I go through a lot of charcoal. Between cooking great food, blacksmithing, historical bladesmithing, and fireworking I burn up a lot of the stuff each year. And while "lump" charcoal has become readily available in recent years, it is expensive and not always the species or type you want, so making your own makes sense. The art of making charcoal isn't lost, far from it, there are many many videos and tutorials all over the net that show you exactly how. But each time I mention how I do it I am met with awe as most of what other people are doing is complicated requiring special tools and furnaces or waste a lot of material and fuel. By simply following the rules of nature my system is stupid simple and produces a great clean burning charcoal in an enjoyable afternoon.
What is Charcoal?
While most people would recognize charcoal many are often confused by what exactly it is and what it is not.
Those briquettes from the bag might make your hotdogs and hamburgers taste good but they really aren't true charcoal - they are compacted pucks of mostly clay and filler with just enough black stuff to allow them to burn. Briquettes will not work in your forge or your fire work star. And it is totally unrelated to coal which is a rock. Charcoal (or as it has become called "lump charcoal" to separate it from the above mentioned briquettes) is essentially incompletely burned wood. Historically charcoal was a main fuel type and used for everything, but first rock coal then gas has now replaced it in our modern world.
Charcoal can be made from any type of wood and other plant and animal carbon sources. Different uses for charcoal benefit from different starting materials with hardwood such as oak and hickory being the most popular for cooking, soft wood such as willow and pine for metal work and black powder, and things like bone for achieving the beautiful rainbow of colors seen with color-case-hardening on antique firearms.
How is charcoal made?
Wikipedia says Charcoal is: a lightweight black carbon residue produced by strongly heating wood (or other animal and plant materials) so as to drive off all water and other volatile constituents.
Charcoal is simply incompletely burned wood. If it was completely burned it would be ash and useless. As we want the carbon we need to get rid of all the volatile oils etc that make up the rest wood. Historically they made huge piles of split wood and covered them with dirt. Once ignited the charcoal burner would control the air entering the pile by opening and closing openings at the bottom. These piles would slowly burn for days and days and many sleepless nights, but it worked and it made a lot of it. More recently people have figured out retorts and other furnaces to make small batches of charcoal, with some form of sealed container the wood is placed inside of, then heated only allowing the wood gas to escape. The wood is thus charred to perfection but not burned to ash.
My system is much easier and doesn't waste as much fuel as with heating a retort. I simply use the physics we are already using to create the charcoal to drive the reaction ie fire. The the only thing you need to remember from chemistry class is the Triangle of Fire: Heat, Fuel, Oxygen (very similar to the fundamental 3 in small engine repair: Fuel, Air, Spark) as seen here:
Triangle of Fire
When wood burns a chemical reaction is happening. For the reaction to occur three things have to be present: Heat, Fuel, and Oxygen. If one of these things is missing or as we know from small engines, in the wrong ratio, fire will not happen. While most charcoal making is accomplished by starving the fire of oxygen, my system turns the equation on its head - I unbalance the equation by feeding the fire too much fuel. Unbalanced triangle = no fire, same if I starved it but without the heat loss and waste of fuel.
Alright already! How do you do it?
I simply add too much wood and pile fire atop fire in a can. It could totally be a hole in the ground, but a 15 gallon can works perfectly. I build one fire and allow it to heat the can, then before it burns to ashes I add enough wood to starve the first of oxygen, then repeat until the can is full. I then seal it off with a steel plate to rob it of all oxygen and leave it for a few days to cool off. Done. Here is a video I recorded of my process in 2017. As you can see it cycles perfectly naturally on its own, and as long as you watch it and don't let it burn down too far you can easily make charcoal and take the guess work or heavy engineering out of your set up.