Once the thermal layer is fired and put into use you will inevitably see cracks. There are two types of cracks I found that happen. One type of crack is the expansion crack from the initial firing. When the oven cools these cracks don’t close. The nice thing about cob is that to repair these cracks you just wet the cracks with some clay slip and work in more cob to fill the crack. Then smooth them out with a damp finger and let them dry for a day or two before you fire the oven again. This technique works on the inside and the outside of the thermal layer.
The second type of cracks is the expansion that appears while firing. These cracks close back up when the oven cools. This I have learned is normal in clay ovens. If the cracks close on the outside of the thermal layer then you don’t have to try and fill them and if you put the insulation layer over thermal layer it helps hold the thermal layer together. You will need to keep an eye on the inside of the oven. If a crack opens with heat and then closes as it cools then you’re fine. However I found that early on some cracks on the inside will start that way and then suddenly they stay open and never close. Those you need to go and fill.
As long as regular maintenance is done your oven should last. That was another statement that I read over and over but, I had no idea what it meant at that time. Well now I can tell you that regular maintenance means checking and filling cracks.
Since my first pizzas I have roasted chicken, vegetables, herb potatoes, hams and baked bread. During all of this cooking, it was so good, I had to begin checking and repairing cracks. Since then the cracks seem to have stopped showing up. I could see how in the future with temperature and humidity changes cracks might occur so we’ll just have to keep a close watch and take care of them.