Burning wood is not a single reaction but a cascade of overlapping chemical events: water boils off, large organic molecules crack apart into flammable gases, those gases ignite and react with oxygen, and whatever carbon is left behind slowly oxidizes into ash. The process starts well before you see a flame and continues long after the fire dies down to glowing embers. What looks simple in a fireplace involves hundreds of individual chemical reactions, and the balance between them determines everything from the color of the flame to the toxicity of the smoke.
What Wood Is Actually Made Of
To understand what happens when wood burns, you need to know what wood is built from. Three large organic polymers make up the bulk of any piece of wood: cellulose, hemicellulose, and lignin. Cellulose is the structural backbone, long chains of sugar molecules bundled into tough microfibrils. Hemicellulose is a group of shorter, branched sugar chains that fill the spaces around the cellulose. Lignin is the rigid, carbon-rich polymer that glues everything together and gives wood its stiffness. In a typical piece of wood, cellulose accounts for roughly 40 to 50 percent of the dry weight, hemicellulose another 20 to 30 percent, and lignin fills in most of the rest. The cell walls have a layered ultrastructure in which cellulose microfibrils sit surrounded by a hemicellulose-lignin matrix.1Journal of Wood Science. Thermal reactivity of hemicellulose and cellulose in cedar and beech wood cell walls
Each of these three components breaks down at different temperatures and produces different byproducts when heated, which is why wood combustion unfolds in stages rather than happening all at once. There is also water, of course. Even seasoned firewood holds around 15 to 20 percent moisture by weight, and freshly cut “green” wood can be half water. That moisture has to go somewhere before the real chemistry starts.
Drying and the Onset of Pyrolysis
When you first heat a piece of wood, nothing dramatic happens chemically. Up to about 100°C, the heat is simply driving off water. This is why a freshly lit fire hisses and steams: energy that could be feeding the flames is instead going toward evaporating moisture trapped inside the wood’s cells. Only once the wood dries out does its temperature climb high enough for the organic polymers to start breaking apart.
That breakdown process is called pyrolysis, and it is the heart of what people mean by “burning” wood, even though pyrolysis itself is not combustion. Pyrolysis is thermal decomposition: heat energy snaps the chemical bonds in cellulose, hemicellulose, and lignin, releasing a stew of smaller molecules as gases and leaving behind a solid carbon residue called char. Hemicellulose is the most thermally fragile of the three and begins to degrade at roughly 200 to 300°C. Cellulose follows, breaking down most actively between about 300 and 375°C. Lignin is the most stubborn, decomposing gradually across a wide temperature range from around 200°C all the way up to 500°C or beyond.2PubMed Central. Characterisation of the fire behaviour of wood: From pyrolysis to fire retardant mechanisms
These aren’t neat, sequential steps. They overlap heavily, and the exact temperatures depend on wood species, density, grain orientation, and heating rate. But the general pattern holds: hemicellulose goes first, cellulose next, lignin last.
What Pyrolysis Produces
The volatile gases released during pyrolysis are what actually burn with a visible flame. When cellulose breaks down, it first forms an unstable intermediate sometimes called “active cellulose,” which then either depolymerizes into volatile compounds or polymerizes into solid char.3Journal of Analytical and Applied Pyrolysis. Levoglucosan formation mechanisms during cellulose pyrolysis The volatiles include a wide variety of molecules: water vapor, carbon dioxide, carbon monoxide, methane, methanol, acetic acid, formaldehyde, and a sugar derivative called levoglucosan that is characteristic of cellulose breakdown. The exact mix depends on how fast and how hot the wood is being heated.
Lignin pyrolysis produces a different chemical signature. Because lignin is built from aromatic (ring-shaped) carbon structures, its thermal breakdown releases phenols and other aromatic hydrocarbons, along with light gases formed when functional groups like carboxyl and methoxy groups detach from the benzene rings.4PubMed Central. A review on lignin pyrolysis: pyrolytic behavior, mechanism, and relevant upgrading for improving process efficiency These phenolic compounds contribute heavily to the distinctive smell of wood smoke and are also the precursors to creosote, the tarry residue that builds up inside chimneys and flue pipes.
Lignin also produces proportionally more solid char than cellulose does, because its heavily cross-linked aromatic structure doesn’t vaporize as easily. This is why the carbon-rich charcoal left after a fire retains the rough shape of the original wood: it’s largely the lignin skeleton, stripped of its volatile components.
Where the Flames Come From
The visible flames dancing above a burning log are not the wood itself on fire. They are the pyrolysis gases igniting as they mix with oxygen in the air above the wood surface. This is gas-phase combustion: flammable molecules like methane, hydrogen, carbon monoxide, and various hydrocarbons react with oxygen, releasing heat and light. The reaction is exothermic, meaning it produces more heat than it consumes, and that surplus heat radiates back to the wood surface to drive further pyrolysis. Once the cycle is self-sustaining, the fire keeps going without any external heat source.
Flame temperature in a wood fire typically ranges from about 600°C to over 1,100°C, depending on oxygen supply, wood type, and how well the gases mix with air. The color of the flame reflects both the temperature and what is burning. The yellow-orange color most people associate with a wood fire comes largely from incandescent soot particles within the flame. A cleaner, hotter burn with better air supply tends to produce bluer flames at the base, where combustion is more complete and fewer soot particles are present.
Researchers have found that spectral analysis of wood flames can even identify the species of wood being burned, because different woods release different concentrations of mineral elements like sodium and potassium that emit light at characteristic wavelengths. Sodium produces a bright emission near 589 nanometers and potassium near 767 nanometers.5Measurement. Biomass flame spectroscopy technique to identify wood species through spectral emission during combustion processes That sodium line, incidentally, is the same wavelength that gives sodium streetlights their amber glow.
Smoke and Soot Formation
Smoke is the visible evidence of incomplete combustion. When pyrolysis gases don’t fully react with oxygen before cooling, they condense into tiny liquid droplets and solid particles. The white or gray smoke you see when a fire is first getting started is mostly water vapor mixed with condensed organic compounds. The darker, dirtier smoke that comes from a smoldering or oxygen-starved fire contains far more particulate matter, including soot.
Soot is essentially tiny clusters of carbon atoms arranged in graphite-like sheets, formed when hydrocarbon molecules in the flame crack apart and then recombine in oxygen-poor zones. Research into wood soot specifically has shown that the formation pathway involves a mechanism in which hydrogen atoms are stripped from hydrocarbon fragments and replaced with two-carbon units, progressively building larger and larger aromatic ring structures. Cyclopentadienyl intermediates, five-membered carbon ring fragments, also play a role depending on the specific compounds being pyrolyzed.6Energy and Fuels. Some aspects of the mechanism of formation of smoke from the combustion of wood
From a practical standpoint, the takeaway is straightforward: the less oxygen the fire gets, and the lower the flame temperature, the more soot and smoke it produces. A well-ventilated fire burning dry wood produces relatively little visible smoke. A smoldering pile of damp wood produces a lot.
The Smoldering Stage
Once the volatile gases have mostly been driven off, you’re left with char, which is mostly carbon with some residual minerals. Char doesn’t burn with a flame because there are no gases left to ignite. Instead, it undergoes a surface oxidation reaction: oxygen from the air contacts the solid carbon directly and reacts to form carbon dioxide and carbon monoxide. This is what produces the familiar red glow of embers. Both the COâ‚‚ and CO surface reactions contribute to carbon consumption during this phase, and the mass loss rate stays roughly constant at a given oxygen supply.7Proceedings of the Combustion Institute. Propagation of glowing combustion front in a packed bed of activated carbon particles and the role of CO oxidation
Smoldering combustion is slower and cooler than flaming combustion, typically occurring at temperatures between 400°C and 700°C. It releases a higher proportion of carbon monoxide relative to carbon dioxide compared to flaming combustion, which is one reason why smoldering fires in enclosed spaces are particularly dangerous. CO is colorless and odorless, and it binds to hemoglobin in your blood far more readily than oxygen does.
The smoldering stage can last for hours or even days in a large log. If you’ve ever returned to a campfire in the morning and found embers still glowing under a layer of ash, you’ve seen how persistent char oxidation can be when insulated from heat loss.
What Ash Is Made Of
Everything that doesn’t leave as a gas or a particle eventually ends up as ash, the mineral residue left after both the organic carbon and the volatile metals have been driven off. Wood ash is mostly inorganic: calcium, potassium, magnesium, and smaller amounts of sodium, zinc, iron, manganese, and other trace metals. The calcium is present largely as calcium carbonate and calcium oxide, which is why wood ash is strongly alkaline. When dissolved in water, wood ash leachate is about 92 percent hydroxide and 8 percent carbonate.8Bioresource Technology. Physical and chemical characteristics of wood ash
The amount and composition of ash depends on combustion temperature. As the temperature rises from about 540°C to 1,090°C, total ash yield drops by roughly 45 percent, because more of the volatile metals like potassium, sodium, and zinc vaporize and escape.8Bioresource Technology. Physical and chemical characteristics of wood ash This is why a very hot fire leaves less ash than a cooler one, and why the ash from a roaring blaze tends to be finer and more calcium-rich than the ash from a gentler fire.
The alkalinity of wood ash is the basis for its traditional uses: making lye soap, adjusting soil pH in gardens, and even as a primitive cleaning agent. A scoop of wood ash in water produces a solution with a pH around 13, strong enough to dissolve grease.
Particulate Emissions and Health
Wood smoke is not just an aesthetic nuisance. It is a significant source of fine particulate matter, the tiny particles that penetrate deep into the lungs and are linked to respiratory and cardiovascular disease. The particles produced by wood combustion are a complex mixture of organic carbon, elemental carbon (essentially soot), and inorganic ions. The combustion conditions exert a stronger influence on the emission profile than the wood species does, meaning that how you burn matters more than what you burn.9PubMed Central. Physicochemical and toxicological properties of wood smoke particulate matter as a function of wood species and combustion condition
In biomass burning generally, organic carbon and elemental carbon together make up the largest share of fine particle mass. Studies of crop-residue burning, a closely related process chemically, have found that organic and elemental carbon can account for more than half of fine particulate emissions, with organic matter dominating the mix.10Environmental Pollution. Characterization of the chemical components and bioreactivity of fine particulate matter produced during crop-residue burning in China The practical implication is that improving combustion efficiency, getting more of the volatiles to burn completely before they escape the firebox, is the single most effective way to reduce harmful emissions from a wood fire.
Nitrogen Oxides From Wood Fires
Most people associate nitrogen oxide pollution with car exhaust or power plants, but wood combustion produces it too. The nitrogen in these emissions doesn’t come from the air (at least not primarily at the temperatures of a typical wood fire). It comes from the small amount of nitrogen chemically bound in the wood itself, locked into the organic structure of the cell walls.
Studies of wood combustion have found that most fuel-bound nitrogen, around two-thirds to three-quarters, is released during the initial pyrolysis stage rather than during char burnout.11Bioresource Technology. NO and N2O formation during the combustion of wood, straw, malt waste and peat It comes off as hydrogen cyanide (HCN) and ammonia (NH₃), which then react in the flame zone to form nitric oxide (NO). Some nitrous oxide (N₂O) forms as well, but it tends to be destroyed quickly by reactions in the hot gas. The conversion rate of fuel nitrogen to nitrogen oxides depends on the nitrogen content of the wood, the combustion temperature, and the oxygen concentration.12Forests. Extensive Operating Experiments on the Conversion of Fuel-Bound Nitrogen into Nitrogen Oxides in the Combustion of Wood Fuel
Wood generally has low nitrogen content compared to fuels like coal or agricultural waste, so its NOx emissions per unit of energy are relatively modest. But in areas where residential wood burning is widespread, the cumulative contribution to local air quality can still be significant.
What Happens to Wood Smoke in the Atmosphere
Wood smoke doesn’t just drift away and disappear. Once released, the organic compounds in the plume continue to react with sunlight and atmospheric oxidants, and those reactions can actually make the pollution worse before it gets better. Laboratory smog-chamber experiments have shown that exposing dilute wood smoke to ultraviolet light under typical summertime conditions can more than double the mass of organic aerosol in the plume within a few hours, as gas-phase compounds oxidize and condense into new particles.13Atmospheric Chemistry and Physics. Laboratory investigation of photochemical oxidation of organic aerosol from wood fires 2: analysis of aerosol mass spectrometer data
The type of burning matters here too. Smoldering combustion releases more volatile organic compounds than flaming combustion, and those extra volatiles serve as precursors for faster formation of secondary organic aerosol when hit by sunlight. Research comparing the two has found that smoldering plumes aged in light develop a higher oxygen-to-carbon ratio and more oxidized aerosol than flaming plumes do, with furanic compounds (ring-shaped molecules derived from sugar decomposition) identified as key gaseous precursors.14Journal of Geophysical Research: Atmospheres. Evolution of Organic Aerosol From Wood Smoke Influenced by Burning Phase and Solar Radiation This secondary chemistry is part of why wildfire smoke events can degrade air quality hundreds of kilometers downwind, well after the original particles have dispersed.
Why Fire Mattered for Human Evolution
The chemistry of wood combustion is interesting on its own, but it also sits at the center of one of the defining transitions in human history. Controlled fire gave our ancestors cooked food, warmth, light, protection from predators, and eventually the ability to smelt metals and fire pottery. Researchers studying wild chimpanzees in fire-prone savannas have documented that even our closest living relatives show a familiarity with naturally burned landscapes, suggesting that passive exploitation of fire likely preceded controlled use by a long stretch of evolutionary time.15PubMed. Investigating foundations for hominin fire exploitation: Savanna-dwelling chimpanzees (Pan troglodytes verus) in fire-altered landscapes
The chemical reactions described throughout this article are the same ones our ancestors stumbled into hundreds of thousands of years ago. Pyrolysis, gas-phase combustion, char oxidation: the physics hasn’t changed. What changed was the species that learned to start and control the process, and then, very slowly, to understand what was happening inside the flames.