After I removed the arch frame I began working on the chimney. I used three bricks on each side of the arch to span from the arch to the thermal layer of the oven. I then used one brick on the front and back to span the gap at the top. This allowed me to have about a four inch flue that runs the width of and behind the arch that goes up into and 8 inch by 8 inch chimney.
You can see in the picture above how I positioned the bricks to span from the arch to the thermal layer. Any gaps I filled with cob and smoothed out. I also used some clay slip to help smooth and seal everything inside the flue.
In this picture you can see how I used the bricks to span the gaps on top. This gave me a square base to work off of. I just filled the gaps with cob the began building the chimney keeping the same 8 inch by 8 inch size inside. I worked the cob up a couple layers at a time. My chimney is about 4 to 5 inches thick all the way around on top of the bricks. I worked the chimney up to 6 feet tall.
Then I went back and started adding cob against the side of the brick risers. From the front tapering it out to the thermal layer. My reasons for adding this was to provide good support for the weight of the chimney as well as to reinforce the risers for the arch. I also added cob around the sides of the chimney to make it very stable so there would be no concern in high winds.
You can see in the the picture above where we began adding the cob on the sides of the chimney and on the sides of the risers. Since you can only put down a couple layers at a time we were taking a break waiting for the cob to set. Charles and Jason thought it would be fun to attempt to convince me they could sleep in the oven again.
This is one of those design elements that you need to decide on. You don’t have to have a chimney but, I wanted to be able to cook with some live fire at times as well as I wanted the smoke to be drawn off so it wouldn’t be in our faces. So in order to do those things I needed a chimney. I am really glad that I built the chimney as it helps a lot. I can honestly say that I won’t build another oven for myself with one but, for you the choice is yours.