I put up the first arch tonight. I experimented around a bit to determine exactly how I want to support the arches, etc.
The picture here are actually of an early trial. It worked well enough, but I decided I wanted a more secure means of support so I took it down and redid it.
I ended up placing a brick to bridge the gap between the arch feet and the outer walls, since none of the other methods seemed to be satisfactory in this case.
Also not seen here iss an important step when laying up dry arches that I forgot (my battery died so I don't have any pictures of the final arch) That is to rub the top of the arch with dirt (or in this case, dirt mixed with some sand) or mortar or whatever is handy to fill in the gaps and imperfections. (The other option is to be incredibly precise grinding the angles on your bricks, but that's VERY difficult. If you rub a little something over it all to fill in the gaps you have a little room for error)
If you don't do this, the arch will have a tendency to go lopsided or even collapse.
If it wasn't currently below freezing, I would have rubbed it over with mud instead, but that will have to wait until later.
You can see the slip form I'm using to put the arches together. I'm taking advantage of the sloped floor. The two 2x4's rest on a sheet of plywood in the rear of the oven and on a 2x4 in the front (accounting for the 1 1/2" of difference in height) When I want to remove the form from under a completed arch, I knock out the support 2x4 in front which allows the arch support to fall down and be removed. THe first arch is the most difficult to do this way, since the 2x4's don't drop down any in the very rear.
A few notes on making the arches for those following with interest of making their own oven:
It's easiest to grind the angle on just one side of the brick. Then alternate each brick as you lay it in (so the angle ground sides butt up against each other, unground sides butt, etc). It's hard to keep 2 faces flat when you grind the bricks down like this (I'm using a diamond grinding wheel, it makes a lot of dust but is pretty slick.)
If you don't think you can do a good job grinding, you can just lay them in without grinding, and pack refractory mortar in the wedged opening. The disadvantage of this is you have to wait for the mortar to set before you can move on to the next arch.
Just to reiterate, when you lay an arch dry and before you remove the form, rub it with dirt or fine sand (or a mixture of both) a smear it over with a slip or mortar mixture of some kind to fill in gaps and imperfections. Unless you make a check gauge and carefully craft each individual brick there are going to be imperfections and inconsistencies (or if you're lucky, you could get your hands on some arch or wedge bricks and then this step is unnecessary). When the support is removed the arch will seek to balance its forces within itself. If imperfections are present, it will average them out meaning the arch will go lopsided, and it will be 'wobbly' which means it might fall apart eventually. If the imperfections are too bad, it will fall apart right away. So filling in the gaps thoroughly makes it stable.
If I remove the template and the arch moves AT ALL, then I'm going to take it apart and redo it.
You really only need (and want) a flat segmental arch for an oven. This arch has 7" of rise over a 49" width, it's based on a 48" radius circle. That's actually more rise than I need for stability, I could get by with like 3 inches of rise to support the structure if I wanted to, but I chose to build an oven with shorter walls and a higher arch mostly because it looks better (even though nobody but me will hardly ever see the arches once it's all done)
The taller you make an arch, the harder it is to support.
Actually wehn I was taking one of my trial arches apart, a brick or two removed it fell down and the tops of the remaining bridges wedged themselves together to form a very strong and stable arch that had maybe 1" or so of rise.
When assembling the arches, make sure to firmly seat each brick as you lay it in, otherwise the arch will shift when you remove the supports. I take my masonry hammer and tap each brick several times until it seems to be firmly seated on the brick below it. Also take care to align each brick in every other direction as you seat it. Once the key is in place, and especially once the form is removed, it is very hard to adjust and move anything.
For these arches, I'm trying to make my keys to where they will drop in about halfway, and then need tapped in with the hammer the rest of the way. This way any slack that's left in the arch is taken out. This also will lift the arch above the form a little bit, which in turn means the top edges are being forced open (or the bottom edges being forced together) aim for consistency in how much the key drops in before being driven and how much wedge they have in them, otherwise your arches will not line up with each other.