An old craft with a practical logic

Walk into any working coppice wood in Britain and you are walking through a system that has been turning sunlight into usable materials for a thousand years. Hazel, ash, oak and sweet chestnut cut on a rotation of fifteen to twenty-five years gives you hurdles, bean poles, tool handles, thatching spars and firewood. But there is always a remainder: crooked pieces, split lengths, brash too thick to leave and too awkward to use. That remainder is where charcoal comes in.

An earth kiln is the oldest and simplest way to make it. You are not burning wood for heat; you are driving off the water, resins and tars and leaving behind a carbon skeleton. Starve the fire of oxygen and it cannot turn the wood to ash. Get it right and you end up with dense, ringing lumpwood that burns hot, long and almost smokeless — the sort that cooks reach for and that artists still grind for drawing charcoal.

Choosing wood, site and season

Almost any hardwood will char, but dense species give the best yield by weight. Oak, ash, birch, alder, willow and hazel all work well. Softwoods char quickly but spit and crumble, so keep them out unless you are making artists' charcoal from willow, which is a craft in itself. Season your wood for at least six months — damp wood wastes heat drying out and produces clouds of acrid smoke.

Cut lengths to roughly 50–80 cm, about the thickness of your wrist to your forearm. Sort as you go: large pieces at the base, small stuff near the top, so the burn stays even. Save anything thicker than your forearm for the firewood stack; oversized pieces leave uncharred "brands" in the middle of your lumpwood.

Site the kiln on level, bare ground at least three metres from anything burnable. You need a water source within easy reach and a calm day. Avoid peat, deep leaf litter and old tree roots — underground fires can smoulder for weeks. If you are in a smoke control area, check with your local authority before you light up, and always have the landowner's permission.

Building the stack

Mark out a circle around 1.8 metres across and lift the turf carefully with a spade, keeping it damp and in one piece; you will put it back later. Set a stout stake, or three poles lashed into a chimney, in the centre. Now build the wood around it, leaning the lengths inwards and packing them as tightly as you can — gaps mean uneven burning. Work up to a mound around a metre high.

Cover the stack with a layer of leaves, bracken or straw, then the turf, grass side out. Finally shovel on 10–15 cm of soil and pat it firm, leaving the chimney open at the top and a few inches of the base clear all round.

Lighting and reading the smoke

Drop kindling down the chimney and light it from the top. Once it catches, rake back the soil at the base to open four or five small draught holes. Air is drawn in low and the fire creeps downwards through the pile — a downward burn that is far more efficient than burning upwards from the bottom.

The smoke tells you everything:

• White and steamy — the wood is still drying. Patience.
• Thick yellow-grey and sharp-smelling — volatiles are being driven off. This is the smelliest stage, and the point at which the kiln runs hottest.
• Thin, wispy blue-grey — the wood has charred through and you are nearing the end.

Watch the mound for a slight sink as the wood shrinks. When the smoke from the base holes turns thin and blue-grey all the way round, act quickly: knock the chimney in, fold the turf over, cover everything with soil and tamp it down. That seal is the whole trick — starve it now, or your charcoal goes up in smoke.

Cooling, sorting and using it

Leave the kiln sealed for at least two days, three if the weather is warm. Then open it carefully and rake the charcoal out, quenching any glowing patches with water. Sort it into three piles:

• Sound lumpwood — black, light and ringing when tapped. Bag it once cold.
• Brands and brown ends — part-charred wood, ideal for the fire at home or a second kiln.
• Fines and dust — brilliant for the garden, for filtration and for mixing into compost.

Good lumpwood lights easily, holds heat and leaves little ash. Cooks value it for the clean flavour it gives; artists choose willow charcoal for its soft, dark line; and anyone with a water filter or a compost heap will find a use for whatever is left at the bottom of the sack.

Working safely and by the seasons

Charcoal burning fits the rhythm of the wood. Cut and coppice in winter while the sap is down, stack and season through spring and summer, then burn on calm, dry days in late summer or autumn. Never leave a kiln unattended, keep a filled watering can or hose beside you, and wear leather gloves and a dust mask when bagging. Smoke from a badly managed kiln will annoy neighbours and waste your wood; a well-run earth kiln gives off surprisingly little once it is sealed and burning clean.

Do it once, and you will never look at a coppice stool quite the same way again.