, the flame will burn down to the wick and reignite the wick and restart the melting and vaporizing of the wax fuel.
Because smoke is flammable.
The the smoke coming from the wick is also catching fire as it’s not fully oxidized, so it’s burning further in the fire coming from the wick.
Smoke occurs when there isn’t enough oxygen to burn the fuel completely. Further away from the wick, more oxygen become available and the smoke continues to burn.
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