Building a Cob Oven

I've always wanted to build one of those dome shaped ovens from cob so here goes. 

 

The size

We decided to make the oven 800mm wide at the base (inside measurement) and some sited suggest that the height should be 3/4 of the base so we made this 600mm but suit yourself.  Just keep the proportions. 

 

The base

However you build the underlying structure, it must be strong.  A cob (pizza) oven is heavy.  The upper level of the base, the part that the fire is laid on, is made from fire bricks.  It is made a little larger than the inside diameter of the bottom of the oven. The rest of the base must be wider still to accommodate the thickness of the walls of the oven. We put down a layer of gravel with just cement powder on the structure before laying the fire brick.  This makes a layer of cement (without sand) that is full of voids for insulation.

 

 The core

The shape
We decided to use the catenary shape.  The catenary is not a round shape (semi-circle) or a parabola.  It is apparently the ideal shape so that the force from the weight of the shell is transferred tangentially down the shell to the base.
https://en.wikipedia.org/wiki/Catenary

It is formed easily with no mathematical knowledge by hanging a chain by it's two ends.  The chain droops down in this ideal shape.  

 

Stand a sheet of ply wood or similar, on its long edge.  Make sure that the upper edge is level.  Put two nails in the upper edge at the distance apart that you want the inside base of the oven to be (800mm in our oven).  Note that you are not going to cut the ply wood.  It is just for laying out the oven.

 

Hook one end of a chain on to one of the nails and adjust the link that you put over the second nail until the bottom of the loop is the distance from the upper edge that you want the inside height of the oven to be (600mm in our oven).   Take a can of spray paint and spray over the chain.  Take off the chain and you have the perfect catanery shape marked on the ply wood.

 

Measure the distance between the two nails and make a mark on the upper edge of the ply wood, exactly half way between the nails.  Drop a vertical from this mark down to the bottom of the curve formed by the spray paint.  You can simply use a builders square to draw a right angle from this mark down to the bottom of the curve.

 

Lay the ply wood down and make  marks on this verticle line every 10cm, starting from the edge where the nails are.  When you get to the apex of the curve draw an extra line half way between the last line and the apex (5cm spacing instead of 10)

Measure from each mark at right angles to the vertical line to the curve.  Wright these down.  You are now finished with the ply wood. Note that the first measurement is right at the base.  In other words from the vertical line you have just drawn, to one of the nails.

Get some material such as carpet underlay.  From this material, cut a series of disks with a radius of each of these measurements you have just made.  You should end up with somewhere around 10 disks of ever decreasing diameter.  Make one more disk at 5cm below the apex of the curve.  Make a small hole in the exact middle of each disk.  

 

Cut a round straight stick (dowel for instance) exactly the height you have decided for the inside of your oven. Make a mark on this stick every 10 cm and 5cm at the very top.

Get some sand.  Beach sand is great.  When damp, it must be able to more or less hold its shape.  The sort of sand you make sand castles with, with your grandchildren down on the beach.

Put the largest disk (800mm in diameter in our oven) on the fire brick and adjust the position so that it is centered.  Put the stick in the hole in the middle and pack sand around it.  Pack a layer of sand out to the outer edge of the disk so that it is level with the first mark.  Adjust the stick so that it is vertical.  Put on the next largest disk and continue this way until you reach the top.  You have now made the core that you will remove when you have totally coated it with cob.  We found it helpful to drape a piece of plastic net, such as is used to cover grape vines before harvest, over the core.  This gives a bit of purchase for the cob and reduces slump.  It burns away in the first fire.

Many cob ovens look like an igloo with the entrance sticking out from the front side.  You make it as a catenary as well but about half the height of the main part of the oven and of a width at the bottom that allows you to insert a tray that you plan to use into the oven.  It is a 'cylinder' rather than a dome and is covered with cob too.  A chimney is often inserted into this entrance just where it meets the dome.  Otherwise all the smoke comes out of the entrance.  A simple stove pipe, wrapped in chicken wire and coated with cob does nicely.

Making Cob

Cob is a mix of clay and sand.  It is rich in sand such that the grains are touching and so as the clay shrinks as it dries, the cob does not shrink.  We got our clay from  a nearby river and the sand likewise.  We did the mixing on the sort of  cover sheet which is used to cover floors while painting walls and did the mixing with bare feet.  The cob should be workable but as dry as possible.  You trample it and then lift the edges of the cover sheet to move the cob back into the middle.  Rinse and repeat.

 

Your cob will slump when it is first applied, so apply it to the outside of the core about as wide as the palm of you hand and about 10cm deep.  Allow a day or so between layers.  The cob will dry enough to support the next layer.

 

Much later, when you are using the oven, the heat of the oven will turn the inner layer of the cob into ceramic.  The outer layer will still be clay so rain will soften it and wash it away.  If you build the oven in a complete shelter this is fine but if it is outside you must add a layer of cement.  We first covered the oven with pink bats and a layer of chicken wire and then applied the cement.

Leave the oven alone for a week or two and then very carefully dig out the core.  Don't worry about the layer of vineyard netting.  Leave the oven for another few weeks.  It has to dry out a bit.

Build a very small fire in the oven and keep it going as long as you can.  This will dry out the cob.  Perhaps do it a few times.  At some point you can build a larger fire but not too big at first.  The vineyard net will burn out leaving its impression on the inside of the oven.  You will also be able to see a line where each disk was located.

Not a bad idea to paint the outside of the oven with a water soluble plastic paint or even lime it with a slurry of Calcium Hydroxide.