Pyrolysis Stage

Part of Charcoal Production

Understanding the chemical stages of wood-to-charcoal conversion.

Why This Matters

Charcoal production is not simply “burning wood slowly.” It is a controlled chemical decomposition called pyrolysis — the breakdown of organic material by heat in the absence of oxygen. Understanding what happens inside your kiln at each temperature stage transforms charcoal-making from guesswork into a predictable, manageable process.

When you know the chemistry, you can diagnose problems by their symptoms. White smoke means drying. Yellow smoke means tars are volatilizing. Blue haze means pyrolysis is complete. A kiln that smells sweet is releasing wood vinegar; one that smells acrid is producing creosote. Each of these signals tells you exactly what temperature range the wood has reached and what action you should take.

This knowledge also explains why certain mistakes are catastrophic and others are recoverable. Opening a vent during the tar-release phase wastes valuable byproducts and overheats the kiln. Sealing too early during the drying phase traps steam that can blow the cover. The chemistry dictates the operational rules — learn the chemistry, and the rules become intuitive rather than memorized.

The Chemistry of Wood

Wood Composition

Wood is primarily composed of three organic polymers:

Component	Percentage	Role in Wood	Behavior During Pyrolysis
Cellulose	40-50%	Structural fiber	Decomposes at 300-350°C into gases and char
Hemicellulose	20-30%	Binding matrix	Decomposes first, at 200-300°C
Lignin	20-30%	Rigidity, waterproofing	Decomposes slowly over wide range (200-500°C), produces most of the charcoal

Lignin is the key to charcoal production. It has the highest carbon content of the three components and resists thermal decomposition more stubbornly than cellulose or hemicellulose. The more lignin a wood species contains, the more charcoal it yields. Hardwoods generally contain more lignin than softwoods, which is one reason hardwood charcoal is superior.

Minor Components

• Water: 15-60% of fresh weight; must be removed before pyrolysis begins
• Extractives (resins, tannins, oils): 2-10%; volatilize early, contribute to smoke and wood vinegar
• Minerals (calcium, potassium, silica): < 1%; remain in the ash

The Five Stages of Conversion

Stage 1: Drying (20-150°C)

What happens: Free water and bound water are driven from the wood as steam. No chemical change occurs to the wood structure itself.

Observable signs:

• Dense white smoke (steam) billowing from vents and chimney
• Hissing and crackling sounds as trapped water boils
• The kiln exterior feels warm but not hot
• Condensation may form on cool surfaces near the kiln

Duration: This stage can take 25-40% of total burn time, depending on moisture content. For wood at 40% MC, the drying phase may take 12-18 hours in a medium mound kiln.

Energy balance: Drying is endothermic — it absorbs heat. The fire’s energy is consumed evaporating water rather than converting wood. This is why high-moisture wood dramatically reduces yield: the heat wasted on drying is heat that could have been converting more wood to charcoal.

Operator action: Maintain full airflow. All vents open. The fire needs oxygen to generate the heat required for drying. Do not restrict airflow during this stage.

Confirming Drying Is Complete

The transition from drying to pyrolysis is marked by a visible change in smoke: it shifts from dense white (steam) to a thinner, grey-white with a slightly acrid smell. This is your signal to begin reducing air intake.

Stage 2: Pre-Pyrolysis (150-280°C)

What happens: The wood begins to thermally decompose. Hemicellulose breaks down first, releasing acetic acid, methanol, and other light compounds. The wood changes color from its natural shade to brown. This stage is also called “torrefaction.”

Observable signs:

• Smoke changes from white to grey with a sharp, vinegary smell (acetic acid)
• Wood begins to darken visibly if you can see it
• The kiln exterior becomes noticeably hot
• A sweet or sour smell may be detectable downwind

Chemistry: Hemicellulose decomposes into:

• Acetic acid (wood vinegar) — the sharp smell
• Methanol (wood alcohol) — volatile and flammable
• Carbon dioxide and carbon monoxide — combustible gases
• Water — additional moisture released from chemical bonds breaking

Operator action: Begin reducing air intake slightly. The wood is now releasing flammable gases that can sustain combustion with less external air. If too much air enters, these gases ignite inside the kiln and raise temperatures too quickly.

Stage 3: Active Pyrolysis (280-400°C)

What happens: This is the critical conversion stage. Cellulose decomposes rapidly, releasing large volumes of flammable gases and tars. The wood structure fundamentally changes from organic polymer to amorphous carbon.

Observable signs:

• Smoke becomes yellow-brown, thick, and oily
• Strong creosote/tar smell
• Smoke may be flammable — it can ignite at vent openings (visible flames)
• The kiln exterior is very hot to the touch
• Earth cover may crack or sag as wood shrinks

Chemistry: Cellulose breaks down into:

• Tar — a complex mixture of hundreds of organic compounds, appearing as brown-black oily droplets in the smoke
• Wood gas — a mixture of hydrogen, methane, carbon monoxide, carbon dioxide, and various hydrocarbons
• Pyroligneous acid (wood vinegar) — a watery condensate containing acetic acid, methanol, acetone, and many other compounds
• Solid carbon — the charcoal itself, which remains in the kiln

The solid residue shrinks significantly during this stage — wood loses 50-70% of its volume as volatiles escape. This is what causes the earth cover to sink.

Energy balance: Active pyrolysis is exothermic — it generates heat. Once this stage begins, the process becomes partially self-sustaining. The released gases burn and provide heat to decompose adjacent wood. This is why you must reduce air intake: too much oxygen causes the exothermic reaction to run away, converting charcoal all the way to ash.

Operator action: Significantly reduce air intake. Close upper vents. The burn front should be advancing slowly through the wood stack, not racing. If flames appear at vents, close those vents immediately — flames mean oxygen is reaching hot charcoal and burning it to ash.

The Runaway Danger Zone

Between 350-400°C, the process generates enough heat to sustain itself without any external air. If vents are too open, the temperature spirals upward and you lose control — the entire kiln can convert from charcoal to ash in hours. This is the stage where most charcoal is lost to over-burning.

Stage 4: Final Carbonization (400-500°C)

What happens: Remaining volatiles are driven from the charcoal. Lignin, which has been slowly decomposing throughout, completes its transformation. The carbon content of the remaining solid increases from roughly 50% to 80-90%.

Observable signs:

• Smoke becomes thin, bluish-transparent
• Tar smell diminishes, replaced by a clean, mineral smell
• The kiln surface may glow very faintly in darkness at vent points
• Smoke volume drops dramatically

Chemistry: At this stage, the remaining reactions are:

• Final lignin decomposition (slow, producing mostly carbon and small amounts of methane)
• Rearrangement of carbon atoms into more ordered structures
• Release of hydrogen and methane from remaining organic bonds

Operator action: Close nearly all vents. The process needs minimal air — just enough to maintain temperature. The burn front should be completing its pass through the final sections of wood.

Stage 5: Cooling and Stabilization (500°C to ambient)

What happens: No further chemical conversion. The charcoal cools and its crystal structure stabilizes. The carbon becomes less reactive as temperature drops.

Observable signs:

• No smoke or only invisible heat shimmer
• The kiln surface gradually cools
• No sound from inside the kiln

Chemistry: As charcoal cools below 200°C, it begins to slowly absorb oxygen from any available air, forming surface oxides. This is why charcoal exposed to air while still hot ignites — the oxidation is rapid enough to generate flame. Below 100°C, the oxidation rate becomes negligible.

Operator action: Seal completely. All vents closed, earth cover reinforced. See Cooling Protocol for detailed procedures.

Temperature and Charcoal Properties

The final temperature reached during pyrolysis directly determines the properties of the charcoal:

Peak Temperature	Fixed Carbon	Volatile Matter	Properties
300°C	55-65%	25-30%	Brown-black, easy to light, smoky, low heat
400°C	70-80%	10-15%	Black, standard quality, good for most uses
500°C	80-85%	5-10%	Dense, hard, rings when struck, forge-grade
600°C+	85-95%	< 5%	Very hard, difficult to light, highest heat output

For most applications, targeting 400-500°C produces the best balance of quality and usability. Charcoal made below 350°C retains too many volatiles — it smokes heavily, produces creosote, and has lower heat output. Charcoal made above 600°C is excellent for metallurgy but is so inert that it is difficult to ignite without forced air.

Byproducts and Their Uses

Pyrolysis produces valuable byproducts that are wasted in most simple kilns but can be captured with modest modifications.

Wood Vinegar (Pyroligneous Acid)

Collected by condensing smoke during Stages 2-3. A brownish liquid containing:

• Acetic acid (8-12%) — preservative, cleaning agent, mordant for dyeing
• Methanol (2-3%) — solvent, fuel (caution: toxic if ingested)
• Acetone (< 1%) — solvent
• Water (80-85%)

Collection method: Run smoke from the kiln chimney through a tube that passes through a trough of cold water. The cooled smoke condenses into liquid that collects in a container at the end. Even a simple bamboo pipe cooled by wet rags will collect usable quantities.

Wood Tar

The heavier fraction that condenses at higher temperatures. Black, viscous, and waterproof. Uses include:

• Waterproofing rope, fabric, and wood (traditional boat tar)
• Preserving fence posts and structural timber
• Adhesive when mixed with plant fibers
• Medicinal applications (antiseptic, treatment of skin conditions)

Wood Gas

The non-condensable gases (CO, H2, CH4) can theoretically be captured and burned as fuel, but this requires more sophisticated retort kilns with gas collection systems. In a simple pit or mound kiln, these gases burn inside the kiln and contribute to the heat that drives pyrolysis.

Reading Your Kiln Through the Chemistry

Understanding the stages allows you to “read” your kiln at any moment:

What You Observe	What the Chemistry Says	What You Should Do
Dense white smoke, no smell	Stage 1: Drying in progress	Keep vents open, wait
Thin white smoke, vinegar smell	Stage 2: Pre-pyrolysis starting	Begin reducing air
Yellow-brown, oily smoke	Stage 3: Active pyrolysis	Reduce air significantly
Flames at vents	Stage 3: Excess oxygen	Close that vent now
Blue-transparent haze	Stage 4: Nearly done	Close almost all vents
No smoke, cover warm	Stage 5: Ready to seal	Seal completely
No smoke, cover cool	Fire has gone out	Re-ignite or accept partial burn

This diagnostic framework works for every kiln type — pit, mound, drum, or retort. The chemistry is identical; only the physical container differs.