How to Make Charcoal (UK): Retort, Drum and the Process

Charcoal has a whiff of ancient craft about it, and it is: wood colliers were making it in earth-covered mounds long before there were barbecues to feed. But the principle is simple, and once it clicks you can turn a pile of coppice poles, prunings and offcuts into something genuinely useful. Charcoal is the fuel that made blacksmithing possible, it is what a good barbecue really wants, and crushed into the soil it becomes biochar, a long-lasting home for water and soil life. For anyone managing a bit of woodland it also closes a loop: the thin, twisted and knotty wood that is no good for planks or clean firewood is exactly what a charcoal burn is happy to eat.

This guide explains what charcoal actually is, then walks through the practical ways to make it at a homestead scale, from a taster in a tin can to a proper ring kiln. If you are producing the wood yourself, it pairs naturally with coppicing for firewood, which is where a lot of good charcoal wood comes from. Read the safety section before you light anything: this is a job with real hazards, done well outdoors and never indoors.

The principle: pyrolysis, not burning

The whole thing rests on one idea. If you let wood burn freely with plenty of air, it goes all the way to ash and you are left with almost nothing useful. If instead you heat wood hard while starving it of oxygen, it cannot fully combust. What happens instead is that the heat drives the water out first, then cooks off the volatile gases, tars and oils as smoke and flammable woodgas, and leaves the woody skeleton behind as almost pure carbon. That black, light, brittle carbon is charcoal. The technical name for cooking a material without enough air like this is pyrolysis.

In practice you need serious heat. Good charcoal forms when the wood reaches roughly 400 to 470°C with as little oxygen present as possible. Too cool and the wood is only half converted, leaving unburnt "brown ends"; too hot or heated too fast and the charcoal comes out fragile and full of splits. That is the balance every method below is trying to strike: get the whole load hot enough, keep the air out, and hold it there until the volatiles have all gone.

There are two families of method. In a kiln, you light a fire in among the wood itself, let it get the whole load up to temperature, then seal off the air so the load cooks in its own retained heat rather than burning up. In a retort, the wood sits inside a sealed inner container and is heated from the outside by a separate fire, so it never touches the flames, much like making charcloth in a sealed tin. Retorts are cleaner and waste less of the wood, because none of the charge is sacrificed to raise the heat and the woodgas can be piped back to feed the fire.

Four ways to make charcoal

Method	What it is	Best for	Effort
Tin-can taster	A sealed food tin of wood scraps thrown into a campfire, with a small vent hole	Learning the principle; a first batch you can hold	Very low, an afternoon
Oil-drum / single-drum kiln	A steel drum you load, light inside, then seal to cut off the air	A useful home batch of barbecue or biochar charcoal	Moderate, most of a day
Two-barrel retort	A sealed inner drum of wood heated inside a larger drum or brick kiln	Cleaner, higher-yield batches; using brash as fuel	Higher to build, cleaner to run
Traditional ring kiln	A large steel ring with ports and chimneys, sealed with soil and sand	Bigger, repeatable batches around 100kg plus	High, a long day plus cooling

The tin-can taster is the honest place to start. Pack an old food or paint tin with dry wood scraps, punch one small hole in the lid to let the woodgas escape, and sit it in a hot campfire or firepit. Jets of flame will hiss from the hole as the volatiles burn off. When the jets die away the charring is done; lift the tin out, plug the hole (a stick or a dab of mud works), and let it go stone cold before opening. Inside you will have a handful of real charcoal and, more importantly, a feel for the whole process in miniature.

The oil-drum or single-drum kiln scales that up. A steel drum is loaded with wood, a fire is started inside to get the load hot, and once it is properly alight the drum is sealed so the only escape is a controlled vent, letting the wood char in its own heat rather than burn to ash. It is the classic small-scale kiln and it is what many people build first, using a scrounged drum and a fitted lid.

The two-barrel retort is the cleaner, more efficient cousin. Here the wood goes into a sealed inner drum, which sits inside a larger drum or a small brick kiln. You light a fire underneath and around the inner drum and keep it fed for about an hour until the wood inside gets hot enough to start out-gassing. That woodgas vents through a pipe, ignites, and from that point the reaction is largely self-sustaining: the gas coming off the wood becomes the fuel that keeps the wood cooking, so you can stop feeding the outside fire. Because the wood never touches the flames, you get clean charcoal with no soil or ash mixed in, and you can burn brash and scrappy wood as the outer fuel.

The traditional ring kiln is the workhorse for anyone making charcoal in quantity. It is a large steel ring, often 1.5 to 2.4 metres across, with a set of air ports around the base and several chimneys, topped with a conical lid. Loaded well it can turn out a serious batch in a single burn. The steps below follow the ring-kiln method, because it shows every stage of the process clearly, and the same thinking scales down to a drum.

Making charcoal in a ring kiln

1. 1

   Choose the site and the day

   Pick flat, dry, fairly stone-free ground, well away from buildings, fences and anything that can catch. A sheltered but not sealed-in spot is ideal: too much wind makes the burn run wild, too little and it drags on. A still, dry day is best, both for control and to keep smoke off the neighbours.

2. 2

   Load the wood

   Stand your best-dried logs on end around the ring like spokes on a wheel, with a heap of kindling and sacrificial wood in the centre to get things going. Pack the rest in as tightly as you can, smaller pieces to the cooler outside, larger to the hot centre, building up into a peak to fill the lid. The less air space, the more charcoal you get.

3. 3

   Light it and let it catch

   Light the central kindling through one of the ports, traditionally with a rag on a stick. Prop the lid up slightly so air can feed the fire. Thick smoke will pour out as the fire spreads through the load. Let it burn openly just long enough to drive the whole load's temperature up, no longer, because everything burning now is charcoal you will not get back.

4. 4

   Seal down to starve the air

   Once the load is properly hot, drop the lid, bank soil around the base and fit the chimneys onto alternate ports so air can only enter where you choose. Seal the rim of the lid with sand to close any gaps. Now the fire is drawn inward through the ports and the load cooks with little oxygen: pyrolysis proper. Swap the chimneys around every hour or so to spread the heat evenly.

5. 5

   Read the smoke and judge it done

   For hours the smoke stays thick and yellowish-white as steam and volatiles come off. Gradually it thins and pales; near the end some chimneys may show small flames of burning woodgas. When the smoke has almost stopped and turns thin, the charring is complete. A faint blue tint means it has gone a touch too far and the charcoal itself is beginning to burn, so do not wait for that.

6. 6

   Shut it down and cool fully

   Pull the chimneys and pack every port and gap with soil to cut off all air at once. Check for leaks after half an hour and seal any you find. Then leave it, a big kiln needs 12 hours or more, often overnight, to go cold. Do not peek: opening a hot kiln lets air in and the woodgas can ignite and blow the lid off. Only when it is genuinely cold do you open up and empty it.

7. 7

   Grade and store

   Sieve the charcoal over a mesh to knock off dust and remove stones and any unburnt brown ends. Larger pieces, over about 25mm, make barbecue charcoal; medium pieces suit a forge or woodburner; the fines are perfect crushed into the garden as biochar. Store it bone dry, as charcoal readily soaks up damp from the air.

Wood, timing and yield

Use dense, dry hardwood. Oak, ash, beech, hornbeam, birch and fruit woods all make excellent charcoal, and mixed coppice such as hazel, alder and willow works well too. Species matters far less than condition: the wood must be dry and sound. Slightly rotten or wet wood gives crumbly charcoal and a smoky, sulky burn, so keep it out. Cut everything to a fairly even size, very roughly 30 to 120mm across, so the whole load chars at a similar pace, and give it a good few months to season, closer to a year if you can. This is where a coppice system pays off, since the thin poles and offcuts it produces are ideal charcoal wood.

Do not expect to get back anything like what you put in, and that is normal. Because most of a log's weight is water and volatile compounds that are deliberately cooked off, traditional kilns turn only roughly a quarter to a third of the dry wood's weight into charcoal. In other words the wood loses most of its weight; a batch that looks generous going in comes out a good deal smaller. Treat any single yield figure as a rough guide, since it shifts with the wood, its dryness and how the burn is managed. Even so the numbers are worthwhile: a small ring kiln can still turn out around 120kg of charcoal in a single burn.

Common mistakes

• Doing it anywhere enclosed. A shed, garage or polytunnel is never acceptable: the carbon monoxide and smoke are dangerous. Open air, always.
• Using wet or rotten wood. It gives poor, fragile charcoal and an unpleasant, smoky, hard-to-control burn. Dry, sound wood only.
• Letting the fire run open too long. Every extra minute burning freely before you seal the kiln is charcoal turned to ash. Seal down as soon as the load is hot through.
• Pieces too big or wildly uneven. The load chars unevenly, leaving unburnt centres. Cut to a fairly consistent size.
• Opening it while still hot. The single most dangerous mistake. Air plus hot woodgas equals a flashback that can take the lid, and your eyebrows, with it. Cool fully, then open.
• Storing it damp. Charcoal drinks moisture from the air and then lights badly. Keep it dry and covered.

Before you light: three quick checks

1. Are you outdoors, in the open, well clear of anything that can catch, with water and a way to fight a fire to hand?
2. Is your wood dense, dry, sound and cut to an even size, and is it a still enough day to keep smoke off the neighbours?
3. Do you have the time to see it through, including many hours of cooling before you open up, and will you not leave it unattended?