Charcoal consists of taking animal or vegetable matter, which is rich in carbon, and of removing the volatile components from it, which essentially leaves only the carbon. Here I will be considering the use of wood as the vegetable matter. To remove the volatile components, the wood is heated to a moderate temperature which removes the unwanted agents. The fuel for this heating is the volatile factors themselves. Only enough oxygen is admitted, or used, to allow these components to burn. They burn at a temperature which is low enough that the carbon does not burn or combine with the oxygen. At the end of the process, only the carbon remains, which is now called charcoal.
From the pictures which I have seen, the trees are cut into about four foot lengths, and split until the diameter is about six inches. These are stacked on end around a central flue, until a dome-like structure, perhaps ten feet high, is formed. Then the pile of carefully placed wood is covered with dirt, preferably damp, to perhaps a thickness of six inches, maybe less. At the bottom, a few small holes are made through the dirt to admit a limited amount of air. At the top center, an exit hole is made for the combustion gases to escape. A fire is started at locations around the bottom. This fire is flameless, as the temperature is low, but still the volatile agents are burned off.
The collier, or the person doing the work, has to look for leaks in the outer covering, as they could become points where oxygen is drawn in which could cause a much hotter fire, consuming the carbon in the wood. I believe the combustion gases tend to be smoky. This and other telltale signs, such as the outer temperature, tell the collier how the process is going. Depending on the size of the pile, it may take a few weeks to complete the process. The danger lies in the pile getting too hot.
When the collier judges the process is done, he seals the vents, to cause the combustion inside to stop. After this, he has to work fast on the next step. He tears the pile apart and watches for any sign of flame where the charcoal might be burning. If it is burning, he has to redistribute the charcoal, and put any burning embers out. While doing this, he tries to avoid rough handling of the charcoal, which converts it to small pieces and dust. Large pieces of charcoal are desired. The charcoal is gathered up and taken to the near vicinity of the furnace, where it is usually stored in a shed. The furnace master feels more comfortable if he knows he has a backlog of charcoal to keep his fire going. Also, the shed serves to keep the charcoal dry. Of the original 100 parts of wood, the charcoal weighs about 25 parts, while taking up about 60 parts by volume.
If someone can improve on this story, please do so. As you can tell, I have never made an ounce of charcoal.
We gratefully acknowledge the work of John Blankenbaker who published over 2,500 Germanna History Notes via the Germanna-L@rootsweb.com email list from 1997 to 2008. We are equally thankful to George Durman (Sgt. George) for hosting the list and republishing the notes via rootsweb.com.