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...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...
They're only four bricks high - this should keep the timber dry and stop any rot. You can see how the ground isn't level - the back wall is only a brick above the ground, while the front is four above the ground. I'll fill all the space with subsoil we're moving from elsewhere in the garden, saving me more money on skip hire.
This will be a 10 foot x 10 foot base - the bolts you can see are 12mm threaded bar embedded in the mortar to bolt the base frame on. It would be a shame to build the thing only for it to blow away when the wind gets up...
Amazingly, it's exactly level. That or I need a new spirit level. The plastic 'skirt' method I used to hold the timber ring in place has made a strange sort of edge to the hearth slab, but it worked!
It's done a good job of holding all the concrete in, and everything is well bonded together on very uneven walls. This gives me a nice flat, level slab to start building the oven on, a bit like a blank page. The little hole near the front of the slab is where the thermocouple wires come up from the front panel - there'll be six thermocouples by the time I'm finished.
When I placed this on the stand and levelled it there were some pretty large gaps (>4" in parts), so the easy way to fix that was to make a black plastic 'skirt' inside the ring, and then trowel on a sloped edge of concrete. The black plastic stops the concrete falling out of the gaps. When this has set I'll pour the rest of the hearth - hopefully with no leaks.
