I make charcoal to sell at the local farmers market. I'm a farmer and prepper. I use two steel drums, retort method, which produces high quality charcoal.
Charcoal has many uses. It used for cooking and heat without flames, water filtration, making pyrotechnics and has some medicinal uses. This is hot, hard work but simple to do. With a little imagination all components can be changed or modified as long as age-old principles are followed. I prefer using red oak. It comes out naturally pitted so there is no need for enhancements for water filtration.
Concept: Small pieces of quality hardwood are placed in a small steel drum. The small drum is placed into a larger steel drum. Scrap wood is then stacked in the gap between the two and burned. This “cooks” the quality wood into charcoal without allowing it to be consumed by oxygen.
Furnace – 55 gallon steel drum with removable lid. Called an “Open Head”.
The lid is held on with a quick lever closure ring (preferable) or the nut and bolt closure ring.
Retort – 15 gallon steel drum with removable lid. The crimp type lid is most common.
Both can be purchased new from and oil distributor or obtained used from trucking companies or large farms etc. First burn out any residual contents of used drums with an open fire.
3 fire bricks or spacers, used to raise small drum off the bottom of the large drum.
Wood: 2 types
Any quality hardwood makes great charcoal. I prefer red oak. It comes out very pitted with large cracks. It is easy to light and produces a very even burn when used for cooking. Great for water filtration without enhancements. For pyrotechnics use muscadine or grape vine.
Perfect scenario – Cut red oak logs and allow it to dry for nine months or so. For making a batch cut pieces, 5 inches long, off the end of logs. Quarter it, and split it into 1.5 inch thick bricks. Use a hand axe and cut the bricks into pieces 1.5 inches x 1.5 inches x 5 inches or smaller. If the pieces are larger then it just adds unnecessary cooking time.
Tip on tree selection – Pick a red oak inside a stand of timber that grew at least 100 yards from any open area among older trees. It would have grown straight, tall and fast, with very few knots, and hence great for splitting! You don’t want a tree that grew near the edge of a field. It would have had lots of limbs in its first 25 years, lots of knots, very hard to split.
Scrap wood, hardwoods produce a steady even heat. Small amounts of scrap pine lumber produces quick heat, helps regulate cooking process. All are split small enough to go in between the sides of the barrels and about 2 feet long.
Note: Pictures of my furnace and retort drum set-up can be found as attachments to my posts at the Eat The Weeds Forum.
A. Furnace Drum
1. Removable Lid: it is used to help regulate air flow during the cook. Raise with wood or rebar just a little while cooking. Most have 2" x ¾" Head Fitting Plug, also helpful with air control. You can also mount a piece of flue pipe with damper in the center of the lid if you want to be creative.
2. Cut vent openings along bottom edge of 55 gallon drum. Cut 3 vents, 3 inches (v) X 8 inches horizontally, evenly spaced around circumference. Leave one end [of each vent tab] attached so they can be partially closed to control air flow. After the burn, cover them with dirt to seal off air.
B. Retort Drum
1. The small drum bottom must be vented. The purpose is to allow gas to escape from the oak while it is being cooked. These gases also burn outside the small drum during the process. This reduces the amount of scrap wood used. These are the same gases used to run a gasifier or woodgas engine. A full small drum will weigh about 55lbs and produces about 18 lbs of charcoal.
2. In the bottom of the small drum drill 1/4 inch diameter holes. Drill about 30 holes
Note: I'm sure at some point early in the process, there is a quick flash burn in the small drum. Oxygen is gone soon, no ash. Gases don't burn until they leave the small drum.
At night you can see 30 blue jets of flame from bottom of small drum. Waste of scrap wood cooking at night. did it once just to see.
Ash from scrap wood starts to clog big barrel vents. pushing it back keeps air flow going straight up (chimney effect) away from bottom of small drum. I rarely see any ash in small drum, then just a little on few pieces in bottom.
1. Put the fire bricks in the bottom of the big drum to support the small drum. This allows space for out gassing. It also prevents the ground from wicking heat from the small drum.
2. When the small drum is loaded and the lid is clamped set it on the fire bricks.
3. Drop kindling down the sides of the small drum and then scrap wood up and over the top.
4. Stuff paper and tinder into the large drum vents and fire it up.
Cooking a Batch
Moisture is always your enemy!
The goal is to hold 700 plus degrees in the small drum for at least 1.5 hours assuming that the small drum is full and the moisture content is low. If the moisture content is high then it will add hours to the cooking time.
Only cook in hot weather, 90 plus degrees and sunny. If the temp is around 70 you will use a lot more scrap to cook the same batch, more work and time for the same return.
When the batch is done put the lid on the large drum and tighten the band. Close the bottom vents on the large drum and cover them with dirt to stop all air flow.
Tip: Don’t allow the scrap to burn out naturally. When you decide the charcoal is done then seal the Furnace. The burning scrap will use up remaining oxygen and prevent charcoal loss.
Before ignition, be sure to fill the small drum to the top and then shake it thoroughly. You'll then be able to add several more pounds of oak. Important - you still have to get the lid clamped on tight--freely without forcing.
There will be very little space for air. When the flash burn occurs it will be rapid. When gas starts escaping from the oak there is no oxygen for it to burn until it exits the vents in the bottom.
Also folks worry about cooking long enough. I tell them, "you will only under-cook one time." When you go out the next morning and find your mistake, you'll have to clean out the barrels, prep all the scrap, and re-cook the same batch. You wont make that mistake again!
Leave the air tight Furnace to cool over night. If you expose the charcoal to oxygen while it is still hot it will ignite and burn up all your work. The next day when the Furnace is completely cool remove the small drum. It should weigh about 20 lbs, if it feels a lot heavier then you did not get a complete conversion.
Pour the contents onto a framed 1/4-inch mesh screen to filter the tiny pieces and dust. Next bag up your charcoal. You should have 18 lbs of high quality natural charcoal.
Tips on Getting it Right:
You have to learn to "read the smoke." There is an art to this!
The first smoke will be heavy and white. This is moisture from the scrap wood and will continue for a while. Next the smoke will almost disappear. A short time later the white smoke will reappear but not so heavy as before. This is the moisture from the oak in the small drum.
This is the most important part of reading the smoke. The amount of white smoke from the small drum tells you how long to burn scrap. Only experience can teach you!
There is a small amount of loss as ash, maybe 1 to 2% at the bottom of the small barrel. Although crude this is a very efficient process for producing high quality organic charcoal.
Note that this charcoal-making process can be scaled down. The aforementioned procedure also works with a 5 gallon metal bucket and a 1 gallon metal paint can. Use you imagination, I’ve seen a pottery kiln used with several 1 gallon metal paint cans.
Activated charcoal is nothing but natural charcoal treated with liquid Calcium Chloride or Zinc Chloride for 12 plus hours. It becomes very pitted. Red oak comes out naturally pitted. (Not as good as activated but close.)
Warning: Use only natural unprocessed/untreated wood for charcoal. Things like pallets have been treated or had a host of chemicals and heavy metals spilled on them that are not consumed by fire.
You’ll get only about 15 to 20 batches out of a set of barrels, as they will deteriorate with high heat over time.