July 21, 2006

7-17 La Carbonera: Fun With Pit Kilns

So I need some charcoal for my project. I wanted it to be made of one type of local wood that would grow fast and also made good char. On the recommendation of several sources, we selected guaba wood, an Inga spp. After two or three years of growth, the trees can be pushing two stories high and are a popular species for shading coffee crops, so trimmings or full trees would be relatively easy to get.

It didnít work. While the story of how it didnít work might be interesting for anyone planning to build a pit kiln, my advice at this point would be not to.* At least not if you donít have time to fail dismally in the beginning.

For those of you who didnít get a chance (and why not, for pityís sake!) to read my 13 page paper on man-made Amazonian soils whose high charcoal organic matter gave them particularly long-lasting properties beneficial to agricultural activities, hereís a brief rundown. The carbon left over after a low-oxygen, slow burning fire is high in super-stable aromatic carbon chains that hold onto nutrients and water. Also, you usually retain 30-50% of the original wood biomass instead of the usual 3% left over after a fire that burns the wood to ash. If you donít then use that charcoal as fuel, it can be added to soil to improve properties like nutrient and water retention, tilth, drainage in clay soils and certain types of biotic activity. So.

I hired Cenůn to build and start two traditional pit kilns to ensure there would be a back-up supply of charcoal and try adding a different treatment to the mix. Due to a miscommunication, by the time I got to the site on Friday, the first kiln was almost finished. However, I ended up spending a fair amount of time over the weekend futilely trying to restart a totally dead fire in a hole in the ground filled with a mix of green and dry wood, that is, when I wasnít trying to patiently explain to the proprietors of restaurantes and carnicerias that I was really interested in carting away any waste bones they had left over.

Monday the new kiln with the bones added, but otherwise exactly like the first, was slated to be started. I was able to assist with the hauling and watch throughout the whole process, ending up with a clearer understanding of why previous attempts to restart the other kiln the way I had been were doomed from the start.

Note: If you want to read on, note that this is a graphics-heavy post that was originally intended to serve as an illustrated instruction manual for the process. And just to ensure you donít think I meant for this to be a complete downer post, I encourage you to enjoy the schadenfreude, or perhaps just to look at this cool local bug whose head used to be made into jewelry Ö

Cornisuelo. Costa Rica, 7-04-06. - natasha

First, thereís the pit. The main body of it is 1.9m long, by 0.9m across at the top and a little narrower at the bottom, by 0.5m deep. The extra flares at the end extend for about another 0.4m.

Charcoal pit kiln. Costa Rica, 7-10-06 Ė natasha

Charcoal pit kiln. Costa Rica, 7-10-06 - natasha

The main pit is then filled with logs cut to about 0.9m long and mostly laid across the width of the pit except for the larger pieces laid lengthwise across the very bottom, four pieces set in upright at the four corners and other pieces used to fill in gaps. Then in one of the end flares, you add kindling. The bones were put in with the logs and then covered with more wood in hopes of sufficiently cremating them so they could be safely used as an additive in my plant growth trials. Itís now prohibited to sell steamed bone meal in Costa Rica because the government was concerned about the potential to spread disease when used as cattle feed. That proved a little inconvenient, but I canít argue with the sentiment.

Laying in the wood. Costa Rica, 7-10-06 - natasha

Pit flare and support logs. Costa Rica, 7-10-06 - natasha

Bones and kindling. Costa Rica, 7-10-06 - natasha

When the main pit is filled with wood, itís overlaid with about 6-8cm of freshly cut weeds and greens to keep dirt out of the fire, then covered with a few centimeters of dirt just a little less thick than the layer of greens thatís walked on until itís stamped down pretty tight.

Laying on the greens. Costa Rica, 7-10-06 Ė natasha

Stamping down the dirt. Costa Rica, 7-10-06 - natasha

It was finally time to light the pile. I wasnít thrilled when it became clear that the method for getting the kindling going was to burn plastic bags over and among it.

Lighting the fire. Costa Rica, 7-10-06 - natasha

Cenůn explained that paper burned too fast to be a good material and if youíve never been here, I can only say that the difference in the amount of paper used in Costa Rica compared to in the U.S. is vast. Bluntly, not much money is spent here on non-food items that are used once and thrown away; there isnít a lot of junk mail and Iíve only been in two houses that used paper towels in the kitchen. I donít really know enough to say differently, but considering that Cenůn used to do this for a living before you could buy charcoal at the market, I suspect that this is simply the most likely way such a fire would be started if this technique came back into wide use. In that light, I didnít argue.

A farming practices manual from the Costa Rican government that described a similar process seemed to call for using a bar of zinc to start the blaze going, but I have to think that the likelihood of someone around here buying such an exotic material, when a free plastic bag from the store would serve as well, is slim to none. Also, itís so damp here that itís hard to get a match going very well or to keep it going once itís started burning. When youíre trying to start a fire in a hole in the ground, this is a major pain in the tookas.

Itís a simple mechanical issue, but I think itís the sort of technical difficulty on which whole development projects could stumble. Things are really just different here from what Iím used to; in ways that I could list in tedious detail without perhaps ever being able to get it across to someone whoíd never been here or a similar country and in spite of the fact that Costa Rica is relatively westernized. Basic questions of what constitutes an Ďavailableí material for a given purpose can have a multitude of answers. If someone wanted to help a community in another country, I think there could never be an appropriate substitute for going thereto live and talk with the people. I can see now that hotel visits barely count, in so, so many ways.

Once the fire was started, Cenůn fanned it to get it going until there was a good amount of smoke coming out of the other flare and about 4 additional holes he made in the top of the mound near the side away from the fire. He followed the same process when restarting a fire with additional kindling in the first kiln.

Fanning the flame. Costa Rica, 7-10-06 Ė natasha

Before the chimney. Costa Rica, 7-10-06 - natasha

This fire was allowed to burn for about an hour before being covered with greens and then dirt. Another hour later, a metal chimney pipe was added to the flare on the other side, then packed around with greens and dirt.

Covering the fire. Costa Rica, 7-10-06 - natasha

Setting in a chimney. Costa Rica, 7-07-06 - natasha

Covering the extra vents. Costa Rica, 7-10-06 - natasha

A longer assembly of chimney piping can be added if you want to capture the vapor condensate. This was done with the first kiln, but was removed to allow greater airflow and the extra piping added to the chimney of the second kiln. The close-ups of the chimney assemblage were taken during the finishing of the first kiln.

Chimney pipe fully assembled and installed. Costa Rica, 7-07-06 - natashaChimney with rain hood. Costa Rica, 7-07-06 Ė natashaCondensate drain spout. Costa Rica, 7-10-06 - natasha

The smoke seemed to be coming pretty well by the time everything was closed up and there was nothing much more to do than wait, check to see if they were still working two or three times a day and hope. The new one quit on Thursday, while the first one didnít stop giving off heat until Friday.

Opening the tomb. Costa Rica, 7-17-06 - natashaNonetheless, when I opened the pits up the first chance I got in between torrential downpours on Sunday, the top layers were clearly nothing but heavy piles of moldy wood. Theyíd been rained on when the following pictures were taken, but when I first dug open the top, clouds of mold spores poured into the wind. Thank goodness it was blowing the other direction, this thing was like the set of a horror movie.

Much as I didnít want to, the only choice left if I wanted to get started on my project was to use bags of charcoal from the store. Any charcoal that might be underneath of all this heavy, moldy, green wood is effectively unrecoverable within the necessary timeframe. That timeframe being approximately, Ďnearly too late to salvage.í Additionally, walking on the piles and stomping at them with my (mumble, mumble) pounds of weight and very heavy hiking boots elicited no significant compression or settling as might happen if some of the logs at the bottom of these half meter piles had in fact burned through as expected.

*@ &^%@#$% ****!

On the bright side, store bought charcoal still constitutes a locally available material and is recognizably composed of a former tree, instead of being mixed with god-knows-what and compressed into identical briquettes. Iím counting my blessings and readjusting my expectations, oh yes. At this point, itís keeping the mellow on, experiment-wise.

Weíve also just entered the canicula period of summer, after the relatively dry 2-3 weeks of the summer of San Juan. This means that we have warm to cool summery weather in the morning and downpours starting from sometime between noon to three that last until eight or nine at night. Iím told that starting around the 15th of August, itís really going to start raining. This isnít too bad, just means that itís better to get things done for the day before lunch. Additionally, I clearly brought too much sunscreen and not enough bug spray.

If you were wondering though, without a trace of irony or sarcasm, Iím having a fabulous time and would gladly stay longer than Iíve planned to.

* It must be granted that a good few people used to make charcoal in pit kilns around here and that this is a widespread practice throughout the tropics. But it clearly isnít entirely simple and it seems that once the immediate, hands-on experience falls into disuse, a learning curve is required. Still, if I tried this again with similarly limited time, I think I might well want to find a more reliable method for beginners and those who havenít tried it for a while.

Posted by natasha at July 21, 2006 11:04 AM | Costa Rica | Technorati links |

Hi Natasha,

As someone who has been reading
your material on Dark Earth with
considerable interest and excitement,
I'm not at all bothered by your
experiement's deviations.

The internals of the kiln are much
less important than the seedling growth
rates. That's because underground kilns
may be too unpredictable and the
quantity of their yields may not be

Instead, I envision a bathtub-sized
steel enclosure that's the shape of
an elongated cube and that can be
towed around behind a truck.

Inside, the portable kiln would have
several layers of wood with a grate
seperating each layer.

At the bottom would be a hatch to remove
charcoal and the thing would be timed
so that removal and adding new wood
on top would yield precisely the right
kind of charcoal.

Ideally, a village could keep the thing
running for days on end. Then it could
be towed to the next site.

Marvelous commentary. Thanks,


Posted by: Jared Scarborough at July 23, 2006 01:16 PM

Thanks for the interest, it's very encouraging.

That's a good idea about a portable kiln, though I don't know that towing is a good means of transport on these roads. They're in very bad shape, though better than many in the region. Definitely though, some far more reliable method needs to be determined and I'll be looking at all the options I can find in the future.

Posted by: natasha at July 24, 2006 11:08 AM

I have a number of pit kiln pieces done by a friend of mine. They are wonderful, very earthy colors and you never know what the outcome will be, very random. It's not an easy process and my friend still loses more pieces to "the pit" than she is able to salvage intact, but she loves this process.

Posted by: SME in Seattle at July 30, 2006 07:27 PM

Hi all! What do you think about conflict in Israel?

Posted by: Gary at August 2, 2006 03:56 AM