I've decided to build my oven dome using a clay mix rather than firebrick. Firebrick and refractory mortar is extremely expensive round here, and I don't want to spend ages cutting firebricks to shape - I don't own a brick saw, and am reluctant to buy one just for this project. I like the idea of moulding the oven walls out of clay - this should be much quicker than the firebrick option.
Kiko Denzer's book (a must buy if you're building one of these things) recommends using a clay/sand mix for the oven dome. A potential problem with this is that the clay and sand have different rates of thermal expansion, which could lead to the sand crumbling away from the clay, into the food. My cooking's bad enough as it is without this... Instead of using sand, I'm going to use firebrick grog made from waste or 'seconds' firebrick. I've done some tests using firebrick grog (0.5-1.0 mm grade - a bit like very fine grit) and fireclay. By mixing these two I can make 'mouldable firebrick' - a mix that when dry will be the same as firebrick, but much easier to make to the right shape.
To find the right mix I started with some recipes from 'Contemporary Ceramic Formulas' by John Conrad. This gives some recipes for refractory firebrick mixes, which I'm modifying slightly here. I'm trying five mixes:
- 80% firebrick grog, 20% fireclay
- 70% firebrick grog, 30% fireclay
- 60% firebrick grog, 40% fireclay
- 50% firebrick grog, 50% fireclay
- 40% firebrick grog, 60% fireclay
I made all these up in small amounts with the minimum amount of water needed to get them to stick together, and had a play with them. Firstly, I tried moulding them into a ball:
Mixes 1-2 were very difficult to mould, while 4 and 5 were very easy - just like working pure clay. I then tried squishing them by planting my thumb in the middle of the ball:
Mixes 1-3 were pretty crumbly, but 4 and 5 were tough and fairly plastic. I don't think you could mould mix 1 very easily, while 3-5 would be workable. Next I formed the clay into little bricks, and stuck a fork in them in the middle. The bricks are labelled 1 to 5 with dents at the the top of the brick. By measuring the distance across the fork mark I can see how much they shrink when they dry:
I dried these at room temperature for a day or so - mixes 1 and 2 dried very quickly. I had hoped the gritty grog would help to speed drying - in old English cloam ovens fine gravel was mixed with the clay to help the clay dry quicker. I finished the drying off in a warm oven for an hour. I don't own a kiln, so instead I fired the centre of each brick with the trusty old blow torch for 10 minutes:
All the pieces heated up in the middle to a cherry red colour that glowed for 10-15 seconds after I removed the torch. None of the bricks crumbled or cracked during this. Once they'd cooled, I tried crumbling the outside (unfired) part of the brick to see how strong they were:
Mixes 1 and 2 were very crumbly round the edges, and not very strong. Mixes 4 and 5 were very strong - I couldn't crush them at all. At this point I measured the shrinkage of the blocks - 1 and 2 hadn't shrunk at all, 3 had 1.5% shrinkage, 4 had 3% shrinkage, 5 had 6% shrinkage. The greater the shrinkage, the more cracking I'll get in the oven walls. Finally I soaked the bricks in water and saw how much of the clay was unfired:
Mixes 1 and 2 virtually fell apart. These were pretty light, and probably didn't conduct heat that well. Mix 5 had a solid fired core that wouldn't dissolve, with 4 almost the same but a little smaller. Mix 3 was somewhere in between. Mixes 4 and 5 felt most dense and heavy.
After all this testing, I've settled on mix 4 - 50% fireclay, 50% firebrick grog. This is plastic and mouldable, only shrinks by 3%, and seems to conduct the heat pretty well. It's also tough when it's unfired, so should prove strong enough for the dome. I now have 100kg firebrick grog and 100kg fireclay in the garage ready to go - just waiting for decent weather now. Currently minus 1 outside today...
After I couldn't get the last brick to fit (three assembly-disassembly cycles, two hours work...) I lowered my support by about 1/2 an inch, then used bits of wood to hold everything in place. Jamming lots of broken brick into the big mortar gaps helped too. The arch itself took 4h, and was the single most infuriating bit of building I've ever done.
The arch is 1/2 a brick wide, and is separated from the brick ledge behing it by a gap of 1/2 inch, which I'll fill with an insulator later on. The ledge behind will support the chimney, and sits on two sections of angle iron which rest on the surrounding brickwork. A second angle iron supported ledge sits about 8 inches behind this first ledge, which you can see best from behind... 
That gap in the middle will form the chimney, hopefully. I'll fill the second ledge in next time.
You can see the front ledge is lower - hopefully this will encourage the smoke to go up the chimney, not out the front arch. The angle iron was the only recycled bit here - from an old bed frame which I salvaged a few lengths from. All that surrounding brickwork will vanish soon, so I've only bothered cleaning up the bricks on the front.
Much head scratching is going into this - I want the chimney to be supported on 4 lengths of angle iron (on the right side of the picture above). I want the oven dome to be completely separate from this masonry with a thermal break, which I'll probably make out of clay/sawdust mix or vermiculite. I reckon some of the ovens I've seen on the web leak a lot of heat up the chimney through the masonry joining the dome to the chimney, plus I'm worried about the thermal expansion of the dome cracking the masonry.
The plan is to have an arch at the front (on the right hand side above), with a span of angle iron and bricks behind it. Another span of angle iron and bricks will sit on the left hand side, above the entry to the oven dome. I'll fill all the gaps in the insulation mix afterwards. This plan may change - it's changed three times so far...
Eventually all the messy masonry on the sides will be hidden by the cladding - the brick entry is set 2" forward from the insulation blocks to allow room for it. I still have no idea and no plan how to do this (render? more stones? pebbles?), but ignoring the problem is working fine for me at the moment.
You can see how the oven sits at the back of the summer house, and is 'half in, half out'. There's a porch roof on the back to keep it dry, and this is only covered with black plastic at the moment as I'll need to get access to the main roof when I sort the chimney out.
I'll clad the sides with some 4 inch board I've obtained from a demolished fence - the roof is corrugated bitumen sheet. Hopefully with the light rigged up inside I can do some work when it's dark...
You can just about see the oven, covered in black plastic so my insulation bricks don't get saturated with rain. It's got about half a roof a roof now - hopefully finish this tomorrow if it ever stops raining...