By Chris Deziel | Updated Aug 30, 2022

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TL;DR

The crackling and popping you hear from a wood fire are caused by rapid expansion and release of combustion gases trapped within the wood’s cellular structure.

What Happens During Combustion?

Wood combustion is an oxidation reaction. Cellulose, the main component of wood, is a polymer of glucose (C₆H₁₂O₆). When it reacts with atmospheric oxygen, the exothermic process produces carbon dioxide, water vapor, heat, and light:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

During combustion, the wood itself doesn’t burn; instead, it sublimates, turning directly from solid into gas. If the temperature isn’t high enough to ignite these gases, they disperse as smoke along with unburned particulates.

Snap, Crackle, and Pop Explained

Wood’s internal architecture is far from uniform. Microscopic cells, bound by cellulose walls, house gases released during sublimation. As the temperature climbs, these gases expand rapidly, increasing pressure on the surrounding cell walls. When the pressure exceeds the structural integrity of the walls, they rupture, causing a sudden burst of gas – the audible pop or crackle.

Variations in log structure—knots, voids, or uneven density—can trap combustion gases in larger cavities. When the pressure builds, the resulting explosion can eject debris, explaining the occasional splash of wood fragments from a fire. For safety, it’s wise to line a fireplace with a metal mesh screen and maintain a safe distance from open bonfires or campfires.