How to Make Charcoal

Timmy asks:

When I make a fire, I end up with little lumps of charcoal, but most of the wood burns to ash. How do they make charcoal without wasting all the wood?

The little bits of charcoal that are left in your fireplace or campfire are made one of two ways. The first way is that the fire goes out, and as it cools, it leaves little bits of unburned wood. The little bits are burned on the outside, but the inside is still partially unburned. To understand this, we have to look at how wood burns. There are a few stages:

  1. As the wood heats up, gasses are released from the cells in the wood. These gasses burn up, and make a lot of the yellow flames that we see in a fire.
  2. Once the wood has released the gasses, what is left is very dry, but solid. When these dry hard bits are exposed to high heat (like the heat produced when those gasses burn), they also burn, releasing more gas, heat, and finally leaving behind ash.

Sounds simple, no? Well, it is – sort of. The confusing bit is that once a fire is lit, all of these things happen at the same time. Fortunately, because the fire on the outside of the wood uses up all of the oxygen, the wood will burn from the inside out. If you have a fire and put it out before a log has burned up, you can cut open the log. You will find that the outside of the log is covered with charcoal that is full of cracks and fissures. This is where the phase 2 burning is taking place. The easy to burn gasses have been released, and the underlying structure is partially burned.

A bit deeper in, you will find charcoal that doesn’t have many cracks in it. This is the part of the log that has released most of the easy to burn gasses, but not all, and the structural part of the wood hasn’t started to burn yet. It hasn’t started to burn either because the heat hasn’t penetrated that deep yet (wood conducts heat very slowly), or because there is no oxygen to allow the structural part of the wood to burn (remember this: it’s important).

Even deeper in, you’ll find wood that hasn’t begun to burn at all.

The part that we’re interested in is the middle bit – the part where the charcoal doesn’t have a lot of cracks in it. We know that we can make charcoal like this by carefully controlling the temperature – getting the log hot enough to release all those easy to burn gasses, but not hot enough to actually burn the underlying structure. This is really tough to do, because if the log hot enough to release the gasses, it is hot enough to ignite the gasses, which will then heat up the underlying wood, and burn it too (this is why fire works instead of just going out).

A much easier way to turn the log into charcoal is to light it on fire, and get it good and hot, then to remove all of the oxygen. If the fire was big enough, the heat that is left over will continue to “cook” the logs, letting them release the easy to burn gasses. If you’re really fancy, you can allow in just enough oxygen to let some (but not all) of the gasses burn. This can generate enough heat to make sure the fire stays hot enough to keep cooking the wood until all of the gasses have been released, but the underlying structure doesn’t burn. This will create whole logs of charcoal.

That’s how charcoal was made in the old days. They’d make a great big pile of logs. I mean BIG: whole trees – lots of them. Then they’d coat the whole pile with a thick layer of mud (or clay), leaving a bunch of big holes around the bottom to let in air, and a hole in the top to act like a chimney. Light it on fire and the fire will bake the mud/clay into a rock-like wall, so that it won’t cave in as the wood burns. When the fire is going really good (you can tell by the color of the smoke coming out of the chimney – when the fire is burning well, there will be no smoke), start plugging up the air holes around the bottom. Watch the smoke. As it turns into a thick white smoke, you know that you have started starving the fire – there isn’t enough air getting in to burn all of the gas that is being released from the wood. The unburned gasses make the smoke white. By adjusting the amount of air that is allowed in, you can make sure that there is enough oxygen to let some of the gas burn (to keep it hot in there), but not enough to actually burn the charcoal that is left. Once the wood has released all of the gasses, you plug up all the holes. What is left of the fire will go out because there isn’t anything left burning – no oxygen.

Charcoal factories used to spend weeks or months building these wood piles, and they would often take weeks to burn. Sometimes they would take over a month to go out and cool completely once the holes were blocked off. Once the whole mess has cooled off, you bust open the baked mud walls, and you’ve got a big old pile of charcoal. In huge pieces – sometimes almost as big as the logs that went in. These are cut up (or broken up) into whatever size you need. Voila. Charcoal.

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