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Mastering Charcoal: DIY BioChar Kilns for Sustainable Fuel and Soil Enrichment

February 12, 2024 | Author blogs, Sustainability & Ecology

Making charcoal, especially biochar, is one of the most effective ways you can use the extra brush and logs when clearing an overgrown area. Master forester and author, Tomi Hazel Vaarde writes in their book,Social Forestry: Tending the Land as People of Place,” about two unique yet affordable ways to build DIY kilns to make your very own charcoal! Below is a compilation of excerpts from Hazel’s book, alongside photos and illustrations, to give you a step-by-step process to make these kilns and burn wood. 

The Types of Charcoal Kiln

The old, simple word kiln refers to a contained combustion chamber where materials can be heated to high temperatures. There are brick-kilns, pottery kilns, board-drying kilns, and charcoal-kilns. The idea is to be able to control temperature, drive-off moisture or brown-gasses, and cook clay-and-glazes to pottery. An oven for cooking or drying food generally does not heat up more than 500°F (>200°C). Kilns can go much higher for longer periods of time. Pottery can take all night.

A retort means a vessel that holds a chemical-reaction. A single-retort kiln contains Fire inside the main chamber. The fire is lit and is kept burning through a controllable oxygen-intake-port. The smoke from the gas-outtake flue (the chimney) is watched carefully for color changes, and the pyrolization is shut-down by capping the intake-port and the chimney port.

A double retort is a chamber within a chamber, with the inner chamber heated by combustion in the outer chamber. The smoke from the inner chamber is watched carefully and combustion is quenched in the outer fire when the inner chamber vent smoke turns clear or blue.  

The charcoal kilns at the ridge-pads and sorting-yards can be burned, quenched, unloaded, and reloaded, with the sorted-by-size wood, over and over, long into the winter. 

Vesta likes fat, freshly-dried Manzanita, one-to-six inches in (3-15 cm) diameter and two-to-four feet (~1m) long. The Fire Pig in Hestia requires two-and-one-half foot pieces (<1 m) to fit in the drum-retort.

Both Hestia and Vesta are kilns built to use portable-by-wheelbarrow parts. The Fire Pig drum in Hestia comes apart, and the bricks and pipes can be moved to a new location someday. Vesta is a simpler but larger kiln with a big sheet of metal-roof that can be rolled-up and moved, various portable stove-parts, and a down-slope metal wall. These are both small-kilns that are nonetheless moveable as fuel-reduction proceeds. What we will have to do is dig new pits and trenches on the new site, somewhere out on the newly extended trail-system.

How a Hestia Kiln is Built

Double Retort Kiln “Hestia” :

Hestia is a one-half cord (64 cubic feet or more than 2 cubic meters) pit, built in an old bulldozed cross-bar drain, left by the 1987 firefighters on the ridge between Wolf Gulch and Little Wolf Gulch. This north-rising ridge was bulldozed in 1987 and has grown back in thick Buckbrush and Manzanita. Twenty years ago, when we first started tending the ranch, we could walk up and down this steep backbone ridge, following wheel-ruts on easy, open, ground that standard fire-prevention wisdom would keep open, as a big fuel-break.

At Hestia, the kiln-site on the ridge, the pit is protected by a wind-wall built of salvaged-bricks and on-site adobe-mortar. In the pit sits the Fire Pig. This is a 55-gallon (280-l) steel-drum, laid on its side in a steel-cradle, to lift it off the floor of Hestia’s pit. The drum-lid has a reverse snorkel,coming off the high-middle threaded-hole in the drum-lid and bending down and under the cradle.

The two-inch (five-cm) pipe, under the drum, has forty one-quarter inch drilled-holes on a staggered one-inch grid-pattern, with the holes facing up toward the drum side. This is the vent-pipe for the inner-chamber of the double-retort. This is also the brown-gas-burner that ignites only after enough of the water vapor is driven out of the wood inside the Fire Pig (the inner chamber). The outer chamber is the pit, Hestia-herself, ready to hold Fire.

To pyrolize the Oak logs or limbs inside the drum, we burn Buckbrush and Manzanita brush (from the ridge-clearing-work) laid alongside and on top of the Fire Pig. The Fire Pig drum has two wings, welded as a wide V on the top, to hold some hot-coals from the brush-burn-pile in Hestia. As soon as the brown-gas-burner ignites, we drag some of the hot-coals away from both sides of the Fire Pig and shovel them into the oxygen-quenching tubs, just as we do with the burn-pile method described above. This brush-charcoal does not have to be ground up and when cooled off can be bagged and wheelbarrow-delivered to the farm-operation compost piles. Once charged with bio-life and nutrients this brush charcoal becomes bio-char and is added, with the compost, to the food production-fields.


makes the best cooking fuel.


is good for composting.

How a Vesta Kiln is Built

Single Retort Kiln “Vesta” :

Vesta is a one-quarter cord (32 cubic feet, more than one cubic meter) trench, cut into a slope so that it can be covered and vented. The trench is about three feet across, one-and-a-half feet deep (less than a meter by less than a half meter) and less than ten feet long (3 m). The slope is at a ratio of one-in-ten rise. The walls are tamped-adobe that was pounded into wooden forms, and the top is a ten-gauge sheet of galvanized, flat metal with three pipes as ribs on top. When the trench is filled with wood, the lid is lowered and covered with up to 6 inches (15 centimeters) of dry soil. The cap-soil is a non-flammable insulation and comes from two storage pits on either side of the kiln-trench.

The 2-foot (60-cm) down-draft feed-tube is at the down-slope end of the trench and enters the trench through a 6-inch (15-cm) pipe-elbow at the bottom of the downslope end-wall. The chimney is a five-foot (150-cm) length of galvanized stove pipe, set on another six-inch elbow, set into adobe at the up-slope lip of the kiln-trench.

The chimney and the downdraft feed-tube can be pulled off the elbows (with heavy gloves and pads) when it is time to shut down the kiln and cut￾off oxygen to the interior-combustion. There are two stove-pipe caps, of the sort that is used to close an unused input-hole in a brick-chimney. When the pyrolization is complete and the stove pipes are pulled off, the elbows are capped and set tight with a mallet to prevent smoke-escape. Then extra cap- soil is added where there are smoke-escapes and tamped down lightly to seal up Vesta all-the-way.

This combustion-system is a hybrid of a downdraft rocket-stove and a single-retort kiln-box. The scale is appropriate for two workers, or one very-competent operator. The lid can be lifted off (after the dirt cap is removed), once the kiln cools off after about a day. We must be sure the fire has gone out before we dare to unseal. We could lose a lot of charcoal to a small oxygen input.


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