The combustion of a log in a fire is a complex chemical reaction, representing a rapid process of oxidation that releases the solar energy stored within the wood. This event requires the classic “fire triangle” components: fuel (the log), an oxidizer (oxygen from the air), and sufficient heat to initiate and sustain the reaction. The visible flames and glowing embers are signs of the multi-stage chemical breakdown happening inside the wood.
The Log’s Chemical Makeup
The structure of a log is built from three primary organic polymers: cellulose, hemicellulose, and lignin. Cellulose, which makes up about 40–50% of the wood, is a long, chain-like molecule composed of linked glucose units that provide the wood’s tensile strength. Hemicellulose, a more branched carbohydrate, wraps around the cellulose microfibrils.
Lignin, the third major component, is a highly branched polymer of aromatic alcohols that acts as a natural adhesive, binding the cellulose and hemicellulose together. These organic compounds, composed mostly of carbon, hydrogen, and oxygen, are the source of the stored chemical energy. The log also contains water, which must be managed before the wood can burn effectively.
The Initial Breakdown (Pyrolysis)
The burning process begins with the application of heat, which first triggers a drying phase where any residual moisture in the wood must evaporate. This initial step is endothermic, meaning it absorbs energy, which is why burning wet wood is inefficient and produces a hissing sound. Once the wood temperature rises past 100°C, the water turns to steam and is driven off, allowing the true chemical breakdown to begin.
As the temperature climbs further, typically reaching between 200°C and 300°C, the solid wood structure begins a process called pyrolysis, or thermal decomposition. Pyrolysis is a chemical alteration of the fuel that occurs without the need for oxygen, breaking down the complex polymers into simpler compounds. Hemicellulose tends to break down first, followed by cellulose and lignin at higher temperatures.
This thermal cracking produces a mixture of gases, vapors, and tars, collectively known as volatile matter, which are forced out of the wood. These volatile products include combustible gases like methane, carbon monoxide, hydrogen, and various hydrocarbons. The flame itself is not the solid wood burning, but rather these highly flammable gases mixing with oxygen in the air directly above the log and igniting.
The ignition of these gases usually begins around 225°C and requires temperatures between 540°C and 1,225°C for sustained combustion. This flaming stage is responsible for the majority of the heat energy released, often accounting for up to 60% of the wood’s total heating value. The heat generated from the burning gases then feeds back into the log, sustaining the pyrolysis process.
The Final Stages and Byproducts
As the volatile matter is consumed, the fire transitions from flaming combustion to glowing combustion. The residue left behind after pyrolysis is a solid, carbon-rich material called char or charcoal. This char is essentially pure carbon, and it burns directly on its surface in a solid-state reaction with oxygen from the air.
This glowing combustion occurs at high temperatures, often reaching between 600°C and 800°C, and produces a steady, intense heat with minimal visible flame. The final products of complete combustion are primarily carbon dioxide and water vapor, with approximately half of the wood’s original mass converting to each.
The solid, non-combustible mineral residue left over is called ash, composed of compounds like calcium and potassium that were present in the original tree. Smoke is another byproduct, consisting of unburnt volatile matter, fine particulate matter, and incomplete combustion products like carbon monoxide. The presence of smoke indicates that the pyrolysis products were not completely burned in the flame before escaping into the air.