Bathtub Biochar

Dear Dairy

This post will answer the question; “How many pictures of charcoal can you fit in one blog post?”

Have you ever had a bath? I’m not boasting when I say that I have. If you’re into biochar, and I hope you are, please allow me to tell you of the benefits of making biochar in a bath.

I’ve tried a few different ways of making biochar over the last decade. I started making retorts from drums and used concrete blocks for an oven, I laboriously cut up old pallets and filled the drum. The advantage of the pallets was; they were dry and free. The disadvantage was the odd shapes of broken pallets needed a bit of work to fit efficiently into the retort.

There are a lot of nails in a pallet. Sooooo many nails. These had to be picked out by hand.

My crushing method was to put the char in a plastic bucket and bash it with a post. This really only had the effect of smashing the bottom out of the bucket. I moved to rolling over the char with a fully laden wheel barrow, but this didn’t crush it much. Finally, I drove over it with the car, then I remembered all the nails that I might have missed and stopped.

These days I make biochar in the bath tub we pulled out when doing up the bathroom. It is pressed steel and was a lovely avocado colour. The enamel has long since fallen off and I noticed the first rust hole today. It’s shape is a bit twisted as I picked it up with the tractor and crushed one side.

Despite all this use and misuse, it still retains the critical features of being a very cheap metal trough shape that can accept 1 meter long pieces of wood and accumulate a reasonable (for my needs) volume of char in a short time.

The tub shape is vitally important. We are trying to char the biomass (wood) with 5 sides of the rectangular prism closed and one open. This way the minimum amount of oxygen gets to the fire, making mostly charcoal and not much ash. It doesn’t have the steeply sloped sides of a Kon Tiki but the ‘flame cap’ still seems to work.

The long shape, rather than square or round like a Kon-tiki kiln, means I don’t have to process the wood very much from my source. I mostly make biochar from sawmill waste and pruned branches. Both can be cut into 1 meter lengths with pretty minimal fuss.

I’m really trying to handle each piece of wood as few times as possible. I’m not making biochar to teach or demonstrate, I’m making as much as I can as quickly as possible with as little work as possible.

And…trying to use recycled equipment to do so.

Starting a fire in the bottom of the bath with twigs is easy. Once embers are forming I can rake it across the bottom and start adding larger wood.

Many times I’ve run out of dry wood and tried to use wet wood. A huge amount of the fire energy goes into drying the wood before it begins to char, and in my experience, that means less char at the end. Now I just use dry wood. Its lighter to carry to the fire and you get more biochar for lees effort.

For a while I used the pit method to make biochar, but I found the amount of material I needed to do the burn was huge. It was a big pit. The amount of wood got bigger every time I dug out the pit and it got a little larger. I got tempted to use huge chunks of wood but they took hours to char, while everything around them burnt to ash.

With a bathtub it’s always the same size and I can pretty much fill it with embers in 6 hours, depending on how dry the wood is.

Time efficiency is important to me. I’ve learned to take the bath to the wood pile, something that was much harder with a pit burn. Even a few meters closer saves me a lot of walking.

Ideally I have another job that needs doing nearby. While the fire burns I can put my chickens in alphabetical order or teach the cows to moo in ‘G#’. Every 20 minutes I load more wood on the fire.  In 6 (or so) short hours, it’s time to drown the fire.

Safety is important, unless you have never been injured, then you might be pretty relaxed about it. Your time will come. Our climate is pretty wet, so fire risk is low in autumn, winter and spring. I could put the bath up on bricks to stop it scorching the grass but I don’t bother. I try to burn on days that aren’t too windy. I never light near trees or anything flammable (vehicles, fire wood stacks, children etc.) Sometimes I remember to wear gloves and I always have a shovel ready to lift things back into the bath if they fall out or throw earth on any problems.

Ideally the bath is where I can get a hose to it. It’s difficult, but not impossible to pick up the bath while it’s still burning and take it to water. Poking a hose in one corner of the bath and filling it from the bottom is much safer and easier. The next day it might have drained (it does have a plug hole) or it might not. If the plug got filled with mud or ash or something, the shoveling is a bit harder, but not too bad.

If there were absolutely NO nails in the original biomass, crushing can be achieved by feeding the charcoal to cattle or pigs. Sometimes a little sweetner is needed (molasses) but pigs will certainly eat charcoal without this. When it comes out the other end, its crushed and inoculated for you and the cattle will even spread it around. Pigs always use the same spot as a toilet so there is a bit of work in spreading it, but since you were cleaning them out anyway, it’s no additional work.

If the biomass we full of nails, I dig a deep hole and bury it, if not, I add the char to compost or feed it to animals or put it in the chicken pen.

So, if you can get an old bath why not avoid costly custom made biochar equipment and have a go at making biochar in it? If you time it right, you can have a bbq over the embers.