Biochar in a bathtub

We became big fans of biochar years ago when we went to this workshop, and we’ve also written about it here. We even held our own workshop on the farm to learn how to make biochar vessels out of 44 gallon drums.

The bathtub biochar method in action
The bathtub biochar method in action

Despite all of that, we’ve never made it ourselves, because it seems like a lot of effort to produce a small amount of biochar.

That may all have changed! We were lucky enough to go to a small, privately run workshop recently that introduced us to a much simpler system for making biochar in a bathtub.

It was hosted for us by Grow Great Fruit members Win Westerhoff (below left) and Clare Claydon, and the workshop was run by Jim Sansom (below right), who started by talking us through the main steps of the method.

Jim Sansom explaining the bathtub method
Jim Sansom explaining the bathtub method

The bathtub method is based on the ‘Kontiki Cone Kiln’, which is a large (about 1.6 m across) kiln for making biochar, but is a much simplified version.

One of the challenges with this method is that there’s no hard and fast rules about when to stop the burn. “Judge when to stop” was Jim’s advice, “when a stick has lost its identity as a stick”.

Another thing to know before you start is don’t use a cast iron bathtub, or it may crack when you light a fire in it. And obviously don’t use a modern plastic or fibreglass bath either – you need to find an old steel bath at your local tip shop.

At the workshop we went through the actual process of making biochar under the expert tutelage of Jim, Clare and Win — and it wa:s surprisingly fast and easy.

Here’s how it works:

Step 1 – Fill the bottom of the bathtub with light, dry fuel.

Starting the fire in the bottom of the bath with light, dry material (and a bit of metho if needed!)
Starting the fire in the bottom of the bath with light, dry material (and a bit of metho if needed!)

Step 2 – Light it on fire. (Pro tip: use a bit of metho if needed). Aim to get a good strong fire going along the whole length of the bathtub.

Step 3 – Once it’s burning, pile up with bigger stuff, but make sure it’s burning the whole length underneath. (Pro tip: this is much easier on a still day than a windy one.)

Step 4 – Stack the bath until it’s 3/4 full. If the fire is burning well, it shouldn’t be too smoky. It’s fine to use bigger wood in the bathtub as long as it’s all roughly the same size so it burns at about the same rate.

Hugh keeping the fire going evenly along the whole bathtub with a rake
Hugh keeping the fire going evenly along the whole bathtub with a rake

Step 5 – Use a metal rake to pack the wood down hard, and to keep the fire burning evenly and everything burning at the same rate. The gases will be pushed out of the wood and are then burned in the bathtub, which prevents oxygen getting down into the fire. If too much oxygen gets into the fire the wood will just burn away to ash. New research has shown that if the biochar is made at a lower temperature you end up with biochar that has a higher level of organic matter.

Hosing the bath to put the fire out when you judge it's finished
Hosing the bath to put the fire out when you judge it’s finished

Step 6 – When you judge that most of the material is blackened to char, quench the whole thing with a hose.

Hugh and Clare admiring the finished product
Hugh and Clare admiring the finished product

It’s OK for sand, dirt, clay and mud to be on the wood and in the cooking process, and in fact this may be beneficial by creating a more highly mineralised final product.

How do you apply the finished product? We’ve covered this before in other blogs, but here’s the methods that Clare and Win use:

  • dig a trench, tip in a barrow load of biochar, add compost and a scattering of blood and bone;
  • broadacre – spread with a fertiliser spreader and then go over it with harrows.

The point is finding some way to incorporate biochar into your soil where it will work its magic.

Of course it’s not magic, it’s science! Biochar has remarkable effects in soil by providing both a stable form of soil carbon, and by providing amazing (and extensive) habitat for soil microbes. If you still need convincing of just what an awesome impact this can have on your soil, you may want to take our Soil Biology and the Soil Food Web short course.

4 thoughts on “Biochar in a bathtub”

  1. This does look easier but the tricky bit of not turning to ash is still daunting. Why not just collect coals from your fire… we have a kitchen slow combustion stove as well as a lounge room Ned Kelly heater. Both these produce quite a bit of coal over the winter months which I collect. Maybe not as good as the Biochar but I’m sure it would still have good qualities and I don’t have to do anything except collect it. Btw you can also use the ash to make laundry liquid! As well as use some (sparingly) on the garden. Win win!

    1. Hi Dora – yes, excellent idea. Coal collected from the fire is still an awesome product to put in the garden, and you can pre-charge it to increase its effectiveness with worm juice or compost tea just the same as biochar (crushing it first might help with this as well). Interesting about using ash to make laundry liquid – do you have the recipe? (And do you mind sharing it?)

  2. Interesting use of old bathtubs, they have many more uses as well! I make charcoal in the cheapest possible version of a “cone kiln”: a cone-shaped depression in the ground. You build a fire and keep adding wood until you have a strongly burning pile; you then let it burn until most of it is glowing red “coals”, then douse with water to put it out. It is imperative that you open up the mass and hose water into the middle to cool it down or it can spontaneously re-ignite and burn way to ash after you have walked away. Having the fire in a cone shaped hole reduces access of air to the bottom of the pile and thus you lose less of it to combustion than on flat ground, while the earth acts as a kind of insulator to keep the retained and radiant heat acting on the partly burnt wood down there, driving off the combustible volatiles. I also crush the finished charcoal by stomping on it over a coarse metal grill to reduce the size to less than 3cm, then grind it through an old electric mulcher to reduce most of it to less than 5mm. Having it still moist (not wet) when grinding it minimises black dust; if it is dusty that fine carbon can get into the electrics and short out your mulcher. The end product is similar to the expensive horticultural charcoal you can buy in Bunnings. Any residual ash in the charcoal will just add valuable minerals to the soil where you use the charcoal, and for preference rather than adding it directly to soil I include around 5% to 10% of the ground charcoal in hot compost to “charge” it with nutrients and microbes. The other minor ingredients I put in the hot compost are lignite (brown coal) and stone dust, both of which are also very cheap and long lasting in the soil. You do have to have enough land to be able to devote an area to burning wood, and a supply of wood, so not so practical for suburban backyards.

    1. Hi Stuart, thanks for this – it’s fantastic to hear about other people’s systems for making biochar, and yours certainly sounds well thought out and tried and tested. We’ll have to give it a whirl. Thanks for sharing.

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