I’ve been making small commercial quantities of charcoal in various steel boxes for a few years, mostly for sale to blacksmiths. So far I’ve used 3 different boxes, all using the same principles and each working well. My current bin (in the pictures) makes 300 – 500kg of charcoal in a burn, and a burn can be done in a day’s work.
1. Hot burn
This design of bin and the method of burning described below is a “hot burn” – a flaming fire in an open-topped container. Most traditional methods of making charcoal use a “slow burn”, which is slower, much smokier, but which can yield more charcoal per unit wood. It may be possible to do a slow burn in these bins, but I’ve never tried.
2. The bin.
The bin pictured is my latest (and biggest).
It is built from an old waste skip, which forms the top half – its bottom was rusted out. The new bottom half was fabricated and added on.
Size: 1.8m long x 1.6m high x 0.9m wide.
Steel sheet: 3mm bottom half, 2mm top half
The bin has a pair of lids (original to the skip) which fit on top, but which are kept separate from the bin during the burn. Channels of angle iron have been added to hold sand or dirt to help seal the bin when the burn is finished.
The bin is just small enough to fit in a 6’ x 4’ box trailer, and just light enough to be manhandled into the trailer when empty.
3. Air holes
The bin has 10 air inlets, 5 on each of the 2 main faces. These are made from short pieces of 2” steel pipe, threaded on the outside so metal caps can be screwed on to exclude air.
These holes help to make a hot fire early in the burn.
A full burn yields 20 to 25 full-size hessian potato sacks (from 14 to 22 kg each), or from 300kg to 500kg in total. This takes around 8 to 10 hours burning time. Bagging the charcoal takes another 3 or 4 hours.
5. Design principles
A charcoal bin is essentially a container which can cope with the heat of a fire and a bit of knocking about, and can be sealed airtight when the burn is finished.
The bin needs to be able to cope with heat distortion, and still be able to be sealed. Bins can warp a lot in the heat of the fire, so make sure the lid joint has a big margin for movement. The sides of the bin tend to bow outwards with time, often 25mm or more.
You need a shelf or trough which will hold sand or dirt to seal the lid. You don’t want the sand or dirt to fall into the bin and contaminate the charcoal.
Bigger bins make charcoal more quickly, and can burn bigger pieces of wood. This reduces the work in sawing, and allows you to burn stumps and branchy timber. It takes much less work and time to make 20 bags of charcoal in one big bin than in 10 200litre drums.
Making Charcoal in a big bin
6. Fire safety
It’s really easy to start a bushfire while making charcoal. Take great care to clear flammable material from around the bin before starting a burn, and to ensure that you are burning in suitable weather.
Remember that hot coals and sparks can fly metres from the bin.
I never make charcoal without a knapsack spray and a rakehoe handy.
7. Starting a burn
I start a burn in the bin with all the air vents open and the lids off, with a big blazing fire of twigs and sticks. I like to have a good pile of light wood (up to 50mm diameter) collected first, and plenty of medium wood (50 – 100mm) easily available. Long pieces which can lean against the sides of the bin are good for establishing a fire.
Once a good hot bonfire is established, then bigger pieces can be added. The purpose of this gradual approach is to avoid big pieces of wood falling to the bottom of the bin without being charcoaled.
Once a good hot fire is established, it’s good to fill the bin quickly, so that big pieces have plenty of time to burn through. Fill it as full as you can, without pieces falling off.
8. Cooking the wood
As the wood burns, it will char (turn to charcoal), break up and fall to the bottom of the bin. This charcoal - your product - will continue to burn to ash if it has access to air, so you want to restrict access of air to the bottom of the bin once the charcoal starts to build up.
If you look through the air vents you can see this progress. When black wood, or black wood with flames around it is visible through the vents, you want to keep air flowing in and cooking the wood.
When red coals are visible through the vent, you know you are burning charcoal and it is time to close the vent.
Close the vents as they show red coals. Your nerves will tell you when you need to use a glove – red coals inside the bin can heat the metal red hot (visible at night).
Within an hour of lighting up it will generally be time to shut all the bottom vents, and within a few hours the higher vents will be shut. Most of the burn will be done with all the vents shut.
As the charcoal rises in the bin, I often use a crowbar to lever large pieces of wood out of the coals and into the air where they will burn more.
9. Finishing the burn
As the bin fills with charcoal, the fire gets slower. As the charcoal gets closer to the air coming from the top, more of it will burn to ash.
For the last stages of the burn I like to throw in incompletely burnt wood from previous burns, and some smaller wood. These pieces fill in gaps and protect charcoal from being exposed and burning away.
When you think you are starting to burn up charcoal, and waste charcoal and wood, then it is time to close the bin. It’s normal to have incompletely burnt wood on top (often called “brands”). This can be put into the next burn.
Once the lid is shut, no more charcoaling will happen. Anything which is wood now, will still be wood when the bin is opened.
10. Closing the bin
Put the lid or lids in place, taking care of your eyebrows in the flames. Sometimes weights or clamps are necessary to hold the lid down as it warps in the heat.
Seal the edges of the lid with dirt or sand. The smoke will show you where the gaps are.
At first, the heat of the fire will create gas under pressure, which will need to escape. This is not a worry, but it is a good idea to check the bin in a few hours, or the next morning, to ensure that the seal is good.
11. Bagging the charcoal
Once the bin is cold to the touch on top, it should be ready to bag. With a good seal this can be achieved overnight, but be very careful. Be cautious about live coals. It’s good to feel the top of the bin before the sun has shone on it, to feel whether heat remains.
After lifting the lids, I like to tip the bin partly over, to make it easier to shovel the charcoal out.
I use a bagger (pictured) for seiving and bagging the charcoal. My favourite bagger has a bag holder from a potato digging machine, with spring-loaded hooks to hold the bags.
The seive shown in the photo has 5mm holes to allow ash and fine charcoal to pass through. This is acceptable for blacksmithing, but I would generally prefer larger holes. For cooking charcoal I use a seive with 11mm holes.
Larger holes obviously allow more fine charcoal to pass, but this provides more fine char for use as a soil improver.
The fines and ash separated from your charcoal are valuable as a soil improver. They can be dug into a vegetable garden, or spread under fruit trees. Ash from the charcoal forge or cooking fire can also be used in the garden, but be cautious not to overload the soil in one place as ash is stronly alkaline.
I then sew bags shut with a bag needle and string.
Danger: it is possible for coals to remain alight in the drum. A bag of charcoal with a live coal inside could catch fire, and burn a vehicle, house or shed, or start a bushfire. Remember you are working with fire!
Store your bags of charcoal out of the rain. Water will make the charcoal sparky and hard to light, and rot your bags.
My bin makes just enough charcoal to nicely load up the Hilux, with enough room left for the family.
12. Have a shower
13. Using your charcoal
The charcoal you make from wood can be used for blacksmithing, cooking, or soil improvement. Charcoal is a smokeless fuel, which will not blacken pots or hurt your eyes when cooking.
Warning: Charcoal fires emit colourless, odorless and poisonous Carbon Monoxide gas, so must not be used in confined spaces. Burn your charcoal in a place with good ventilation (open windows, open doors, or outdoors).
Charcoal and firewood are “greenhouse neutral”, or “carbon neutral” biomass fuels, when produced from sustainably grown forest. The carbon dioxide released when charcoal is burned, is equivalent to what would be released were the wood to decompose naturally, and equivalent to what the forest will absorb when re-growing.