Sunday, 2 September 2012


Growing up I knew charcoal as the square, chemical-soaked briquettes people bought in bags and poured into the barbecue grill once a summer. Like so much else in our lives it came from a store, wrapped in plastic and pre-treated for shelf life, with no sense that it shared a name with something amazingly useful, which hundreds of generations had made themselves.

Charcoal is simply wood that has been burned without oxygen, either by being heated but sealed away from oxygen or, more commonly, setting it on fire and then cutting it off from the air, keeping the wood from burning completely into ash. Most other substances in the wood are driven off, leaving a porous shape of almost pure carbon, lightweight and easy to transport.

It can purify water by soaking up impurities, as in many kitchen sink filters, and treat poison victims when crushed and drunk in a fluid, and filter distilled alcohol before drinking.  It allows people to burn fires hotter than wood, enabling people to smelt iron or shape glass in a way that wood fires cannot. It can be added to soap for abrasion, crushed to make ink or paint or mixed with minerals to make gunpowder.

Perhaps the most surprising use, one that gained a burst of attention in recent years, involves trapping carbon from the atmosphere. Frequent readers of this blog might have already heard of this and can feel free to skip ahead a few paragraphs – but for the unfamiliar, I will recap the basics.

Farmers in Brazil have long known about the “black earth,” or terra preta, found over vast areas of the Amazon. In the last decade or two archaeologists have begun to realise that the terra preta was not a naturally occurring phenomenon, but had been cultivated over centuries, if not millennia. What’s more, they began to realise that much of the sparsely inhabited Amazon rainforest was once densely populated with humans, continually enriching the soil as they farmed.

Like many Stone Age societies, they burned land to clear it for farming or hunting, but unlike many others they turned some of the wood into charcoal and then worked it back into the soil, creating an unusually rich and fertile ground; according to a 2006 article in Nature, bio-char – the charcoal folded into the earth to make terra preta -- is three times richer in nitrogen and phosphorous than ordinary soil and 20 times richer in carbon.

This information might have remained a curiosity, part of the amazing new research in pre-Columbian natives, except for one thing: the same technique could work for us to offset carbon emissions. Burning plants may seem like a strange way to combat climate change, but merely charring wood into charcoal, rather than letting it burn away into ash, locks much of the wood’s carbon away in a stable form. 

According to researcher Bruno Glaser at the University of Bayreuth, Germany, a hectare of meter-deep terra preta can hold 250 tonnes of carbon, as opposed to 100 tonnes of carbon in unimproved soils. In addition, the bio-char itself increases soil fertility, which allows farmers to grow more plants, which allows more bio-char to be added to the soil. Johannes Lehman, author of Amazonian Dark Earths, claims that combining bio-char and bio-fuels could draw down 9.5 billion tones per year, equal to all our current fossil fuel emissions.

Whether or not such people are correct, the technique has drawn admirers like climate scientists James Lovelock and Tim Flannery. As I mentioned last week, climate change is hitting people around the world, yet we struggle to reduce our pollution even a little, and our goal of a zero-carbon world seems ever more hopeless. Bio-char, however, offers everyone a way to be, not just carbon-neutral, but carbon-negative, with almost no technology.   

What we don’t know, however, is whether such a technique would truly work for most people; what works in the Amazon rainforest does not apply everywhere. The redoubtable Albert Bates is experimenting with such techniques on his farm in Tennessee, and I have heard from other people doing the same. We would like to, but first we needed to learn how to make charcoal.

I tried three ways of making charcoal, with varying degrees of success. Since charcoal can be created when wood is inside a heated and sealed container, I put two metal buckets together, one upside-down and atop the other, and inside I placed one or more pieces of wood. The crack of space between the two buckets I plastered with clay, and around them I started a fire of some of our rotting lumber. (Never burn treated lumber for charcoal or even in a fireplace; it has chemicals that can be poisonous when inhaled, and I would not want to filter water through its charcoal.)

I tried to make small amounts of charcoal, one at a time, but it never worked. If the clay plaster held, the logs inside were merely signed, and if part of the plaster fell off – as happened more frequently – the fire caught inside until I was left with only a few small pieces of charcoal and a lot of ash.

For the second method I took a page from Waterford farmer and author John Seymour and dug a trench, lit a fire in it, tossed in some logs and covered it with corrugated iron sheeting. Then I packed the cracks tightly with clay and plants to seal in the oxygen, and uncovered it a few days later. This worked better, as I did get some charcoal out of it, but the amount was still tiny.

The best method, I found, was the one charcoal burners used from ancient times until just the last century. They made every sword and ploughshare possible, yet historians say they tended to be a reclusive lot, with their own jargon and secret tricks, spending much of their time sleeping rough in the woods like gypsies.

Trying to replicate their methods, I stacked logs in a triangular pattern and leaned more upright pieces of wood around them, until I had a small and dense ring of wood about a metre high. Then I filled the interior of the triangle with tinder and kindling – sawdust, mulch, twigs, anything that would light easily and create an intense heat that would burn the rest of the wood.

Then I covered the wood with recently-cleared weeds, spread clay over the weeds, and shovelled earth over those, until I had a mound open at the very top, with a “chimney” that looked down into the tinder-filled space between the logs. 

Next came the big moment – I lit a fire-starter and dropped it down the middle, and within moments had a raging fire inside the mound. I covered the top of the mound with strips of weeds and shovelled more earth on top – the weeds and roots served to block the entrance, so that I wasn’t simply shovelling loose earth into the hole and putting out the fire.

The result was a strangely smoking hill, and when it smoked too much when it cracked and too much oxygen got in. When a hole or crack formed, I plastered more mud and earth over that part – carefully, for the escaping steam can get quite hot – until the leak was stopped.

Two days later, I broke it open, and began fishing out the charcoal, and got about five kilos from an estimated 36 kilos of wood. Most text say the charcoal can be as much as 60 per cent of the wood by volume and 25 per cent by weight. I probably got less charcoal because I let it burn through the night; I had to spend part of the day building it and light it in the evening, as the constant threat of rain here meant I couldn’t leave it overnight. Charcoal-burners, though, were said to watch their mound for hours until the smoke turned from white to blue, indicating they were beginning to burn charcoal, before putting the fire out to maximise the amount of charcoal from a single burn.

With more careful measurements, amateur scientists around the world could try such techniques on some kind of fast-growing wood, like willow, and see if we could do with terra preta in temperate climates what Amazonian tribes did in the rainforest. On paper, it looks like it should work: willow can yield ten tonnes to the acre, the charcoal would retain a quarter of that mass of the wood, and should remain stable in the soil for decades while new tonnes are grown. All this, though, is theory, and we won’t know unless we experiment.
Even if we can’t offset emissions with terra preta, however, we can certainly use the charcoal for many other purposes. Once all industry, from metal to glass to gunpowder, came from this craft, and while I don’t believe we will return to such a medieval level any time soon, this is one craft I want to learn and pass down.

Top photo: One of the pieces we made.
Second photo: The remains of the bucket experiment. 
Third photo: The remains of the trench experiment.
Fourth photo: The log pile for the mound experiment. 
Fifth photo: The mound just after being lit. 
Sixth photo: The mound sealed on top and smoking. 
Seventh photo: The takings from the mound experiment, with a lemon for scale.
Eighth photo: My daughter helping me sort charcoal from the remains of the mound. 


Greg Yurash said...

Thanks for an interesting post Brian. It sounds like a key to efficient Charcoal production is knowing when to stop the process, or put the fire out. Are there known traditional ways to do this? Collapsing the pile? Dowsing with water? I imagine you don't want to just open the pile up while it is still hot as it may reignite violently. Any sugestions?

Brian Kaller said...


It is very important, and I'm told charcoal burners camped by their mounds keeping an eye on the smoke, stopping the burn when it changed colour. I read that they stopped it by covering it with more earth to entirely cut off the oxygen, although that seems like a tricky business, and I would probably hose it down first to make the earth stick.

Eric Hacker said...

Thank you for the excellent write up on biochar. I have been making charcoal for a couple of years now and have been meaning to post something similar, but writing is not easy for me.

My current technique produces more charcoal for probably less effort and certainly less clock time. I will try very hard to put together an in depth write up soon. Here is a brief version.

I use an outdoor fireplace (not a pit) and a water bath. The water bath is a plastic 5 gallon bucket filled with water. I recommend a metal bucket, as the plastic melts some.

I build up a good fire, and when some wood has burned to coals, I shovel it out and dump it into the water bath. I add fresh wood to keep the fire going. It is a bit of an art to balance.

I primarily burn softwood, mostly thick branches, to get a high production rate. If I am actively managing the fire I can produce 16 lbs an hour. Usually I am not that productive, but I'm also not spending the whole time in front of the fire.

There are a lot of little tricks that help out, which is why I need to write up more later. A water bath is fine for charcoal that is destined for the garden, as it doesn't have to be dry. It is also accessible to suburban dwellers who might not be able to use a retort or build a mound that takes days to burn.