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
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
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
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
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
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.