Why Charcoal Belongs to the Coppice

Coppicing and charcoal have been partners for a thousand years. Cut a stool in winter, let it regrow, and you get a crop of poles every seven to twenty years depending on species and rotation. Those poles — hazel, sweet chestnut, oak, alder, birch, willow — have all manner of uses, but the crooked bits, the small diameter stuff and the tops make excellent charcoal. It was the way a woodman turned a low-value by-product into something worth carting to market, and it still is.

The golden rule is dryness. Green wood boils off its moisture before it can char, wasting heat and time and wrecking your yield. Winter-cut wood, stacked under cover and seasoned over the summer, burns far better than anything fresh. Most charcoal burns in Britain happen from late summer into autumn, when the timber has had a season to dry and the fire risk in the wood is manageable.

Earth Mounds: The Traditional Method

The earth mound — the "pit" of old, though it was nearly always built on the surface — is a slow, skilled, entirely hand-and-eye operation. You pick level, sheltered ground, often an old hearth site where the soil is already blackened. A central stake goes in, then a chimney of three or four upright logs around it, and the rest of the wood is stacked in leaning tiers, radiating outwards and upwards, keeping that central flue clear.

The whole pile is covered in turf, bracken, leaf litter and finally earth, leaving a ring of small vents near the base. You light it from the top of the chimney, cap it once it catches, and then spend the next three to five days watching smoke.

• Smoke tells you everything. Thick white is steam, grey means the burn is working, thin blue haze means that patch is done and its vent should be plugged.
• You chase the burn round, opening one vent as you close the next, patting earth onto the outside as the mound subsides.
• Too much air and you get a flash of ash and lost product. Too little and you end up with "brands" — half-burnt wood and dross that has to be picked out by hand.
• Quenching is done with water and earth, raked into small heaps, cooled, then sieved. The fine stuff is breeze; the good lumps are bagged for sale.

A mound of three or four tonnes of seasoned wood will give you roughly a tonne of charcoal — call it 20 to 25 per cent by weight, on a good day.

Retorts: Sealed, Controlled, Cleaner

A retort works on a different principle. The wood sits inside a sealed vessel and is heated from outside, so no air reaches the charge. As the wood passes about 250°C it starts throwing off volatile gases; between roughly 250°C and 400°C the bulk of the devolatilisation happens, and you need only a small kindling fire to get things going, because those gases are piped back to the firebox and burnt to drive the process.

That single trick changes everything. Instead of smoking the neighbourhood, the retort consumes its own smoke. The condensable fraction can be captured as pyroligneous acid, wood vinegar, tar and methanol, and the heat is there for the taking — for drying the next batch, heating a workshop or running a kiln. Yields typically climb to 25–35 per cent of dry wood weight, and the charcoal is denser, more consistent and much freer of soil and grit.

There are simple two-barrel retorts that will fit in a small yard, and there are purpose-built steel or brick retorts for serious coppice-scale production. They need capital, thermocouples, a bit of fabrication and a grasp of how gases behave. They are not, frankly, a beginner's project.

Comparing Them Honestly

• Yield: an earth mound gives around 20–25 per cent; a retort 25–35 per cent, plus recoverable liquids and heat.
• Quality: retort charcoal is uniform and high in fixed carbon — the choice for smiths, artists and serious cooks. Mound charcoal is perfectly good for the barbecue, but it varies from batch to batch.
• Skill: the mound demands days of judgement and a good eye. The retort demands instrumentation and a method.
• Smoke: the mound is visibly and smellily smoky; the retort is far cleaner, which matters enormously near houses or a smoke control area.
• Cost and scale: a mound costs almost nothing but your time and suits small, irregular batches. A retort is an investment that pays back on regular volume, and it is the obvious route into biochar.

Choosing What Suits Your Wood

If you look after a few acres of mixed coppice, cut a couple of hundred bundles a year on rotation, and want charcoal for teaching, for local sales and for your own use, an earth mound is a fine place to start — cheap, instructive and deeply satisfying. Once you are producing regularly, or you have neighbours within complaining distance, or you want the by-products, a small retort starts to look like common sense. Plenty of woodmen run both: a retort for the bulk, a mound now and then to keep the skill alive and to show people where the trade came from.

Safety, Smoke and Selling It

Charcoal burning produces carbon monoxide — colourless, odourless and lethal. Never leave a mound or a retort unattended, never burn near a public right of way or where children and dogs wander, and keep water and a rake to hand. Season your timber properly, never burn treated or painted wood, and check the rules on smoke and burning with your local council and the environmental regulator before you light up.

Grading matters for selling: separate out the large lump, the standard size and the breeze, bag it dry, and store it somewhere genuinely dry, because charcoal left in the rain is just a wet apology. Get that right, and a well-managed wood can pay its way in charcoal for as long as the stools keep regrowing.