Building The Thermal Layer

The thermal layer is the layer that makes your oven work. I read that same line over and over again as I was doing my research. I kept trying to figure out exactly what this meant. To put it simply it means this layer is the oven. The insulation layer helps hold the heat and the outer layer seals and waterproofs everything but, the thermal layer is actually the working part of the oven. This layer is central to the entire oven project. It will be the layer that holds and radiates the heat of the oven. This layer will also be a monolithic dome that will be the structural support for everything else on the oven.

Due to the exposure of fire and extreme heat I chose to build the thermal layer with no straw. This is essentially the same mix as a clay brick or terra cotta. Remember the mix is 2:1, two parts sand and one part clay. I settled on this mix with no straw because when I made a test ball with some straw and fired it in the barbecue it was weak and crumbled but, the one with no straw was hard as a rock. I wanted the strength in the dome.

OK so why am I making such a big deal about this one layer? Well there are several reasons for it. One is that this layer needs to be as even and constant in thickness as possible so that the heat is always even inside the oven. Another reason is that it is the most difficult layer to make due to the multiple steps required for this type of construction. With that said the truth is if you pay attention to the details it's not that difficult it's just a lot of steps.

Step one was to make lots of cob balls. Now we made a bunch of the balls close to the same size so two rows of cob balls made the wall between 6 to 7 inches thick. I would highly recommend not using the balls but making cob logs. I ran into some trouble with the cob at times due to the double layer of cob balls that would have been alleviated if we had made the the logs 6 to 7 inches long at the start.

Step two is to begin placing the cob. Your sand form is not going to move so don't be timid or too light handed when placing the cob. You want to place the cob with a little slap / force always angled in and slightly down towards the sand form. You also want to pat down to make sure of a good bond but, remember that a patting a little goes a long way. If you pat too much your hand will become sticky from the clay and will pull the cob up and open gaps. I learned to use a motion that was like a flat handed karate chop. The hard part in this process is to continue adding the cob so that the top of the cob stays perpendicular to the sand form and consistent in thickness. This gives a good bond and makes for a solid dome.

I know all this sounds strange but, after you put down the first couple of rows you'll get the hang of it. A technique that I used was to place the cob with my left hand while pressing and shaping at the same time with my right hand. This technique also allowed me to keep the thickness consistent. I had to measure the width of my left hand to determine where to hold my right hand. This seems a little awkward at first but, I got used to it very fast.

So when the sand form is all completely covered and you are happy with it then you need to give it some time to set. This was tough to judge since the cob seems to set and solidify in no time at all and because it took us two days to build the thermal layer since we ran out of sand. Once the thermal layer is set then on to the next step.

One side note here is that when we began building the thermal layer I very quickly realized that the brick risers that I had planned to build as part of the arch would have to be put in as the thermal layer went up. I didn't have enough room on the base where I was putting the door. So I had to include the first six layers of the risers. My risers were mortared together with the same cob mix as the thermal layer. This also embedded the risers into the thermal layer which made them very solid.