Rocket stove

Rocket stove kits

No, I have not entered the world of rocket engineering.
A rocket stove is a small but powerful cooker which runs on small sticks. Astonishingly it can reach white hot temperatures, despite its simplicity. And because it burns so hot it is incredibly efficient, burning everything in the wood which is combustible and leaving very little waste indeed.

The rocket stove was invented back in the 1980s and looks deceptively simple. It's basically an elbow shaped tube, which can be fairly simply knocked up out of tin cans, which sits in a bigger can full of insulation. The ratios though are important. Without boring you with the measurements and engineering theory, the airflow and height of the flue need to be just right to ensure that just enough oxygen is supplied to the fire, not too much and not too little.

The rocket stove has big implications for developing countries. Wood is a precious and declining resource across much of the world, so the more efficiently it can be used the better. Every move towards making wood sustainable is a move in the right direction. Cut the amount of wood needed for fuel, cut the time and energy used in collecting it. The fact that only small sticks are required is a bonus too.
The rocket stove is now used extensively in disaster zones. It is cheap, easy to produce, portable and green. A triumph of alternative technology. The central flue, the most important part, can be cheaply manufactured from ceramic, which is the best material.

So when the chance came up to make our own rocket stove at the Green Backyard in Peterborough, for free, well we jumped at it. I am not pretending that we will replace our gas cooker, electric oven, microwave and kettle all with a rocket stove, but I find the Green Backyard an inspiring place to go and if I could come back with my own rocket stove, all the better!




We arrived to find out that we didn't need to improvise the whole thing out of baked bean tins, which need regularly replacing as they disintegrate under the phenomenal heat produced within. For we were getting specially made stainless steel elbows. Posh ones! - which should hopefully last several years.
Our tutor for the day, Bob, clearly knew more than we needed to know, but he kept himself to giving us a very interesting history of the rocket stove and just enough technical information to be of interest.
I'd never used metal nibblers before, but I want some now. They made short work of cutting holes in the outer can to fit the flue. A few adjustments and we were ready to fill the void with vermiculite. The three bolts coming out of the top are for the pot, kettle or griddle to sit on. They are adjustable but basically need to sit about an inch above the top of the flue.

Anyway, enough of the technicalities.


Renee from the Green Back Yard
chats to 2 of the course participants
as they head home, rocket stoves in hand.

Enough to say that we had a lovely day and came away with not one but two rocket stoves. I feel some outdoor cooking coming along.

Back to The Green Backyard for Straw Bale Building

A few weeks ago we spent a very enjoyable weekend at The Green Backyard in Peterborough, building a cob oven.
This proved to be quite a task which continued into a second weekend. Unfortunately Sue and I could only make the Saturday, so we missed the final day putting the finishing touches to the outside of the oven. This is done with a looser mix of cob, mixed with finely cut straw.

Anyway, I'll cut to the chase.
Here's the rather amazing finished oven.

When Sue and I learned that Alan Eley, who usually works at Hill Holt Wood, was coming back for a third weekend to help build a straw bale wall, we were keen to return. Again, we could only make one day of the weekend, but this time we missed the right day! For a hardworking band of volunteers and course participants spent Saturday laying the foundations. These were basic, just a treated wooden frame into which earth was compacted... and more earth... and more earth. It's surprising how much earth you can fit into a hole! Again, it was subsoil which was required. You don't really want to be building any sort of wall on top of seed-filled topsoil.

So when we arrived here's what we found.

We also found Alan mixing up a wheelbarrow of strange white powder and beseeching us to stay upwind of the dust which was flying everywhere. It has to be said he looked like a wizard!













This was hydrated lime, being mixed with water to form a putty. Later this would in turn be mixed 1:3 with soft sand to make the lime render. Using this is preferable to cement as it allows the walls to breathe. It's also not quite so energy inefficient to produce as is cement.

Membrane between foundations and bales



The hazel stakes which secure the bales in place

We then proceeded to carefully place straw bales onto these foundations, secured with hazel stakes. It's just like giant lego really.


However, if you've ever built a wall (even lego) you'll know that you need half bricks at the end. Fitting straw bales into an existing frame requires various fractions of straw bale, but it's not as simple as just cutting them. For as soon as you cut through the baling twine which binds them so tightly, they quickly expand out into the available space.


Alan demonstrates
splitting a bale

Therefore some technical skill is required. Alan explained how to thread a straw bale needle through to split the bale into two and secure it tightly. At first this seemed like a baffling art which only the initiated could succeed at.














However, the key was to actually have a go at it ourselves. With a bit of a lead and some common sense, it all came together.





Having mastered all the requisite skills, the wall was erected in no time.

Before it could be rendered,
a quick hair cut was in order.

The final bale goes into place



After sharing a communal pot of soup, we returned to begin the rendering. There were plenty of experts on hand, but it was really a case of just getting it to stick however you could.

Sue and I had to leave before the first coat was completely on. Altogether the wall will get three coats of render. We'll return to see the finished product when the cob oven gets fired up.

Meanwhile I am working on plans for a straw bale pig house, possible even with a living roof. I was impressed with the cob, but the straw bale walls blew me away. Maybe that's the wrong phrase for a house made of straw!

Cob oven. Part Two.

Day 2 of the cob building weekend.

All twelve members of the Veg Group turned up last night and our gathering went on until quite late into the evening. But I was up before the sun as temperatures again sunk below freezing overnight and there was more rotavating to be done.

I managed to get an hour done before the soil started clogging in the tines of the rotavator.
At least there'll be plenty of clay in the soil when I decide to make my own cob structures on the smallholding.

We turned up at the Green Backyard only to be involved straight away in a hunt for a lost wedding ring. The guy had taken his ring off the day before and put it in his pocket. You can guess the rest. We didn't really know him well enough to joke, but mention was made that maybe it was embedded into the walls of the oven! Anyway, we explained about washing machine filters and sent him home to explore. We set to work mixing more cob. Today the cob was to contain straw, which is more normal. This gives it strength but it meant that we had to stomp and twist and stamp and dance even more to incorporate it into the cob.

.



The outer layer of cob on the oven was made thick, very thick. On the plus side, the oven will be well insulated and retain its heat. On the other hand it meant that we had a lot of cob to make. I danced more today than I have in the past twenty years!

After yesterday's painful lesson, I wore gloves today to pack the cob onto the oven. It took five barrowloads of cob in the end and we didn't get it finished until gone 3 in the afternoon. A communal meal of soup and bread was much needed and much appreciated.

 

 

The oven still has one more layer to go, but this will have to wait a while now. It will be a fine layer of cob, containing cut straw, more as a decorative layer than anything else.

 

Oh. The wedding ring. Well I'm happy to say it was retrieved from the rubber seal of the washing machine, unscathed.

Dancing the cob stomp. Building a cob oven. Part One.

The Green Backyard
An amazing project which brings together community, art, sustainability and self-sufficiency

This past weekend we were booked onto a cob building course at the Green Backyard in Peterborough.
Before I talk about cob, a word or two about the Green Backyard. I have been there a couple of times before and really could not imagine a more welcoming place. It is a community garden right in the middle of Peterborough run along green principles, recycling, sustainability, creativity etc...
There always seem to be people popping in and something going on.

At the moment this precious project is unfortunately under threat. After 6 years of hard work, in which time their achievements have been amazing, there is a threat from the council to sell off the land to developers. Peterborough calls itself an Environmental City, so it is difficult to see how this can square up.

If you'd like to find out more, or support the GBY in their bid to buy the plot of land they sit on, please visit their website here.


So, back to the cob course. It was to be a busy weekend, starting with Blokes Baking (Shortbread and Doughnuts) on Friday evening. Another sharp frost predicted for Saturday morning meant that I was keen to get out for an hour or so with the rotavator. This is the only time I can easily turn the soil surface at this time of year.

Then we were booked on the cob building weekend from 11 a.m. followed by a dozen people coming round for the Smallholders Veg Growers Group which I was hosting in the evening.

We turned up to find that about 20 people were already there, all warmly wrapped up ready for a hard day's work (and socialising and learning).
Alan was our very expert leader for the weekend. He is based in Sherwood Forest and has spent many years working on sustainable building, green woodworking, woodland management, blacksmithing, charcoal kilning... basically, all the stuff I wish I'd got into many years ago.

Alan is well into double figures in building cob ovens. After a little background information, he led us outside where we all started demolishing an old strawberry patch. The idea was to clear away the topsoil in order to get at the clay/silt/sandy mix that is the subsoil.
This was to be our raw material for making cob.

Now that I think about it there are many meanings of the word cob. Loaves, webs, swans, nuts, corn, a sulk... All of these and more. But the sort of cob I'm talking about is the building material, made by mixing clay soil, sand and straw. It is much like the adobe walls I have seen in many tropical countries.

Today we were to be building a cob oven, but I have my mind set on using this building method to construct a new pig house and maybe as low dividing walls to divide up the chicken pen.

It's amazing how quickly 20 people can dig a big hole in the ground. It turns out this was the easy bit! Fortunately the subsoil had just the right mix of clay and sand, so we could work with what we dug, unadulterated. For the first layer of the oven there would be no straw mixed in. These fires can easily reach several hundred degrees centigrade, so the straw would not last too long and would just burn away to leave holes in the cob which would weaken it.

So we barrowed the subsoil onto a large tarpaulin, mixed in a little water and started stomping the cob dance! It's quite a simple process really, just stomping and jumping and twisting on the soil until it mixes. Roll it up and over in the tarpaulin and repeat...and repeat...and repeat...

Eventually you end up with your building material. After carefully shaping an igloo mould out of sand, we started the task of building up the first layer of the clay oven.


Alan explains to us
where the oven will sit

We gave the sand igloo
a coat of newspaper

For the future, it was maybe not the best idea to choose such a cold day for this. Shaping 'bricks' out of wet clay with your bare hands sucks the heat out of them very efficiently. My palms were caked in mud but the back of my hands was blue!

At that very moment the call went out that pizzas were now ready to be made on the rocket stove. I dipped my hands into a water butt, breaking the ice as I did so. My hands STUNG! They really STUNG! But the clay took some washing off and was I glad to find a slightly less cold bowl of water waiting over in the outdoor cooking area.
Anything hot would have tasted good right there and then, but the pizza which Sue had knocked up was especially delicious.

I retired inside for a while and took the chance to have a chat with a few of the Green Backyard regulars and to pick Alan's brains a little more. When I finally emerged, the rest of the group were just putting the finishing touches to the first clay layer.

The completed first layer,
artistically scored as a key for the next layer

It was time to bid our farewells for the day. Sue and I had really enjoyed ourselves. Hard work, fresh air, a bunch of like-minded people and learning a new skill had made for a very, very good day.

By the time we got home and sorted out the chickens, it was five o'clock. I was pleasantly shattered and had one hour to veg out before the Veg Group started arriving. More on that later.