The Thermal Layer, The Cracks Again

Today while I was working on plastering, blending and filling the outside of the oven my son Charles climbed into the oven to work on the cracks in the thermal layer.

Before we get to the work Charles did I want to make some comments about my last post about the cracks in the thermal layer.

“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.” 

Well this still hold true for he most part. The only thing that I didn’t expect was that after using the oven this entire time that some of the cracks would open and stay open after cooking with live fire a number of times. It seems that just building a fie in the middle of the oven for a couple of hours to heat the oven for baking is fine but, when making many pizzas or cooking anything else with live fire has more effect. I think that because the fire is moved against the walls more heat is transferred creating more cracks. However the cracks are still fairly easy to repair. 

“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.” 

These cracks start to go away after you’ve been using the oven for a while. We’ve been using it for almost a year now and when a crack shows up it stay until we fill it. I think this is because most of the moisture has been removed and the inside of the thermal layer is now more of a terra cotta.

“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.”


This is the key to making your oven last. Like I said we’ve been using our oven for almost a year and the door opening has gotten hit and damaged, the walls have been hit it pans and damaged and the heat has caused some damage. If the cracks and damage is filled and fixed then the oven just goes right on working but, if the damage is left unchecked sooner or later the oven will fail.


OK so with all that said, on with the show. I took a few pictures of the cracks and damage as well as after the repair.

You can see some of the cracks here with one of these being pretty big and deep. The large crack was too long to get it all into frame.
Two more large cracks in this picture as well. Also if you notice the color of the thermal layer. This is caused by the firing. It’s all a light red to light brown color. You should also note that these were not the only cracks. there were quite a few that had showed up after so much use.
This was the damage done to the door. Some of these pieces were beginning to get loose. You have to be careful not to hit the door opening walls or this could be your door opening. I have been very careful but sometimes my children have hit it and I have accidentally as well. Also the inner door hits it all the time if we’re not careful.
So here’s Charles inside the oven working on filling in the cracks. He is using fairly heavy slip to work into the cracks as deep as he can. Then when he’s all done with that he’ll use the paint brush to smooth out the filled cracks and paint a whole new layer on the inside of the dome.
Here is the thermal layer and door after the repairs are done. I’ll give the oven a few days to dry while I finish the outside. Then I’ll fire the oven slowly building up heat to fire the repairs to the thermal layer. Also to see if anything else cracks so that I can make the repairs.