Oval clay dome

[baba]

Well, hello all
Weeks of reading, watching videos, research, taking notes, drawing sketches, hunting for materials... You know, you all have been there. Never sure if I've got the right stuff or amounts, right ratios, right measurements... But here I am, tomorrow is the day, I'm finally confident enough, I think, to kick start the thing and take the plunge.
It's going to be a 700x1050mm oval dome, utilising an existing concrete counter top that served as a charcoal bbq base for years which got made redundant once I contracted the incurable pizza oven bug.
The 100mm 4,1 perlite and portland cement mix will form the insulating base on top the 150mm existing concrete countertop. 25mm of sand layer will keep the 250x350x65mm fire bricks in place to form the floor.
The door will be on the norrow end with a 280mm height reflecting a 63% ratio of the 450mm dome height.
I'm planning to cast 40mm of home brew refractory clay mix 2,2,1,1 silica sand, powdered clay, portland cement and dehydrated lime as thermal layer. Shop bought Sodium silicate added to the mix. Still unsure how much of the stuff I should use though!! Any ideas?
50mm of 6,1,1 perlite, portland and clay wrapped up with 50mm of rockwool will do the insulating layer. Chicken wire to keep things in place and to provide something to hold on to for the 10mm white cement based rendering. I'll decorate it with a 200mm band of rustic red brick cladding to go around the base of the dome.
I know, sounds great eh? 😉 It all came to me in a moment of clarity after
wrecking my head and losing sleeps over it for weeks. Now saying that, I know tomorrow it'll be a completely different story when I have the trowel in my hand standing there and gazing blankly into the empty wheelbarrow but hey at least I've got a plan!!
Let's see, wish me luck 🤞

[downunderdave]

Two or three 12mm holes drilled through your 150mm supporting slab, near the middle will help in the elimination of moisture from your cast vermicrete insulating slab. This is useful for drying it out both after it's been laid and also in the future should it get wet and most likely will.The holes can be covered over with some insect screen glued on to the top of the slab to prevent the vermicrete falling through and insects getting in.The usual mix for underfloor insulation is 5:1 (strong enough but also adequate insulation) see attached table 

The usual dome thickness for a cast oven to provide adequate strength and sufficient thermal mass is 50mm. The home-brew mix being 3:1:1:1 sand, powdered clay, portland cement, hydrated lime without the addition of sodium silicate. It acts as a deflocculant with clay and may make the mix far too fluid for a workable application, but I'm not sure on this point as I've not tried it in conjunction with the homebrew mix.

A 50mm 6:1:1 perlite, cement,clay mix is quite dense and will not be a decent insulating layer. If you want it to be insulating a 10:1: 0.25 perlite or vermiculite, cement, clay is far superior (see table again)

Rockwool is not easy to render against as it's pretty springy.  Ceramic fibre blanket is much better although a bit dearer. It is also usual to place your higher performing insulator over the dome with the lower performer (vermicrete) over the blanket. This also has the advantage of providing a firmer substrate onto which you can render more easily.
Hope this helps.

[mikeb21]

Aug 2, 2020 10:04:19 GMT downunderdave said:

Rockwool is not easy to render against as it's pretty springy.  Ceramic fibre blanket is much better although a bit dearer. It is also usual to place your higher performing insulator over the dome with the lower performer (vermicrete) over the blanket. This also has the advantage of providing a firmer substrate onto which you can render more easily.
Hope this helps.

I will shortly be adding the ceramic fire blanket & render to my build.  Interested in the above advice.

Seen quite a few builds with the ceramic fire blanket held down with chicken wire then rendered with various different materials (perlite, clay, etc).  I've got lots of blanket cheap so was going to use a couple of layers, don't really want to go out and buy vermiculite/perlite, but worried if render direct that the moisture from the render will just make the blanket wet.

Thoughts?

[downunderdave]

Yes it does, and that creates a problem with the render drying too fast. To overcome that problem I always wrap the oven in clingwrap and leave it for a week. This holds the moisture in the outer rendered layer providing damp curing which enhances strength. Regarding rendering straight onto the blanket I find it far better to do a layer of 10:1 vermicrete first. This provides extra insulation, allows you to restore the hemispherical form because the blanket leaves a lumpy/bumpy form and also the vermicrete dries hard and is a good substrate to render against.