WHY does the wood in the fire crackle?

In simple terms, the combustion process consists of two stages: pyrolysis of wood and combustion of its products. Pyrolysis is the decomposition of complex organics at temperatures up to 450°C. Approximately one quarter of the products are released immediately in the form of gases: carbon dioxide, hydrogen, methane and carbon monoxide. Another half of the wood mass combusts to form a liquid consisting of water and various organics.

The rest of the mass forms charcoal, which is 80-90% carbon. It is highly porous and smolders by interacting with the oxygen that enters its pores. The resulting gases accumulate between the layers of wood and tear them apart with a loud crackling sound.

The wood also crackles constantly, as it shrinks from the heat and deforms, causing the brittle charcoal that formed on the logs during pyrolysis to collapse.

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