The Oven Form

At this point we were starting to get excited thinking how we were so close to being done. Wow we had no idea how much work this was going to be but, it was worth it. Well the first thing you need to do is to find the center of your oven floor again. I found that a string lined up with the center of the door bricks and the first coarse worked well. Then I used a combination square to run the second string to verify I had good right angles. I knew my planned my internal oven size was 48 inches in diameter. So after finding the center of the oven floor I had a starting point. I marked an X so I knew where the exact center was. Then I took a short straight piece of 12 gauge copper wire, a pencil and some string. I tied a small loop on one end and slipped it around the wire then measured out to the end and the other tied loop at 24 inches and slipped it over the pencil. This gave me a way to mark a 24 inch radius to give me a 48 inch diameter. I just held the wire in the middle and stretched the string taunt and marked the circle on the bricks.


OK so why am I doing this? This was to define the internal oven space. This has to be done because in order to create the void that will become the internal oven space we made a sand form. A word of warning here, this will take more sand than you expect. You will need to wet the sand so it sticks together. I started by building a small pile and slowly adding to it. I worked the mound of sand out to the line that I drew. I also made a mark on a 2×4 at 26 inches. I used this as a guide for the top of the sand mound. The reason I chose 26 inches is that the most efficient flow of air through the door is when the ratio between the height of the door is 63% of the height of the internal oven space. So with that to get a 16 inch door I needed an oven height of 26 inches. That takes into consideration a slight shrinkage as the dome cures.

I found that by using a trowel I got a nice, smooth and even finish on the sand form. This is important because the smoother the sand form is then the smoother the inside of the oven will be. Once we had the sand form filled out to the line we marked and to the correct height we then began covering the sand form with wet newspaper. The newspaper acts as a separation layer between the sand and the cob. The wet newspaper will also help keep the sand damp so it will be easier to remove than if it was dry. Once that was all done we were ready to start building the thermal layer of the oven.