Fire in a Hole

Credit: Maxpixel

For the last week, several LEAP students have been working towards creating a working forge in the Green School. While this experiment did start out as a pyromania-induced hobby, it quickly evolved into an opportunity to test chemical knowledge and to research and experiment with metals and alloys. However, before we could begin our foray into furnaces, we needed to inventory our supplies, and source our materials. After some searching and scavenging, we were down one thing: Fuel.

Furnaces can take many forms, from propane burners to coal fire ovens, but we decided on a middle ground. Bio-charcoal. Furthermore, we decided to go the whole way, and make our own. Charcoal can be made many ways, but the method we decided on works a bit like this. By heating the wood in an almost sealed container, certain gases called volatile hydrocarbons, such as hydrogen, can escape through holes in the container. However, once the wood reaches a point where all of the volatile hydrocarbons have been burned off, the container is fully sealed, trapping the other gasses such as oxygen within the container. Bio-char made in this method is called ‘lump’ charcoal, which is characterised by it’s high temperatures as well as it’s lack of ash, allowing for a cleaner fire. Charcoal made in this manner is perfect for smithing, cooking, campfires and other heat-based fires. They don’t release any flames, making them ideal for barbecues.

To accomplish this, we decided to seal our wood within old paint cans, allowing us to seal them airtight, and also to move and stack them easily. To prepare them, all we needed to do was remove the plastic handles, and drill 2–3 holes in their lids so the volatile hydrocarbons can escape, instead of exploding. To heat the cans, we built something called a rocket stove. These furnaces a comprised of two main parts: the base, and the funnel. The base was comprised of a single layer of bricks, with a grill on top, followed by another few layers of bricks. The fire sits directly on the metal grate, which allows for air to oxygenate the fire from underneath, and also for ash to drain out the bottom rather than smother the embers. For a funnel, all we used was some pieces of corrugated tin, wrapped round into a cylinder, and placed upon the brick walls. Heated air naturally travels up, and the super-heated gasses that fires release travel even faster. Because the air cannot escape sideways from the fire, it instead travels directly up, through the funnel and out the top. Due to the gasses ‘heated’ escape, other gasses are caught in the vacuum, creating a cycle of suction, which accelerates the air’s journey, and creates a rocket-like noise, hence the name. By suspending the charcoal cans in the funnel, they are constantly blasted by the super-heated air, cooking them far faster than the fire itself could.

After an hour of rocketing, small flames can be seen emerging from the pressure release holes. This is the result of the other gasses within the wood beginning to ignite, and escape from the cans. When this begins to happen, the containers are removed from the heat, and turned upside-down in dirt or sand, blocking the holes. This forces the gases to remain inside, and continue to cook with the wood. Again, after an hour of this, the charcoal is finished and ready for use.