Under the Choko Tree By Nevin Sweeney

Making a Cob Oven

My Backyard one

And on the Farm

Introduction

I have made two cob ovens in my time, the first one didn’t really work very well, not because of the process but rather my design. The space I had to fit it into at the time was rectangular rather than square so my choice was either to make it exceptionally small or to make it more like an ellipse rather than round. I decided to build it in the longer ellipse shape, and that turned out to be the wrong decision!

The oven never did draw very well, even after I added a chimney. It did get hot and we mucked around with it but it needed an insulating layer it never got and after sitting around and looking more and more dilapidated the cob part was broken up last year. I am currently working on a few different ideas because I still want a homemade wood oven in the backyard. I did make another one for a local farm with a group of volunteers and it was round, and worked well, but again due to the vagaries of chance, the farm has a new manager and that oven too has been broken up.

 

While my track record with ovens still in existence has not been good, I have had an opportunity to learn the process of actually making them pretty well. This is what I have

Making a Cob Oven

Contrary to popular belief, building with cob has nothing to do with corn, it is making up a mix of clay and straw and forming it into hand sized lumps called “cobs”, so the name pretty much becomes self explanatory. So I suppose the next question you may ask is, “why bother?” Wood fired pizza or bread ovens have become a popular back yard accessory, they produce great tasting food and make a great centrepiece for back yard living, but they also use a readily available (even in suburbia) and sustainable fuel that costs virtually nothing. So they must be a great thing to have. You can pick up a DIY oven kit for $1500 (ouch!) or even get one built on your site for a little over $3000 (ouuuuuuch!!), so they do have drawbacks.

It is possible, however, to build your own from scratch for very little cash and you can lure family members into helping with the promise of the previously mentioned wood fire cooked bread, pizza and roasts etc. and rest of this article will show how you can do it.

This is the process that I used -

1. Select your site

2. Gather your raw materials – sand, clay, straw, bricks and material to make the base out of.

3. Set up the base

4. Install fire bricks

5. Set up the inner sand form

6. Make the oven itself

7. Decorate the oven


1. Site Selection – You may have some choice in your site or if your place is like ours, there is not a lot of room so you fit it in where you can. In the best of all possible worlds your site should be –

  • Flat,
  • Close to where you are going to prepare the food for cooking,
  • Sheltered,
  • Located so that smoke will not bug your neighbours, your family or (trust me here) smell up your wet washing,
  • Located so that the oven opening faces away from the prevailing wind, and
  • Be free of fire hazards

Let’s face it, no site is going to be perfect, but tick as many of the above boxes as you can manage.

2. Grabbing the Raws - There are a number of things you will need to get together to make your oven and it is much better if you can have them all ready to go before you start. First off you will need something to make a base for the oven out of such as stone, concrete blocks, wood, bricks..... I think you get the picture! You want the oven to be high enough up to be easy to work with so waist height for the cook would be ideal (say 900mm to 1000mm). Clay is a no-brainer, you may be able to dig it from your own property but failing that see if you can find someone willing to let you dig out a few garbage bins (quite a few probably) of clay from around their dam or any other open section of the property.

I had a connection with a clay tile and a brick making company so I was able to put the bite on them for some clay. The brick clay was very good but the tile clay was more like pottery clay, very pure with not enough sand. It shrank a lot during drying so I had to add a lot of sand but it still shrank quite a bit. The second oven was made from clay dug from a dam on the property hence my earlier comments.

You could try talking to any local brick or clay tile manufacturers to see if you can get access to their quarries for a bit or as a last resort talk to your local landscape supplies place, that is likely to be the most expensive option. You could possibly dig some out of any cuttings at the side of the road, but some of these government types may take a dim view of this practice.

Brickies sand and straw can be bought from hardware and gardening places or rural suppliers. You will also need bricks to form the floor of the oven, the best kind are fire bricks and to get these you will need a specialist wood fired oven or barbeque supplier, try your local yellow pages to find one. Normal building bricks can be used but they will not stand the heat as well and will eventually burn through, if you do not intent to use the oven that much they may be a reasonable, low cost option.

3. Setting up the base – How you do this really depends on what materials you have earmarked for the job. I used concrete blocks left over from the destruction of an old incinerator/ barbeque with two appropriately sized concrete pavers on top and for the farm we used bush rock cemented together with a formed concrete top to act as the base for the oven. Just make sure that it is strong enough to hold the (considerable) weight of clay that will form the oven itself.

This is One Solid Base!

4. Installing the fire bricks – There are a number of ways to set this up, insulating bricks on the bottom and fire bricks in a second layer on top, just a layer of fire bricks or just a layer of normal house building bricks. In both cases we have just used one layer of fire bricks as the oven base, but if you were using normal bricks install them on edge so that there is a flat surface to cook on and more mass of brick to withstand the fire.

Levelling the Base

Installing end Levelling the Firebricks

Put a layer of clay over your base about 50mm thick to set your bricks into, making sure you have enough bricks to cover the entire baking area of the oven. We started by putting a couple of bricks at the outer edge of the base in the area we expected to put the oven opening. Once you have a brick located in the clay where you want it, install others by sliding them down the face of any brick(s) already installed, this ensures that you get a close fit and avoid getting clay in between the fire brick faces. In that way you can be sure of a smooth and gap free cooking surface, and not have to worry about ash getting lodged in any gaps between the bricks.

5. Setting up the inner sand form – The outer shape and size of the sand form will set the inner dimensions and contour of the interior of the oven. It is best to use a fine brickies sand that will hold its shape when moist. Draw a circle on your bricks that sets out where the inner wall of the oven will be, then pile on your sand making sure it is moist and will hold its shape. Form a nice round high dome such that the dome is 65% to 75% of the dome diameter, or, if the dome is 1 metre in diameter it should be 650mm to 750mm high. In practice, if you think you have it right by eye, measure it. Odd on it will still be too low, so the hint here is to actually measure it. Once the dome is roughly formed, smooth it off to a nice smooth surface with a steel concrete trowel which we found gives a great finish. Once the firm is to your liking cover it with one to two layers of wet newspaper, this will help you tell where the form finishes and the cob starts when you dig out the sand form.

Building the Inner Sand Form

Finishing the Sand Form

Applying the wet Newspaper

 

6. Making the oven – Now the fun starts! This process is fun anytime but will be more comfortable in warmer weather; wet clay can be bloody cold!

a. Mixing the Clay - The process starts by laying out a decent sized tarpaulin, at least 3 metres square. The first layer will be the thermal mass layer and so contains no straw, so put a couple of garbage bins-full of clay onto the tarp and one or two of sand depending on how much sand is in your clay, if there is already lots you don’t need to add much sand, if there is very little, add more.

Mixing the clay

Likewise

Roll the mix in the tarp and get it as homogenous as you can, then get your volunteers and do the barefoot stomp and twist on the clay mix to incorporate the sand. As the mix spreads pull up the edges of the tarp and roll it back to the centre. At this point looooots of volunteers would be handy, the original oven was made by just my son-in-law and myself but the second one had lots of volunteers and this made the job much easier and more fun. Add water if the clay is too stiff for the sand to be incorporated properly but not so much that you get a sloppy swamp.

b. Forming the Cobs - When you have a batch of clay/sand mix ready to go, shovel it into a wheelbarrow or two, to make it easier to get at and start forming the clay into cobs by moulding it with your hands into a form that is slightly bricklike about 100mm wide, 100mm in height and around 300mm long. Place this around the edge of your form at the base, then place the second one behind it and so on one after the other.

Part way through the backyard build showing the two different clay layers

Make sure you smooth them in to form a continuous layer and continue the process until you have the first layer finished. Subsequent layers can be applied in the same way, but keeping the tup surface of each cob at right angles to the sand form so that by the time you get to the top the last cob is like the keystone in an arch, helping the dome of the oven support itself when the sand form is removed.

c. Cut the Door – Cut a semi-circular hole in the side of the oven that you intend to use as the access. This will form the doorway and should be 63% of the height of the inner sand dome to allow for correct draw for ventilation. You can use a long knife to cut through the cob and mark out the shape of the door, then scrape out the soft cob material until you get to the inner sand form covered by newspaper.

Door cut out and shaped - ready to remove sand

d. Scrape out the Sand – The sand has done its job of supporting the cob to form the dome of the oven and now it has to be removed to leave the inside ready for firing. If you are building a large oven or the cob is really wet and sloppy, you might want to let the oven dry out for a couple of days before attempting this! You can use anything available to scrape the sand out, but your hands will work the best. Work slowly and scrape the sand into a garbage bin or similar for later recovery.

Work with your hands slowly to remove the sand, you will be able to feel when you hit the layer of newspaper over the top of the sand and the big hint when you do, is to stop digging. Keep going and clean out as much of the sand as you can, leaving the inside of the oven to dry out ready for firing. Once the inside is dry it will make it much easier to sweep out any sand residue.

Making the Door

One of the doors I made myself, it was just some timber bolted together until it was about 50mm thick then cut to shape. It charred up pretty quickly and I don’t know how long it would have lasted in heavy use. The other one was made for us by a fitter and was black painted steel with a hand/bracket on the outside to keep it upright and it worked very well.

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