31" Cast Oven (Santa Cruz, CA)

[zackliscio]

Hello! My first post, though I've been lurking for a while contemplating a build.

We live in Santa Cruz, CA, and the house is a rental but already has a thick concrete slab patio. When a stainless stand around the dimensions I had in mind popped up on CL, I decided it was time to attempt a build. The previous of the stand had intended to build a WFO, but never made it past the stand.

The plan is a 31" homebrew cast oven since that's the largest diameter I can figure out how to fit (and that's pushing it since the stand is 50" x 40").

Hearth:

I'm copying the Cucina Stand approach, and poured 2" concrete slab reinforced with 3/8" rebar (spaced @ 6" grid) over a hardie board backer. I've sourced a 2" ceramic fiber board cut ready to accommodate the firebricks I cut for the floor. I drilled 4 weep holes through the concrete, and plan to layer ceramic tile under the ceramic fiber board.

Dome:

Planning to cast the dome, still waiting on my stainless needles and polypropylene fibers to arrive. Then I will build my sand mold, flue gallery mold, and cast the dome. I have 50' of 1" insulated blanket to wrap the oven. Current plan is to finish as an igloo.

[zackliscio]

So far the forum has answered almost all my questions, so thank you for providing such an awesome trove of knowledge.

A few things I could still use some clarity on:

1. Sand type: does it matter what kind of sand is used for the sandcastle mold? What about for the castable mix? I've seen silica sand referenced most commonly, but I'm still a bit unclear.
2. When placing tiles under CaSil insulation board, should I just grab the cheapest tile I can find from Home Depot etc? Does it matter whether it's ceramic / porcelain / patterned?

Thanks!

[david s]

1. For the sandcastle the smaller grains hold together better. Adding a little powdered clay to the sand also helps. You can place some polystyrene boxes or similar in the middle to save on sand.I read somewhere that it should contain 8-10% water, but as most sand won't be completely dry this is hard to calculate.

2. Get the cheapest, but just make sure they are all the same thickness.

[zackliscio]

Thanks so much david s! I picked up these mosaic tiles, they should cover the area under the CaSil board. Does it matter whether they're upside down / right side up? Should I remove the mesh holding them together so it doesn't melt?

Is it a problem that my concrete slab is a bit uneven? Added a side profile pic to show a sense of the uneven troweling job I did. The slab is wet because it just rained last night, but it had been dry for a week already.

[david s]

The concrete slab should be kept wet for a minimum of a week after placement to allow for adequate hydration/ This can enhance its strength substantially. The uneven surface can be ground back a little if it's more than a millimetre or so. there is a bit of give in the cal oil which should even things out a bit too. don't remove the mesh as it holds the tiles in place with am appropriate gap between them. The mesh will not see temperatures high enough to melt it as it's under the insulation.

[zackliscio]

I cast the dome today and while time-consuming, it was fairly uneventful. I had missed the advice about not removing mesh from the tiles, but I placed them fairly carefully and used some sand to level under the CaSil board. Hopefully this doesn't cause issues with moisture down the line.

Misted the dome and covered it with sheet plastic, where I will let it slowly cure.

1. How long does the dome need to damp cure? 1-2 weeks?
2. Should I keep removing the plastic to mist it? Or is it better to just leave it covered and assume the water in the sand and paper will keep it wet?
3. When should I remove the sandcastle mold from under the dome? After the full cure is complete?

[david s]

The whole idea is to provide little channels between the tiles so water and steam can escape. Atm you've blocked all those up. Looks like it's too late to get rid of it now, so forge on.
I'm not sure if you have used the homebrew castable or castable refractory. They have different curing requirements. Your first post says homebrew so I'll assume that what you've used. Because it contains portland cement, considerable strength can be gained by keeping moisture in the casting for at least a week. There is enough give in the sandcastle to accomodate the minimal shrinkage in the homebrew, but it can be removed after 24 hrs. You can cover the casting in some wet sacks and then cover that in plastic to hold in sufficient moisture for a week, at which point you want to dry the casting out as much as possible. Looks like you've made a pretty good start.

[BraveBoy]

Hello! My first post, though I've been lurking for a while contemplating a build.

We live in Santa Cruz, CA, and the house is a rental but already has a thick concrete slab patio. When a stainless stand around the dimensions I had in mind popped up on CL, I decided it was time to attempt a build. The previous of the stand had intended to build a WFO, but never made it past the stand.

The plan is a 31" homebrew cast oven since that's the largest diameter I can figure out how to fit (and that's pushing it since the stand is 50" x 40").

Hearth:

I'm copying the Cucina Stand approach, and poured 2" concrete slab reinforced with 3/8" rebar (spaced @ 6" grid) over a hardie board backer. I've sourced a 2" ceramic fiber board cut ready to accommodate the firebricks I cut for the floor. I drilled 4 weep holes through the concrete, and plan to layer ceramic tile under the ceramic fiber board.

Dome:

Planning to cast the dome, still waiting on my stainless needles and polypropylene fibers to arrive. Then I will build my sand mold, flue gallery mold, and cast the dome. I have 50' of 1" insulated blanket to wrap the oven. Current plan is to finish as an igloo.

Good luck with it, looking very decent!

[zackliscio]

Thanks as always for the valuable info! Yes it's homebrew (3:1:1:1) with fibers and needles added. I'll continue to keep it wet, and dig out the sand after 24 hours (later today) and report back.

[zackliscio]

Making some progress. The dome seems to be pretty solid--been keeping it wet by misting it a few times a day and covering it. I cast the flue gallery, which isn't very symmetrical or pretty but I think will do the job. Will continue to keep the whole thing damp until this weekend.

As a next step, sounds like I should wrap it in my fiber insulation blanket, then cover it in 2" of insulating vermiculite/perlite concrete (10:1 with the 10 being 50/50 vermiculite/perlite mix?)

Then start the drying fires after it's fully insulated?

[Kris S]

looking good!

[zackliscio]

Added a collar on top of the flue gallery to support the chimney since the original relief I left for it wasn't very big or secure feeling. I wrapped the chimney in cardboard and added more of my homebrew castable around it for support.

Next step I'm planning to wrap with fiber insulation, then cover in 2" of perlite / vermiculite insulating blend.

Is the correct ratio 10:1 with the 10 being 50/50 vermiculite/perlite mix and the 1 portland cement?

[david s]

Coming along nicely, good job.

[zackliscio]

2" of ceramic fiber insulation and 2" coat of vermi-perli-crete have been applied. Now to wait for at least a week while it dries out.

The ceramic fiber board beneath the hearth is very wet--will this just be a matter of time for it to dry out?

[david s]

Hi Zack,

sorry, you're going too fast for me to keep up!
"Is the correct ratio 10:1 with the 10 being 50/50 vermiculite/perlite mix and the 1 portland cement?"
Yes
You can make it stronger by adding more cement, but it won't insulate as well.

The point you have reached now is pretty exciting and you'll be wanting to crack on. Beware! Leave the thing alone for a couple of weeks to allow the sun and wind to dry it. Not sure how much vermiculite and perlite you used, but as the mix is 30% water, that's a lot! When it all goes white you'll think it's dry, but it won't be deep inside. You could buy a cheap garden moisture meter and use it in reverse. ie when it flashes red and says there's little water there, then you'll know it's ok to start pushing the water out slowly by fire. The meter is best pushed in to about the depth of the vermicrete, not at the top, because that will be the driest, but about half way down the dome. Alternatively some sheet plastic thrown over it during drying fires will condense on the underside if there's plenty of moisture there. You can actually cook something, maybe a chicken during these drying fires, but not pizzas...yet. The under floor insulation moisture is the hardest to eliminate, but eventually it will dry. You must be making the brick builders jealous with the speed of your build. It wouldn't hurt to build some more vermicrete around the pipe higher up (use the cardboard trick again). This will support the flue pipe better as well as being a little further away from flame which can destry even high temperature silicon. This extra vermicrete can be little stronger, say 5:1. Also clean the cement off the flue pipe asap. a little oil applied to it first helps.

Dave