1.When I read Kiko Denzer’s book ‘Build Your Own Earth Oven’,
(Handprint Press, USA, 2000) I was immediately hooked; I knew it was my next project. The book mainly focuses on the use of cob, which is not earth as we know it, but transferring the principles is easy enough. Manipulating any sort of soil is fun, whether it is sand, clay, chalk, mud, or humus. It is a fundamental experience packed with learning and creativity. Hence references are made to earth since time immemorial because, quite frankly, it is the building block of life used practically by every species on the planet.
Rather than go in-depth about the constituent parts of the substance of earth, it suffices to say that whether soil is encountered under or above water it generally is
a mixture of the geological substrate from which it grew, and biological debris accreted over thousands and millions of years. There is some science to it, and the quicker learnt the better, but there is no harm just playing with soil to get a feel for larger projects.
Exampled here are a some of ovens I have built, and each one better than the last. My very first oven above, you may have to screw your eyes a little, was a guerrilla garden project on the railway line at the back of my garden. Countless officials walked by without seeing it. The idea arose when the major gas works at the front of the house unearthed huge amounts of pure London clay subsoil. I bent over a wire mesh in the shape of an arch and tilted at one end, put in a wire mesh shelf, threw lots of clay on the outside, and bunged loads
of dry wood on the inside. I then invited my anarchist friends around for a party the following day, in the meantime lighting the wood so that the clay would cure. I have that picture somewhere, an image of Mount Sinai for
British Rail to get their teeth into. It was quick, easy, and fun, and undoubtedly a culinary failure — smoky dough. But I learnt, and the oven became an animal’s den. Now look at the various images scattered about this page and you will see what children and adults alike can do also. The owl became a donation pot at Hope Festival. Hanuman the monkey god became an anarchist icon of resilience and bonding at the Spike. The dome became something of a marriage gift at Sigité in Catalonia. And the 'reluctant boar' at Solteriologic Garden in Honor Oak Park had more than just a face lift, it also smoked a pipe and inspired a song.
This article was taken from the August 2018 and 2019 May edition of our newsletter and you will find more images to amuse you there. Firstly, the oven must be situated downwind, as you would an eco-loo, but generally in a sheltered spot so as not to cause excessive cooling down. In wet areas it will require a roof of sorts, and be located next to a dry wood stack. The way an oven works in general is that the heat is stored in the structure — the walls, ceiling and floor — and this way it is more controllable as now the baking process relies on convection, radiation and conduction. Depending on what materials you have at hand the floor should be constructed of fire bricks and a well-fitted door of thick wood. Again, one uses what materials are to hand. More on this later. Let’s consider the first stage:
3. THE FOUNDATIONS.
Damp will travel through a continuous stratum, and will eventually damage the cob or adobe as would rain. Likewise moisture originating in the structure needs to evaporate out or travel down into a drainage area. Kiko also talks about a frost line, all factors contributing to how well an oven fires up. Remember, only pizzas are cooked on an open flame. Bread is proofed and then baked on heat alone. In the photos showing yours truly I first consolidate the clay soil, put an insulating layer of soft sand, and then build up the sides with salvaged block paving. I infill with old rubble consolidating as I go along, and then finish with more soft sand, about 4in. deep, in order that I can lay a floor. In Sigité we built a concrete floor which afterward was laid on top of a wooden table. Ideally you would build the oven in situ, but it was a tight corner with lots of hands available. It’s not recommended — we could barely lift the oven and concrete together into the corner, let alone test the wooden frame for strength. A raised structure also conduces to ease of use — one frequently checks the flame or heat before baking commences.
4. THE FLOOR.The floor should be level. In the above example I am using fire
bricks salvaged from old night-storage heaters which used cheaper electric at
night to heat up the bricks for use in the day. They get extremely hot. Use a
spirit level to ensure they are flush and even with the bricks next to them.
This is why soft sand is used as once tampered in place the bricks will not move when the walls are in situ. In actual use they are swept and knocked with wood and tools quite frequently. For a 27in. baking floor the oven
should be 48in. diameter in total. Bear this in mind when building the foundations. Likewise there should be a tongue where the door will sit. The interior of the oven is a perfect dome, so we are only dealing with circles here.
5. THE DOME.The dome sits perfectly over the floor. The ratio of diameter to height is 60:75. With a bit of foresight the height of the door is designed at 63% the height of the interior ceiling, not the exterior. This will allow the smoke to naturally eek its way out through the top of the door. You only have to be approximate here. In the monkey god oven I built a chimney with a grill over the top so that any escaping heat could be utilised in heating up a pan. A good option here is water which can then be stored in a flask. In the boar oven a flue came out of its snout.
A dome has straight walls, which maximises the amount of loaves one can bake. It is the strongest structure. For a mould I use soft sand again, which is later scooped out through the door. In Sitgité and the Spike project I used a perfectly-formed old basket which was later burnt out. A sand form must be damp in order to hold its shape. It is smoothed off and covered with wet newspaper which facilitates removing the sand later. In the below example I place blocks on the outside since once excavated the interior ceiling will have more surface area to radiate heat from.
6. MIXING THE COB.Cob needs to be 75-
85% sand, and 15-25% clay. You can do some test runs before you commence by excavating ready-made mixture from the garden. All particles of the soft sand must be covered, and this can either be a lot of fun with kids and volunteers, or very monotonous, especially if the clay is already rock-hard. I designed an innovative way of using a cement mixer to put in heavy dry lumps with water added that when spun, created a clay slip. This was poured off and left to settle. Eventually more water segregates and you
can then pour this excess off too. Then add it directly to sharp or beach sand.
The first layer is the insulating layer, about 3 in. thick, so it is important not to add any other material to this mix.
When applying the mixture do not pat it on as this weakens it. Instead press the cob into itself until the whole dome is covered. See the example above. Once this is achieved then the rest of the layers go on relatively fast.
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6. LAYERING.This is the sculpturing stage. At this point various designs can be incorporated. To strengthen the mixture a bulking agent is used. Normally this is hay or straw, whichever is cheaper. Anything fibrous actually can be used, from pine needles to horse hair. Slap on this layer with lots of fun, before the first layer dries out. At no point should the walls or ceiling be compromised — put more rather than less to between 3 and 6 in. thick. Make sure it is nice and soggy when applying it. Likewise it should sit pretty under gravity. In my original boar oven the head eventually cracked and I had to remould it. This was simply a case of rewetting it and applying it to a damp facade. It worked even better as the mixture was worked even more producing a super cob.
The final layer is the finish, and like the first, should not have any added material unless it is finely chopped. Where a sharp sand or a beach sand was used during the sculpturing stage, here we are creating a fine texture and revert back to softer mason’s sand. Carving knives and tampers are allowed. But before the oven is left to dry naturally the door will need to be cut out if it hasn’t been already at the beginning of the second layer. The sand mould will also need to be removed.
7. THE DOOR.You can think about this way in advance and have a door specially built and moulded perfectly into your design during layering. With a sharp knife a doorway can also
be cut out of a sculpture. A door gets extremely hot, but one salvaged from a wood burner with a viewing window works wonders. The other option is to use a large rock that won’t splinter, or cut a door from wood, and preferably a deciduous hard wood like oak that will burn slowly. It should taper so as to fit like a plug. Bear in mind that you will need to pull it out and push it in with ease, so a handle of sorts is required. This can be simple thumb and forefinger hole grips, a knob, or a protruding bolt. The height of the door needs to be 63% of the height of the ceiling, according to research, in order to draw the smoke away through the top.
8. THE FLUE.This needs to be thought out also in advance. Look at the examples above. I use the flue here to draw smoke into a hot box, which has to be well-made. Here you can proof your loaves. With Hanuman the monkey god I built a grill as a chimney hole. This loses vital heat if not closed during redundancy, but generally it boiled or heated a liquid food of sorts.
9. THE DECORATION.Kids just love this stage, and the more they get involved the more likely they are to want to learn how to bake. Try and use ecofriendly materials but more than likely anything that is not perishable. The outside of the oven can develop hot spots accorded to how well the material has been mixed and the different levels of thickness.
10. BAKING.This is an art in itself and will probably require another article to explain the pros and cons (any contributors?) The thing to remember is this:
We do not cook on an open fire (only pizza) but after firing the oven and letting it burn hot we remove the ash and any other material left over.
Cooking happens through convection, conduction, and radiation.
You will require wet rags to create steam.
A selection of implements are also needed for handling the bread.
The initial burn should make 2 bakes and the bread needs to be spaced enough for expansion. It can be re-fired afterward.
Continue using the heat as it drops in temperature for other foods