Thanks to being busy at work, and some pretty foul weather I've not done much on the oven recently. This meant that it's had even longer to dry out with the 500W halogen light - another 3 weeks almost. Two days ago I mixed up some vermiculite concrete (1 part Portland cement to 5 parts vermiculite), mixing it up well when dry and then slowly adding water until it was wet, but not runny. I packed all this around the dome and the chimney supports. I didn't fill the gaps around the entry arch - that would come later.
You can see how the bit of the dome that's had the halogen light aimed directly at it has turned almost white (the patch in the middle). This bit is too hot to touch right now. I'm glad to say that there haven't been any new cracks over the last three weeks, so I'm hoping this means it's all dry.
I left this to dry for a couple of days, then made up a mix to fill the gaps between the oven entry and the chimney support. These are uneven, anything between 1/2" and 4" depending on the area, and I wanted to fill them with an insulating mix to stop heat from leaking out of the dome. The mix I settled on was 5 parts vermiculite, 2 parts dry fireclay, 1/2 part Portland cement and 1 part sodium silicate solution (all by volume). The addition of the sodium silicate and fireclay made a mix that was very sticky and could be packed together even when fairly dry, and could be moulded very easily. The addition of the cement meant that it started to set after about an hour, and hopefully shouldn't shrink and crack. It was fairly hard after 2 hours.
I used this mix to pack into the gaps, smoothed it all off, then washed it with a wet sponge to get rid of the excess.
This worked better than I'd hoped, and ended up being quite a neat job. Notice that I've filled the gaps between the firebricks and red bricks with the same mix too.
I filled all the sharp corners, and made things as smooth as possible to help the smoke go upwards. The gap between the two black bits of angle iron is the chimney. In the chimney itself I used the insulation mix to mould a nice smooth transition between the rectangular brick chimney and the metal chimney pipe:
This was a very easy way of filling the gaps, and an easy way to get a smooth transition between the brick chimney and the inner clay dome. It really smartens up the oven entrance - just need to wait for this to dry now.