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Wood stove biochar experiment

Bucket of charcoal and ashesOnce I started damping our Jotul F 602 wood stove down at night, I realized that the stove makes copious amounts of first class charcoal.  Thirteen days of fires filled up our bucket beyond the brim --- time to figure out how to filter the charcoal from the ash.

Wood ashes


Both wood ashes and charcoal are good for the garden, but they have different uses.  Ash is high in potassium, and also contains quite a bit of phosphorus, magnesium, and various trace minerals.  More important, though, wood ashes are a quarter to a half calcium carbonate (aka "lime"), so you have to be aware that you will raise the pH of your soil by applying the ashes.  Since our soil already has a pH around 6 and our potassium levels are naturally very high, I don't want to apply too much ash or I'll raise the pH above the optimal 6.5 recommended for a vegetable garden and possibly bring the potassium up to toxic levels.
Sifting charcoal from ash
The charcoal is what I'm really excited about.  We've been sold on the benefits of biochar to soil, but haven't been ready to put in the time to build a biochar production chamber.  Sure is nice of the airtight wood stove to do that job for us while also producing plenty of heat to keep my toes warm.

Glowing coal

I built a sifting box to separate the charcoal from the ashes, a process that went pretty smoothly but needs a bit of tweaking.  First of all, even though the coals had been sitting in the ash bucket for nearly twelve hours, some were still burning, and I scorched the wood Pouring charcoal into a metal bin for storageon the inside of my sifter.  Mark suggested adding flashing to the inside, which is a great idea.  The job was also pretty messy, with ash flying everywhere --- maybe I need to add a lid to the top of the sifter?

Despite needing some optimizing, the sifting operation went quickly once I realized I should only dump in about four inches of stove debris at a time and jiggle the sifter a lot to get the ashes to fall through the holes.  The really difficult bit will be figuring out how to crush the biochar into powder.  We could use the chunks as they are, but my understanding is that we get more surface area and more microbial action if we crush the biochar into much smaller pieces.  While we put our thinking caps on, I'll be storing our charcoal in a metal container so that it can't burn through the bottom and start a fire.

Our homemade chicken waterer kits work all of the kinks out of the process so that your chickens can be drinking clean water in half an hour.


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You could use wood ash to make lye for making soap or curing olives. Be careful though, concnetrated lye is caustic.

Comment by Roland_Smith Thu Dec 9 15:59:13 2010
Got a baseball bat or anything similar? Leave in that container in the picture and use bat as part of mortar and pestle. Just like anything else that needs ground down to dust, and it's therapeutic. Work out your frustrations while making little ones out of big ones. Then throw into your compost pile and let it cook.
Comment by vester Thu Dec 9 16:13:00 2010
Great info, as always. Thanks.
Comment by J Thu Dec 9 17:27:42 2010

Roland --- good idea! We don't use much soap, though, and can't grow olives. We'd still have a lot left over. Any other ideas?

Vester --- Addition to our compost is the goal once we get the biochar ground down (although I'll probably actually add it to the bedding in the chicken coop so that it'll get the high nitrogen manure, which is supposed to jumpstart microbial colonization.) I tried mashing it with a four by four a bit like you're talking about doing with a bat, but that didn't work very well. Perhaps it would have done better if I'd poured out most of the charcoal and just been mashing a few inches worth --- this way, the charcoal mostly drifts out of the way of the pestle rather than getting ground down.

J --- Thanks! As always. :-)

Comment by anna Thu Dec 9 19:34:46 2010

By heating the calcium carbonate you can create calciumoxide (quicklime) which can be used to make lime mortar or even limecrete, which are eco-friendly building materials.

There's at least ten more uses.

Comment by Roland_Smith Fri Dec 10 05:22:52 2010
Now those ideas I really like! We may have to look into that once we've stored up enough ashes to make it worth our while.
Comment by anna Fri Dec 10 08:42:20 2010

Anna,

How to you make your sifting box. I was telling my brother about your homestead and he was interested. He's currently just roto-tilling the ash and charcoal into the garden all together.

Comment by Edward Mon Jan 3 08:24:19 2011

There isn't much too it as currently constructed. I used one 8 foot 1 by 6, cut it down into five pieces, and screwed the box together. You could just make a four-sided box, but the pointed side is useful for pouring out coals. Then I stapled hardware cloth (a wire mesh with small holes) on the bottom. In retrospect, I might have added a coating of metal flashing to the inside to keep the wood from charring when I sift hot coals (but it's also pretty easy to wait and just sift the coals once they cool.)

There's room for improvement (especially needs a lid!), so I'd be very curious to see what your brother comes up with. Also, we've mulled over several ideas for grinding the charcoal, but haven't come to any conclusions yet.

Comment by anna Mon Jan 3 08:51:56 2011
how is the biochar project coming, and have you made any modifications to your sifter?
Comment by Steven Mon Dec 26 19:42:28 2011
I actually posted a followup just a few weeks ago. The short answer is --- still using the same sifter and loving how easy it is!
Comment by anna Mon Dec 26 20:06:34 2011

When sifting ash it is light, becomes airborne and is readily inhaled. It contains silica as well as lye and is very dangerous to breathe. One should wear a respirator, a mask does not filter out micro-sized silica particles that lodge deep in the lungs. I am a potter, I make glaze from wood ash, if you can wash the ash out once it is wet it is no longer breathable and in a much safer form. But keep your hands out of what will then be the caustic water. I found your post because I am looking for other uses for the wood ash and in particular want to make mortar or concrete and had a sense there would be an application here knowing something of the chemistry.

Comment by Donna L Tue Aug 19 13:50:39 2014