Charcoal is a substance that we use a lot of on our channel, and we're not alone. You've probably used it for a barbecue and lump charcoal used to be the fuel of choice for industrial processes before the advent of coal and coke. So what is charcoal and how is it made?
Chemically speaking, charcoal is almost pure carbon. It's made from wood by driving off everything in the wood that isn't carbon, e.g., water, tar, and volatile gases. This process, induced by heat between 600-1100 degrees Fahrenheit, is called pyrolysis. The product is charcoal. It's worth noting that the pillow-shaped briquettes you buy in a grocery store are not pure carbon; they have a binder and other additives (hence why they leave so much ash behind after burning; good quality charcoal should barely leave any ash at all when burnt).
So how do you make it? Again, pyrolysis happens around 600-1100 degrees Fahrenheit. Obviously this occurs partially whenever a wood fire is lit, so you can make charcoal just by lighting a wood fire and collecting the charcoal that is left afterwards (what's called the "direct" method). However, it also isn't terribly efficient since large quantities of charcoal are made and then burned in the same fire. Another option is the "indirect" method: seal wood into a nearly airtight container and bake it inside a second fire (you may have made charcloth in a similar way using a small metal tin and your kitchen oven). This leaves all the wood in the sealed chamber as charcoal (minus the volume of water, tar, and gases). However, it requires a secondary fire, so although it yields a cleaner "harvest" of charcoal, it still seems to lack some desired efficiency.
Whether you use the direct or retort method, your charcoal making venture basically comes down to two variables: 1) heat creation/retention and 2) oxygen. Unless you cut the oxygen flow to the fire down to nothing or near nothing, the wood will pyrolyze into charcoal and then combust and burn away (i.e., what happens in a normal fire). And whatever you do, there has to be some kind of heat source to raise the temperature of the wood between 600-1100 degrees Fahrenheit and then keep it there for a long enough period of time to complete pyrolysis. How you accomplish those two tasks is up to you, but they both have to occur.
On the channel, we've used an old barbecue and a 55-gallon drum as ovens and retorts for the direct and indirect methods respectively. The 55-gallon drum we use has a steel pipe flue welded onto it that in theory should allow the volatile gases to exit the retort then combust and contribute more heat to the retort, but we've had mixed results with the flue. Even a simple earth chamber, above or below ground, can provide the insulation to retain heat as well as cut off oxygen flow.
Remember the two golden rules: Heat/insulation and oxygen flow. Go and may the charcoal be with you.