In his latest scheme, Mr Marshall is casting back to more ancient technology still, but his biochar retorts are highly modern in their aim.

They will pryrolise a tonne of wood and other organic material at a time under a controlled temperature 400-500 C to produce a skeleton of pure carbon - "an apartment block for microbes, basically", Mr Marshall said.

"I have fed it all sorts of things into it, from eucalyptus coppice, offcuts from sawmill, bamboo, roadkill," he said, referring to a kangaroo carcass that he turned into "high spec blood and bone".

"If you biochar a kangaroo, its skull comes out all beautiful and opalescent and full of phosphates, all sheeny and shiny and beautiful," he said.

"I'm putting coppiced logs through the retort. I take them out, put them in the garden, jump up and down on them, and they break up into smaller pieces."

Mr Marshall's father made charcoal the old-fashioned way, producing it for use as a poison antidote, But Mr Marshall said the way his father did it was not environmentally sound, producing pure charcoal but at the expense of pollution.

His biochar retorts - he's demonstrating three at a public launch in Braidwood on April 22 – not only avoid particulates in the air, but retain 30 to 40 per cent of the original weight of the wood as intractable carbon, compared with just 10 to 15 per cent in old-fashioned kilns.

Mr Marshall is known for the black truffles he grows. A forester and farmer, he runs the military and outdoor equipment firm Crossfire. He is also a persistent critic of growing practices at the ACT arboretum which he has dismissed as "the most expensive firewood heap in history", and he has set about planting his own. Mr Marshall says he is planting 117 species of rare and endangered oaks in his arboretum, and he comments that the only place the National Arboretum's two species of oak are endangered is at the arboretum itself.

The need to improve his arboretum soil, degraded by timber growing, then wheat, then grazing, inspired his three years of experiments with producing biochar. He has now linked up with Exeter retort maker Robin Rawle and Sydney deck maker Steve Hardiman to make and sell retorts, which he is calling the "Carboniser", and which will sell for $30,000 to $35,000. Mr Marshall imported one of Mr Rawle's retorts and has "Australianised" the design, strengthening it for easier transport over rough roads and upgrading it to cope with the hot-burning Australian hardwoods.

Mr Marshall said while other biochar-makers on the market were designed for either very small amounts or hundreds of tonnes, his were built big enough to cope with one tonne, and small enough to be transportable.

He envisages it as useful for bushfire brigades clearing fuel loads, councils, farmers and people who want to make and sell biochar onsite.

"We want to see these things on the farm, people using them to turn eucalyptus sticks into biochar to reduce fuel loads, in municipal dumps to deal with bones and heavy organic matter," he said.

"The idea of biochar is that you've created intractable carbon which means the carbon structure is resistant to further decay. If tree falls in the forest and rots, most of the carbon will eventually go back into the atmosphere as carbon dioxide, but if you make biochar out of the same piece of wood, when you put that in the soil it will last hundreds if not thousands of years … We're engaged in carbon capture."

Mr Marshall has made different kinds of carbon, including binchotan white charcoal, a favourite of chefs and the health industry. But he says most binchotan is made in kilns, highly polluting and also using mangroves in southeast Asia, which is environmentally destructive.

Mr Marshall will have three retorts on display in Braidwood on Saturday. He has cooked pizza on top, but they'll be running too hot on bamboo on the weekend - so the pizza is being made in a gas oven by the Italian backpacker farmhands. Renewable energy and biochar expert at the University of NSW Prof Stephen Joseph, and New Zealand biochar expert Don Graves will speak.