Insulation, insulation, insulation...

I've decided to build the rest of the oven as a cylinder - it's the easiest option for me, and means I can get lots of insulation in. I'm using more of the Thermalite blocks - many of these I've bought for cheap because they were damaged. They are fragile, but helpfully very easy to cut to shape. The top part of the oven will look a bit like a patchwork quilt though until I finish the surface off.

This will give me a nice insulating outer surface (4" thick), and I'll fill the space with loose fill vermiculite. This is a cheap and easy insulating option - you can see the bags of vermiculite stacked up on the left hand side below, ready to go in.

This gives me a good 6" gap between the dome and the thermalite blocks at the bottom, and much more above, so this should keep the heat in well.

I'm tipping the loose vermiculite into this gap - this will eventually fill up all this space. I've filled all the cracks in the dome with powdered fireclay, and have coated the whole dome with ~1/4" of dry fireclay. You can just see this in the picture below. The idea is that this will sift into any cracks that form in future and fill them up - a good tip from Alan Scotts book, if I remember right.