To better understand my question, imagine the following experiment: you roll a cigarette, but you put in just half a tobacco and leave the rest of the cigarette empty. When you light up the cigarette (or actually the empty paper), it will start burning downwards, but the flame will stop once it reaches the line where tobacco starts. So how does the tobacco prevent the flame from continuing downwards and burning off all the cigarette paper around tobacco? I imagine it’s the same reason why the paper just doesn’t burn off before tobacco when you are smoking a normal cigarette.
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Paper can burn very fast, and the main thing which limits how fast it can burn, is how fast the fresh air can get to it.
When the inside of the cigarette paper is empty space, there’s open air on both sides and this is a very favourable situation for a flame.
When the burning gets to the part of the cigarette which is packed with tobacco, two things happen. First, the tobacco takes up space and inhibits the movement of air. Second, the tobacco starts to burn and smoulder along with the paper, and this releases CO2 and evaporates the moisture in the tobacco. The CO2 and water vapor both take up space, which makes it harder for more oxygen to get into the burning material.
And then, if you take a drag, you’ll see the combustion gets much hotter and faster for a moment, because you’re artificially forcing more air into it.
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