For a long time I have wanted to build a pizza oven in the back yard. I shied away from spending $2-3k on fire bricks and cement and decided to try and build with what I had already in the back yard.
I watched quite a few videos and blog posts about building ovens with cob, including this very helpful video by Lizi Qi. These photos track the progress of building the oven.
The first step was to dig a hole for the base and fill it with cement.
The bricks were laid out on a piece of half inch steel that was on the property when we moved here. Then a base brick structure for the steel sheet was laid up to about waist height. 
Then the steel was mortared in on top and a layer of bricks added to hold it in place. The cavity was filled with sand and glass bottles to insulate the base from the layer of bricks in the oven.
With the layer of bottles complete, the oven base bricks were laid without mortar on top of the sand, but overlapping the brick base to create a sill.
The shape for a brick arch entrance was laid out on the grass and then a wooden frame created to support it while the mortar set.
The arch required some additional wooden supports as the width of mortar between the bricks wasn’t measured properly when they were laid out on the ground.
Prior to constructing the oven, a shelter was built to protect the earth while it dries. I had a piece of corrugated iron left over from a renovation, but the wood was purchased new and is probably the most expensive part of the build.
Wet sand was mounded into the desired shape of the interior of the oven. Usually ovens are dome shaped, but this oven is based on an oval.
Wet paper was used to keep the first layer of mud from drying too quickly, and also to ensure that it didn’t get stuck to the sand.
For the inner layer, a mix of roughly half sand and half clay soil was used. Where we live, there is red clay soil around a foot below the topsoil.
The inner layer was made around 1 inch thick.
Mixing the cob with bare feet can be disgusting or fun, depending on your outlook.
The first layer was dotted with holes to help the next layer stick to it after it had dried for a few days.
Mixing the cob (sand, clay soil and straw) takes a bit more effort.
The cob is quite rough, so a final layer of clay soil and sand without the straw was used to get a smoother finish.
A few 1mm cracks appeared as the cob dried out, and these were filled with more clay mixture.
After drying for a week, the sand mold was dug out by hand.
And the first fire lit.
The fire was kept going for about 3 hours.
And the first pizza cooked.
The oven retained enough heat for 4 pizzas, but as the door was not a tight fit it was clear that a lot of heat was being lost. With a tight fitting door, it should be possible to keep cooking with the coals removed.
No chimney was installed, as the goal is to cook with heat retained in the earth walls and brick base, rather than continuous radiant heat from the fire.