A moldable refractory material that can be used to cast an oven in place is called “castable.” Kiln makers, amongst others, use castables to form side walls and arches to ovens that must withstand high temperatures.

Oven forms can also be made with mud and straw — adobe or cob — or with clay. Clay ovens can be built with clay you dig from the ground, or clay you buy. A mix of powdered clay and sand can also be used. A sticky clay such as “fire clay” and sand in a 1:1 mix should work. Use sand that ranges from fine to coarse for maximum strength. Clay shells that lack the reinforcing quality of straw may crack — but if allowed to dry thoroughly before firing, and if brought up to temperature slowly, the first time over a period of days, they should retain their structural integrity. However, as I do not have personal experience with earth or clay ovens I defer to the expert, Kiko Denzer, and his book Earth Oven.

The simplest castable to make for a bread or pizza oven is one part calcium aluminate cement to three parts sand. The measurement is volume — typically the shovelful. Thus, to every shovel of calcium aluminate cement you add three parts sand. As always, mix the dry ingredients thoroughly, and then add water. You want to add enough water to form a trowelable mix — but not so much water that the oven castable becomes slushy. You have added enough water when you can make slice the oven castable with a shovel, and the shovel makes clean verticle cuts that can retain their shape. If the cuts made with the shovel slump then you have added too much water. If the mix is crumbly, you haven’t added enough water.

All there is to making a bread or pizza oven with this basic castable is to trowel it over a sand form that is mounded to interior dimension of your oven. After the castable has set, but before it is hard, cut out the door. As always when working with cement, it is best to work when the weather is cool.

One bag of calcium aluminate cement that weighs approximately one-hundred pounds will mix with approximately three-hundred pounds of sand — three one-hundred pound bags — to cover an oven that is three feet in diamter with a shell approximately three inches thick.

Calcium aluminate cement dries quickly. It can cure in as little as 24 hours. After 4 hours you can then remove the sand form. While you can apparently also begin firing the oven at low temperature, I recommend that you let the oven cure for a longer period — escpecially if the oven cured at less than 70F. Kiln makers fire their castable very carefully, over a period of days, slowly slowly raising the temperature of the castable shell before bringing it up to full temperature. Bread oven temperature is approximately 1000F. This is not the temperature at which one bakes bread — of course — but it is the approximate temperature that the oven ceiling will reach at the point the oven is fully fired.
My advise is to wait a week, or more, before heating the oven. Then bring the shell to approximately 225F for a long time — even 24 hours — to drive out all the water. The most controlled way to do this is with a propane burner, or a small charcoal fire. After that, slowly raise the temperature. I advise working with charcoal before switching to wood.
The castable that I have just described is the one I use for my own oven. However, there are infinite possibilites for making a more sophisticated mix. An example of a more sophisticated castable, in this case one created for kilns, is found at artistpotters.com.

Does castable require a wire mesh frame? The answer is no — but pushing chicken wire into the shell as you are forming it is probably not a bad idea. I did not use wire for my most recent castable shell and there is severe cracking — though the cracking is not structural.