The Insulation and Plaster Layers
Next came the 4in insulation layer:
This consisted of wood shavings (horse bedding) mixed with Hawthorne 40 clay slip, that is, clay mixed with water to a milkshake consistency. This stage of the process required a lot of clay, more than we expected, and ended up stretching over two days. We initially started with 2.5 50lb bags of clay, but that only got us about halfway up the dome, so we had to go buy more. We ended up using a little over 5 bags of clay in total and 2.5 bales of wood shavings.
Here's the finished insulation layer:
There was a small mishap at the end of this: I stumbled getting down from the oven base and broke part of the arch. Rebuilding it turned out to be pretty easy though.
We added some sand to the insulation mix, but not a significant percentage. Fairly large cracks developed in the insulation layer as it dried, so if I did it again, I would add more sand and/or use different insulation material, like chipped wood or straw (also to try and reduce the amount of clay needed). This took a long time to dry; even after several months there is still a spot in the oven where moisture bubbles out from the insulation layer when the oven is fired up.
During this time we also finished up the roof, and installed a 4in chimney tube (just regular air duct material):
The last step in the process was to add a plaster layer. This is just "oven mud" (i.e. clay:sand at the 1:2.5 ratio) with chopped up straw and some wheat paste mixed into it, again lots of cob dancing:
We borrowed a weed whacker from a neighbor and chopped up the straw in a large garbage can; got some curious looks from passersby. We made the plaster layer about 1in thick:
And here, finally, is the end result again:
We put some corrugated plastic sheets over the top of the roof to protect it. As you can see here, I also usually put two shaped bricks into the archway to reduce the width of the opening during firing. So what do you get for all this work? Well, you get to play with fire and eat awesome pizza, bread, and flammekueche of course: