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Indirect or Direct Method?
There two methods of making charcoal: indirect and direct. Each involves burning wood in oxygen-starved conditions. The indirect method burns wood in an airtight container using an outside heat source; the result is higher yields but the burning setup can be elaborate. The website saveourskills.com explains the direct method as starting a fire in a container, adding wood and then restricting the flow of oxygen to the fire. The dearth of oxygen means the fuel is not consumed completely but reduced to a nearly pure carbon state.
The Chemistry of Charcoal
Gary Gilmore, a Pennsylvania Department of Conservation and Natural Resources forester, wrote at puffergas.com that charcoal can be made from anything containing carbon, but the traditional material has been wood, which is cellulose fibers bound by other chemical ingredients. Hydrogen, oxygen and carbon atoms are the chief elements of cellulose, and the object of charcoal-making is to remove the hydrogen and oxygen, leaving the carbon. This is done by heating the wood to at least 500 degrees Fahrenheit, at which point the chemical bonds start to break apart and gases are released.
Which Wood To Use?
Charcoal is carbon no matter what type of wood it comes from, Gilmore says, although charcoal varies in density depending which wood is used. For instance, a bushel of hickory charcoal may weigh 25 pounds; a bushel of pine charcoal, 13 pounds. But while one weighs more, a certain weight of pine charcoal will produce the same amount of heat as an equal weight of hickory charcoal. Charcoal does not add flavor when cooking no matter what type is used because the volatile oils have been burned away, Gilmore added; to obtain flavor, use wood that has not been burned. It is recommended that seasoned, not green, wood be used for making charcoal as more energy is required for converting the former.
Top and Bottom
The direct method involves burning wood in a container where oxygen can be controlled. A container called a retort is used to hold the burning wood. Any size container may be used; Gilmore suggests a 55-gallon metal drum. He uses the same size drum for an afterburner, which is used to contain the smoke. The retort must have a lid that can be sealed to control the oxygen. If the drum has no lid, cut a large hole in one end of the drum and cut a piece of sheet metal large enough to cover the hole completely. Slots or windows should be cut in the bottom of the container; during the burning process these openings will be covered as necessary to regulate oxygen flow through the drum. The top, or afterburner, will have both ends open. Similar openings must be cut into the top as well to allow oxygen to burn off the smoke.
The Process
The bottom container may be filled with either hard or soft wood that may be as long as the container is deep, but keep the thickness to less than three inches to facilitate even burning. Pack the burner with as much wood as possible, shaking it as necessary to pack it in. Place a pile of kindling on top of the wood and set it ablaze. Place the afterburner on top of the retort with the slotted end near the top of the bottom drum. This allows oxygen to be pulled through and consume the smoke. A general time frame for charring wood is three hours but check the fire after an hour and frequently thereafter. If one side of the container seems hotter than the other, cover the slots on that side with dirt so slots on the other side will pull more oxygen and better distribute the fire.
Checking the Char
Check at some point the charring process and shift the wood around to ensure even burning. This involves removing the afterburner, and Gilmore cautions that the drum will be extremely hot and will give off volatile gases. Wear heavy gloves and natural-fiber clothing when removing the afterburner. Check the direction of the wind to avoid gases and smoke blowing in your face. To finish the charring process, seal the slots in the burn container with dirt, packing it on firmly to provide a solid seal, eliminating the flow of oxygen and preventing further burning of the carbon. Remove the afterburner and put the sheet metal cover over the retort opening. Let cool overnight and don't put the charcoal in a flammable container until after least 24 hours have passed.