I started on more courses a day or two later. Everything was going pretty smoothly -- I thought! -- until I noticed that the open ends of the vault were sagging inwards. Luckily the refractory mortar was still plastic and some wedges and props got everything pretty much back in line (or circle). I had to add some squinches to make everything fit correctly.
Messy but virtually invisible -- as this area will be right inside the oven door. I was ready to do my first completely closed ring. It came out slightly eye-shaped, but no worries: I figured I could fix it on the next course. I was now on the last few courses, and the angle of the bricks meant that I needed some more supportive forms.
I also built the side walls of the oven landing/flue arch so that they would have plenty of time to cure hoping to thus avoiding the thrust issue I encountered with the oven opening.
I cut a plywood circle with the jigsaw and marked a bunch of concentric circles for guides. This was propped up in the center of the vault with a scrap of 2x. Wedges laid on the plywood supported the bricks at the correct height.
At this point I fixed the out of roundness issue with two more squinches. (covered with mortar on the right of this image.) A few rings closed the top. I immediately popped out the form and cleaned up the mortar on the inside of the dome. Dome done!
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