Scratch build thread - 36-inch inner diameter dome oven

[mnagy]

It’s an 2.5’ insulated stainless steel tube. 6” inner diameter, 8” outer diameter. I was going to make a flat top for the outer arch and mount it right on there.

[UtahBeehiver]

SS chimneys are the lightest option. Is the vent chamber going to be flat too in addition to the outer arch? If so, how are is it going to be supported. Steel lintels are problematic due to the different expansion ratio to fire brick and potential cracking will be a concern.

[mnagy]

Great questions. I don’t really know. The original plan was to pour a curved platform with Portland and fireclay to make a flat square on top of the outer arch that the chimney mount would screw into. I was going to just attach that with refractory mortar.

[mnagy]

I’m making a flue gallery. A big custom brick with a hole in it that the chimney can attach to.

I’m thinking this doesn’t need to handle crazy temps like those in the cooking chamber, so maybe I should use normal concrete and just put some refractory mortar to line the outside?

I have Portland, sand, and fireclay that I can mix to make a refractory cement, but I’m worried I’ll screw it up somehow if I mix ingredients myself and it won’t be strong enough.

what do you guys think?

[david s]

I think you should at least use the homebrew recipe 3:1:1:1 sand, Portland cement, hydrated lime, powdered clay. The lime in the mix provides a more heat tolerant cementious property.

[mnagy]

Ok. I might have screwed this up, but I guess we’ll see in a few days. I can always reset the form and do it again.

I used equal parts Portland, fireclay, sand, and perlite.

[mnagy]

Fire pit and bourbon night. Can’t wait to add pizza to that mix!

[mnagy]

Almost finished with the dome. One more row with the radius tool and then 2-3 more with this upside-down dish-looking platform for the middle.

next step is fire blanket, chicken wire, and rendering. Does anyone have a recommendation for the rendering? Shouldn’t need to handle much heat since the fire blanket will be on there. But it needs to be strong enough to hold the tile, so stucco isn’t an option.

right now I’m leaning towards using a simple mix of Portland and perlite 1:3 ratio. Thoughts?

[david s]

If you are looking for strength perlite addition is a poor choice as it reduces strength, although making it really thick can compensate for the weakness. The other problem with perlite is its ability to retain moisture, so eliminating this is time consuming. My solution, which is certainly not the only one, is after a layer of 10:1 perlcrete or vermicrete to even out the blanket bumps and to restore a nice round form which is firm enough to act as a suitable substrate to render against, a strong rendered layer of 12-15 mm containing lime, which has both elastic and crack healing properties, as well as reinforcing fibres. My brew is 4:1:1 sand, Portland cement, and hydrated lime, with 500 ml of 19mm AR glass fibres/10 litres of dry mix. If the moisture is held in this layer for a week it results in a very strong, but not waterproof layer.

[mnagy]

Thanks David. I’d love something waterproof on the outside under the tile. Perlcrete first with something stronger as a shell sounds like a good option. Is there anything I can just buy in a bag for that outer layer? Like a profinish5000 or something? I’d rather avoid mixing.

[Giovanni Rossi]

mnagy, there are a lot of options for dome renders presented on this Forum. My approach was somewhat of a hybrid of ideas. It's probably overly complicated with marginal benefit, but I'm happy with the result.

I wanted to increase the insulation properties but not sacrifice strength, So I did 3 different layers. 3/4" of 8:1:1:1 (Perlite:Portland:hydrated lime:washed sand); then 3/4" of 5:1:1:1. The final 3/4" layer did not have any perilte and I used 1:1:4 (Portland:lime:sand) and added 150gm of alkali-resistant polypropylene fibers to each 10 liters of dry mix.

That gave me a smooth, hard base for any final covering.

[mnagy]

I like that approach Giovanni. I might do the simple version of that and just go with 1” of the 5:1 perlcrete, and then cover that with the finishing mix you used (or something similar).

[david s]

Most commercial renders/stuccos have a degree of waterproofing added to them which repels water but still allows some to pass so moisture is not locked in. If you
make your own mix it won’t contain any waterproofing, so will probably require a top waterproof coating.before tiling.

[mnagy]

Thanks David! What commercially available (Home Depot/Lowe’s) rendering would you recommend for this application?

[david s]

I’m in a different country, so no doubt available products vary, but I find Davco PM
ptetty good. My own preference is to make my own because as I said, the commercial renders don’t contain lime or the reinforcing fibres.