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...