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. 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 us in a variety of ways, 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.   The redoubtable Albert Bates is gathering research on the merits of bio-char, and gave us a presentation on making it at the permaculture course I took in County Tipperary. Before I started his more sophisticated techniques, however, I needed to learn how to make charcoal, and some years ago I tried my hand at doing so.

I tried three ways of doing so, 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 lumber for charcoal or even in a fireplace; it has been treated with 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 used by the charcoal burners that worked Irish woodlands until just the last century. Before the discovery of smelting with fossil fuels, charcoal was needed to make metal from ore, and once the metal was broken or warped charcoal was needed again by the blacksmith. Any glass that existed came ultimately from their craft, as did gunpowder and any sword or ploughshare, yet historians say they tended to be independent and reclusive, with few surnames referring to their craft as with other crafts – wright, miller, tailor – and their own jargon and trade secrets.

Mimicking the historical accounts of the burners, 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.

Then I covered the logs with our recently-cleared weeds, plastered clay over them and shovelled on more earth.  When done I had a mound of earth open at the very top, and the top hole—the “chimney” — looked down into the hollow space between the logs, filled with tinder and kindling.

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 more plants, in this case strips of weeds with earth still attached to the roots, and lay them upside-down over the top. I shovelled earth onto the top of the mound – 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 I knew to look for a crack where oxygen was getting in.  When I found it, 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 enough to easily make water filters or distil spirits. As an aside, I had hoped that these experiments would be useful for killing weeds as well as making charcoal; after all, they would be in there with the burning wood for two days. No such luck; an amazing proportion of the weeds in there remained alive, green and uncooked, apparently protected by mud or moisture.

One thing I neglected to do, which must wait until the next round of experiments, was take careful measurements of exactly how much wood was put in vs. how much charcoal we got out. I do know that we burned 15 small logs of about two kilos each, and got about 5.5 kilos out of it, so an average of 18 per cent of the wood became charcoal. Most experts say that the charcoal should be 50 to 60 per cent of the original wood by volume and 25 per cent by weight, so that’s not far off – and of course the smallest charcoal pieces I just left in the ashes and burned clay in the centre, and scattered those over the recently-harvested and newly bare sections of our garden.

With more careful measurements, I or others could see if we could use these 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, and the wood does not need to be dried for a year or more before burning into charcoal as it would for firewood. The charcoal would retain as much as 25 per cent of the mass of the wood, in theory, and should remain stable in the soil for decades.

There’s a lot more to learn about this, and I’ve barely scratched the surface. In the meantime, though, we have fun experimenting, and now I want to learn to use the charcoal to filter toxins out of drinking water. More on that in future posts.