Outdoor cob oven: experimenting with materials

Hello.

I am building an outdoor oven for cooking and baking in the garden. I have poured over different designs online and am using the "20$ cob oven" design with some twists.
I dug into the slope and, to my surprise, found red clay underneath 2 feet of topsoil. I am using that red clay for the cob. There was a plenty of boulders laying around for the foundation.
Since I am missing on sand to mix with the clay and straw to make cob, I would like to use some materials that I have at hand.

One is a coal slag used for sandblasting. Here is its chemical composition:
The chemical composition of bottom ash and boiler slag particles is controlled by the source of the coal and not by the type of furnace. Coal ash is composed primarily of silica (SiO2), ferric oxide (Fe2O3), and alumina (Al2O3), with smaller quantities of calcium oxide (CaO), potassium oxide (K2O), sodium oxide (Na2O), magnesium oxide (MgO), titanium oxide (TiO2), phosphorous pentoxide (P2O5), and sulfur trioxide (SO3).

The other one is ferric oxide (Fe2O3) that I collected from a dry bed of a creek using a strong magnet. They look similar, but one is magnetic and the other is not.

So, the stove design calls for use of perlite to cover these empty bottles to create a layer that retains heat underneath the brick floor of the oven chamber. I believe I can substitute perlite with a coal slag. It went through 2400 degrees burning in the burning chamber and is quite inert.  I think that in the building of the dome, I can replace sand with ferric oxide. I am not sure if anyone has ever done it before and would like to hear any pro or contra opinions about its use.  Could it help in making a better heat shield?

Another idea that I want to try is to embed several powerful neodymium magnets within cob. If the heat will not destroy its magnetic capability, I can attach steel pots and cast iron pans magnetically, without using any hooks.

Substituting slag for sand will probably work fine.  Any aggregate that doesn't expand or contract significantly through wet/dry, hot/cold changes should work.  Some slags will expand over time through chemical changes caused by devitrification.  There probably won't be enough moisture (or time) for significant devitrification.  I'm not sure about the ferric oxide.  However, if you have access to a dry creek bed for the ferric oxide - can't you collect sand from the creek bed and not have to make substitutions for sand?

I don't think the magnets will work simply embedded in the cob.  Cob has a low tensile strength.  The tension created by the magnet and food-filled pan would likely pull the magnet through the cob or if there is enough cob between them to prevent break-out, the magnetic field wouldn't be strong enough to suspend the pan.  You could attach the magnet to an armature built into the cob, but if there is an armature why not hang a hook from it?

Hello.

That creek is far from here. I collected this ferric oxide years ago...
I came across a local supplier and can buy 1/2 a short ton for $20. He sells smooth and coarse sands - which one is better to mix for the cob?

And, I underestimated the amount of clay that I need. I am down to the last 3 buckets of the red clay and it will not be enough for a dome.
Alas, the soil that had been trucked in when the house has been built is almost pure white clay. I was digging hole to plant trees and exposed that clay layer. Can I mix red and white clays? Or, at least, can I make part of the dome from red- and part from white clay- cob?

Thank you,

Hello.

My cob oven is almost complete.

I would still like to add more insulation and use another material that I have at hand.
I bought these fiberglass boards to insulate basement walls and there are a few of them left: FIBERGLASS SOUND BOARDS
It is 1" thick material. I wonder if there are any arguments against using it in insulating the outdoor oven? I can skirt the oven around the bottom, maybe 2' high and cover the rest with triangular pieces, kind of Buckminster Fuller polygons...

On the outside of these panels I would like to use red bricks and completely enclose the oven. The  bricks would create a hard and heavy shell that will push the fiberglass boards against the cob and is not going to slide or peel off.

Do you think it will work?

I am attaching these pieces of mineral boards with garden stake. It seems to be needed to be wrapped in chicken wire next. I am not sure if I could handle so much brickwork since I have never done any mason jobs... would you think another 3-4" layer of cob on top will keep these layer securely in place?

Slava

The oven is almost done. I mixed 10 parts sand, 6 parts clay, 2 parts Portland cement and 3 parts garden lime for the outside "shell". chicken Wire was wrapped around the mineral panels to provide something to grab to for the concrete shell...

I am a high school teacher about to build a cob oven with students in Homestead PA (outside Pittsburgh). We want to use materials form our site, but I have concerns. WE are within sight of the former Homestead Works - once the largest steel mill in the world. For about a century it pumped all kinds of nasty stuff into the environment, and I worry about our clay. Local soils are notoriously contaminated. Our site tested positive for arsenic. Is it safe to use clay form our site in our cob oven?

Hi Brandon.

I can speculate that if your clay is deep enough and at a location where there is not much running surface water, the contaminants would not penetrate into the clay.
How deep is it buried? Just discard the first 1' and you should be fine.

Thanks - Slava On!
We will be digging a test pit soon to see how deep the clay is.
What exactly do you mean by 'running surface water'?
The site is on a large hillside (everything around here is), if that helps.

Sometime clay is collected from the bed of the streams. This is the place that would see a lot of surface runoffs that do carry pollutants. Luckily the clay is impervious to water, but when the water stays in contact, it will be absorbed by clay - pollutants and all...

Thanks for clarifying. Ours is definitely not form a dream bed!
We dug today and found clay a few feet down - so I'm feeling like we should be good.

Yes, I would be comfortable using the deposit you describe.

Update: I have added one more layer of clay+sand with 1 gal. if natural iron mixed in (hence the darker color on top). To finish it off, I liberally applied some synthetic stucco material that I scored at Lowe's for $10 per 5-gal bucket.

I think I am done with the outside.
On the inside, I put a couple of firebricks and placed a ceramic grill rack from an old grill. That allows me to pile wood on top and slide paper and cardboard underneath to light a fire quick and easy.

The door definetly needs a handle and a pipe - a damper.

So, what do you cook in them outdoor ovens, besides pizzas?

I would like to stack the wood all around the oven. Like, cover it completely. The heat will dry the logs and they will burn easier.
Is there a danger of getting the firewood too close to it???