Instructions for Gallon Paint Can Biochar Kiln

wae paint can kiln
More pictures of Biochar can
Tools: Hammer, spike, “church key”, tin snips, pliers, welder’s gloves, metal stir rod.
Parts: Gallon paint can, #10 tomato sauce can, tall bean can, (3) ~2″ bolts + (6) washers & nuts, metal pan

The build: Start with a gallon paint can. Discard the cover. Punch the bottom of the can full of holes. I use a “spike” (large nail) available from a home discount warehouse store for around 50¢. It’s about 3/8” in diameter. A large flat blade screwdriver or a round punch that makes holes about the size of a drinking straw would also work. The idea is to go across the bottom of the can in a cross pattern then fill in the segments with some holes down the center of the segment and then some in the remaining area of the segment. It’s not really critical how many holes you have. (template) You want to provide plenty of breathing space but not so many holes that the bottom of the can is weak. None of the holes should be closer than ¼” to any other hole. I also suggest adding three ~2” bolts through the bottom (spaced 1/3 of the way around the outer edge of the can) by punching holes and holding them in place with a nut and washer on the outside and another nut and washer on the inside of the can. This provides legs to raise the can far enough up to provide room for air to circulate around the bottom and also to keep it a little cooler under the can (it gets hot!).

Get a (or many if possible) “gallon” #10 cans from an Italian restaurant. These are the cans that tomato sauce comes in and any Italian restaurant should have plenty of them. If needed, wash the can and, if one end isn’t totally removed from the can, remove it with a can opener and set the can aside.

“Bush Best” and “Van Camps” have beans in a can that is a large size but taller than standard cans (28 oz). Cut both ends out of the can. This will be the chimney. (I’ve also successfully used the tall cans that tomato or other kinds of juice comes in.)

Back to the tomato sauce can. Punch holes in the side of the can at the open end with a beverage can opener commonly called a “church key”. As before, punch the holes on the quadrants and then fill in between with another punch evenly spaced between (a total of 8 holes). If needed, squeeze the turned over parts up against the side of the can with pliers. Place the bean can roughly centered on the unopened end of the sauce can and trace around it with a marker. Punch a hole in the center of this circle on the tomato can and with tin snips, cut to just inside the circle you drew. Cut enough times so that it looks like a cut-up pie. Bend each of these tabs up. You have just built the afterburner. The punched holes will allow secondary air in to finish the combustion and the tabs at the top will align the chimney.

Additional equipment needed: 1) Leather gloves usually called ‘welder’s gloves’. They sell at Harbor Freight for $5.00. 2)  Something (that won’t burn) to stir the char. I use a 2 prong weeding tool I bought at a Dollar Store for $1.00. 3) A metal pan larger than the paint can that you can put water in to cool the char.

Before you fire your kiln, be sure to clear it with your fire department. There shouldn’t be a problem, but it’s best to get them on your side first.

The Burn: Fill the paint can with whatever you are going to use to make charcoal. We like the chippings from tree trimmers but any wood based material will work. Make sure it’s not packed too tightly so you get air circulation through the material. The trick is to have the pieces not too big or too small. Too big (like a piece of 2×4) will take too long to char through; too small (like sawdust) won’t allow air to flow through. Twigs, cut up small branches, etc. work well. I’ve even used short pieces of 2×4’s by splitting them into smaller pieces about the size of a pencil. Light the material in the can. I use 90+% rubbing alcohol from a pharmacy as a starter fluid. Once it’s burning well, put the tomato sauce can on top of the paint can, open end down, followed by the chimney can. Relax and let the fire do it’s thing. You know the process is done when the yellow flames stop. Put the gloves on and lift the afterburner and chimney off and give the char in the paint can a stir to make sure it’s finished. Dump the char in the pan with water to extinguish it and … that’s it. You’ve made biochar. Each burn should yield approximately one quart (depends a lot on how loosely the starting material is packed and the size of the pieces). These kilns are very easy to make and the materials are almost free, so make several of them. They operate with minimal effort and it’s easy to have 4 of them burning at once.

Charging: Mix some nutrients into the water and char and store in a bucket for about a week to “charge” the biochar. That way, it is ready to go into the soil without absorbing water and nutrients from your plants.

Technical description of the process: This is a TLUD (top load, up draft) stove. As the air flows up from the holes in the bottom, it meets a flame front in the material that is being charred. All of the oxygen is used up in this flame front so that as it flows further up, there isn’t enough oxygen to burn the char that was made when the flame was higher in the material. Once all the flaming is done, the char will continue to smolder. (The traditional “burning coals”) But that’s when we dump the char into water and all burning stops, leaving behind the biochar you’re after. The afterburner is very important in the process as the burning in the paint can yields mostly smoke and soot which the afterburner will consume. Plus that secondary burning provides a strong updraft through the chimney that keeps pulling air through the holes in the bottom of the paint can.

Wae Nelson
Florida Gardening Magazine

Thanks Wae !