Mortar Thickness

[CapePizza]

I'm in the planning stages of building a Pompeii "igloo" style oven. The hearth slab I have to work with is 52 inches wide and I'm trying to figure out the largest diameter oven I can construct on that slab. My question is the mortar (or stucco) that goes over the FB insulation blanket, how thick is that usually? How thick is that material built up to?

One other question. What is recommended for that material (the mortar that goes over the FB Blanket Insulation). I've seen it referred to as mortar, stucco, etc..

Thanks very much for the help.

[BeanAnimal]

Your primary insulation should be a minimum 2" of ceramic blanket - adding more is generally considered better if you want to cook with residual heat.

The outer "shell" or "render" is usually a mixture of perlite (or vermiculite) and portland cement. The higher the ratio of perlite to portland, the more insulating properties it has. But it is nowhere near the insulating value of ceramic blanket.

Common ratios are anywhere from 3:1 up to 15:1. The higher the perlite content the hard (weirder) the mixture is to work with. I am going to guess 5:1 or 7:1 is common here, but others can correct me if I am wrong.

Adding lime and sand at a 1:1:1 with the portland can make the mix easier to work and stronger, but also significantly reduces the insulation value.

I think most people end up with 1"-2" of perlite render over the blanket and do it in several layers using only portland and perlite or vermiculite (no sand or lime), with the outer layer having a lower ratio of perlite to make it a bit more durable.

An acrylic or other waterproof sealer is then applied once the dome is cured.

Not asked: But an important step is waterproofing the dome to hearth interface, as water migration under the render can soak the oven insulation. I think most folks with exposed domes cover their ovens in inclement weather to prevent water intrusion.

[CapePizza]

Thank you for the reply. I'll do more research on the "outer shell" material and adding perlite ...
any idea how thick that outer shell will wind up being? Thanks.

[BeanAnimal]

Thank you for the reply. I'll do more research on the "outer shell" material and adding perlite ...
any idea how thick that outer shell will wind up being? Thanks.

Sorry if I was not clear.

Your oven wall (brick) will be 4" in most cases
That will be covered by at least 2" of ceramic blanket insulation
That will be covered by 1"-2" of perlite/portland render. Some folks here call it perlcrete or pcrete.
That will be covered by a thin layer of paint on or roll on sealant - thickness in mils or fractions of an inch, not inches.

[Kvanbael]

I’m currently covering my dome with vcrete. But it is soo weak and likely to crumble that i suspect it will need more than a touch of paint to have some strength. (Want to finish with mosaic)

[david s]

The weak vermicrete layer works to both insulate and restore the perfect form. It is not suitable to glue tiles on to. It does however provide a sufficiently firm substrate onto which you can do an outer rendered layer, then glue tiles onto that.