My biochar kiln doesn’t work correctly because the door does not seal tight, and too much air can get in, and smoke and gasses get out. To make charcoal or biochar, you need a fairly air-tight container, or the contents will just burn to ash.
First burn of the biochar kiln, showing escaping smoke around door
For a fix, I decided to use the old trick traditional cooks use when they want to keep moisture from escaping from a clay pot — more clay. Wet clay can be used to seal the lid. That should work on my kiln too. It doesn’t need to be 100% effective, just 90 or 95% will be enough.
But where am I to get clay? I’ve seen layers of clay in soil profiles in a lot a places I’ve lived, but we haven’t been here long enough to know where those kind of places are in this area. Probably plenty up in the mountains that are a 15 minute drive away, but I don’t even need to go that far.
Soon after we bought this place I tested the soil to see how much clay, silt, gravel and sand was present. That is a simple process (if you don’t need super-accurate results). I put a handful of dirt in a glass jar, added enough water to fill it, put on the cap and gave it a good shake. Then set it down. Most of what settles to the bottom immediately is gravel or sand. Above that is a layer that falls in the next 30 to 60 seconds, and that is silt. Everything else is clay, which can take hours or even days to settle completely. Our soil has about 50% clay content, 30% silt and 20% sand.
So the same process can be used to extract the clay from the soil. I filled a bucket half-full with soil, then added water and stirred it up good. I let it settle just 60 seconds, then poured the slurry into another bucket, without the sediment. After a few hours, the second bucket had settled (the clay is not so fine as to take days), and I poured as much of the clear water off as I could, which also takes away most of the plant rootlets and other floating organic materials. The thick slurry that was left I poured into a flat tray and set it out in the sun to dry off. That is all fairly-pure clay. When the water content gets to the right level I can twirl a chunk between my hands and make a rope-like strand, just as you would if you were making a coiled pot. That will seal the kiln just fine.