21" portable cast WFO

[SoCalZa]

Hi FB folks,

Long time lurker, first time poster. I've been interested in building an oven of my own for a few years and I'm ready to take a crack at it. I acknowledge I have zero experience with the materials and techniques, but I do love to make things, and I love to cook. I've sunk about 50-100 hours into these forums - you collectively have inspired me to take the plunge and build one of these things... Thank you to everyone for letting me learn from you, and thank you in advance for any constructive feedback on the below.

I am posting my plans here in the event you feel like sharing any feedback - I hope to avoid any egregious mistakes this way, so feel free so I can consider and address.

I credit WaterDog's build in 2024 as my main source of inspiration - I'll mostly be following this blueprint as it turned out fabulously and it all checks out with all the rest of the seasoned members of the community here.

Goal & purpose:
Cooking pizza for 1-10 people, with flexibility to cook steak on a Tuscan grill, slide in a chicken or loaf of bread, etc.

I need an oven on the small side so it's manageable in the limited space I have to work with, offers quicker fire times, and keeping in mind I am a novice, so approachability is a factor. I started out thinking I'd use fire bricks but pivoted to casting after considering feedback from members on time + experience, especially considering this needs to be a tighter build.

Considerations:
- I live about a half mile from the beach in southern California. Humidity is natural, if not constant, here.
- It needs to be portable. It'll rest on gravel in an uncovered, outdoor garden area. When in use, I'll pull it out a few feet to give some buffer between the oven + flue and the plants/trees. I do not anticipate being able to easily move this, but I need to be able to move it. When not in use, I'll cover with a weatherproof cover not unlike those on BBQs.


Design basics worth mentioning:

• 21" dome, cast with homebrew. Including burnout fibers to encourage evaporation.
• Weep holes drilled in the base refractory concrete (through the steel base).
• Steel frame with four pillars on caster wheels. Frame and caster wheels will be capable of supporting ~800 lbs; far more than I anticipate this weighing to air on the side of caution, and enhance mobility. I will coat the entire frame in Rust-Oleum.
• Frame is being custom built by a friend with a metal fab shop.
• Caster wheels: mounted to each of the four pillars, going with a 5" diameter solid rubber wheel. Considered pneumatics to aid mobility but decided against it as it adds complexity (murphy's law...). I may consider 6 wheels instead of 4.
• 5" stainless flue pipe, 4 feet in height.

Construction method:
Dome: I am purchasing the below plastic semi-sphere to use as my dome mold. Designed as a planter, if has an external diameter of 21" and height of 7.5". I will cast on top of this by hand (covering mold in saran wrap for easy removal). The flat base of the planter, which would form the dome, will be topped off with sand or other moldable material to give the top of the internal dome a uniform dome shape, and bring the ultimate interior dome height to 8".

I'll follow in WaterDog's footsteps by utilizing a rod through the roof of the mold with a template to ensure uniform wall thickness.



Flue gallery: I will create a mold out of wood to cast the gallery separately.

Floor: Fire brick in a herringbone pattern. I will cut the perimeter bricks with an angle grinder, knowing these do not need to be perfectly executed and will be enveloped by ceramic blanket + external layers.



Some open questions I am considering, which I'd welcome your thoughts on:

• 21" diameter x 8" height, with a door height of 7". This goes against the traditional ~66% principle, but I've read that's not necessarily gospel in practice from some of the senior members on this forum.
• A big design choice I'm mulling over, thinking creatively - what if I extended the dome height by establishing a perimeter of fire bricks (on their sides), and building the dome wall upon that? It would give me a height boost of 4.5". This would mean the dome perimeter is not perfectly round, but enables me to scale things up a bit, and allow for a door height in line with ~66%.
• Any tweaks you'd suggest given I'll be moving this several times a month about 2-3 feet back and forth over gravel?
• Should I consider 6 wheels instead of 4?

Three additional questions about the below illustration from DavidS on another thread last year:

1) Does it matter what material I use for the supporting slab? The slab will sit on the steel frame.
2) Can I get away with 3" of 5:1 vermicrete instead of 4 to cut weight?
3) I am projecting total wall thickness of 3.5 inches: 2 inch inner dome, 1 inch ceramic blanket + wiring, .5 inch for stucco. Does this sound about right?


If you've made it this far, thank you for your time.

[david s]

I see some problems with using the plastic planter dome in that it will be very difficult to cut away and remove out the oven mouth once the casting has set. I presume because of its size that the plastic is fairly thick. Or, did you plan to cast the dome over the plastic planter upend it, remove the planter and relocate the casting over the floor? This will be difficult as it will be very heavy. It is nuch easier to cast in situ. Also at 7.5 inches high it's not high enough IMO a hemisphere would be10.5" high, so the resulting decrease in volume is substantial. This might lead to an insufficient oven volume chamber for decent fire circulation (like removing one cylinder on a small four cylinder car engine). Do the math 4/3 x pi x r3.
Also the sand castle method is easier than you think. Although Waterdog used a template to create his hemispherical form it is not really needed. A stick in the middle of the sand castle of the length of yor desired internal height and a line pencilled around the perimeter on the brick floor is all you need to create a free form sand castle with surprising accuracy by just eyeballing the profile, it's really east.
The brick on edge idea to raise the dome height has the disadvantage of creating vertical joins at the base of the dome which is its most structurally weak point and at only 3' thick is inviting problems there. (a 3' thick brick dome has proved to be inadequate)

When you say "gravel surface, not sure how rough it is, but remeber that you'll have a high centre of gravity, so anything rougher than smooth concrete can become problematic, particularly with small wheels (eg think dangers of E scooters) Make sure your steel stand is fabricated from galvanised steel.

Sorry, I couldn't read your little drawing: too small, can you choose a larger size?

1. Usually 4" of reinforced concrete is the standard, but I think this is overkill for a 21" oven. Remember that a 2" thick slab is only 1/4 the strength of a 4" one as the strength is proportional to the square of the thickness. I've trimmed mine down a lot where it's not bearing weight but is thicker where it needs to be. I also use steel reinforcing, extra cement and fibres to make it as strong as I can to compensate for thickness reduction.

2. You could do that but I believe 2" of cal sil would be superior.

3. That's minimal. I use 1" blanket, 1.5" 10:1 vermicrete 0.5" stucco and while adequate for purpose is good, but not really good. For me this all fits on a square 910 x 910 mm footprint.

[david s]

If your plan was to cast the dome over the plastic planter, upend it to remove the planter then lift the casting and relocate it back over the brick floor, remember that it will be really heavy and moving it may damage it. It is easier to cast in situ.

[SoCalZa]

Many thanks for the quick, detailed responses DavidS. That is indeed what my plan was, and I'd enlist the help of a friend to assist with the heavy lift. I am more discouraged from this idea by the importance of proper height, than the weight while turning it over...

My initial design had a 10.5" height before I adopted the current idea of using the plastic mold, mostly driven by perceived ease of use. I see your point in your first note - the plastic mold-assisted design creates more problems and doesn't really solve what I hoped it would. I'll ditch the mold and use sand to get the ideal 10.5" height.

Confirming galvanized steel on the frame. Wheels will be solid rubber with tread, at least 8 inches in diameter. To make this all a bit easier, I'm now considering laying down pavers on top of the gravel to make the frequent 3-4 foot moves a breeze. The big wheels will someday serve their ultimate purpose when I move homes, getting into and out of a truck.

[SoCalZa]

Hi DavidS, if you're browsing the thread I have a few remaining questions as I get ready to receive the metal frame and take this thing forward! Getting excited now.

10:1 mix for the internal dome? or 3:1:1:1? I've seen 3:1:1:0.5 as well...

What sort of sand can I use for shaping the dome?

Lastly, anybody familiar with burnout fibers I can purchase in the US? I've seen all kinds and don't quite know where to begin.

Many thanks!

[david s]

The much quoted 3:1:1:1 homebrew mortar recipe also works well as a castable although the fairly high proportion of clay in the recipe does lead to shrinkage and possible shrink cracks on drying. I prefer to reduce the clay content for this reason and hence the 3:1:1:0.5 formula which I find is more suitable for a castable mix.The addition of burnout fibres is most important for a thick castable, as they provide a measure of reducing the danger of steam spalling as they leave behind a very fine network of tiny pipes through which steam can pass once they have melted and burnt out. They also help to reduce shrinkage cracking as the mix dries. In fact the very fine polypropylene fibres used for concrete are added to perform this very function. It just also happens that they also work as burnout fibres for refractory castables., melting at 160C. I buy them as part of a two pack system, of very fine19mm pp fibres and the longer/thicker 0.5 x 47mm pp strength enhancing fibres (reserve these ones for a normal concrete mix). For fibre reinforcing of a castable refractory, the recommended fibres are melt extract fibres (stainless steel needles) . These are hard to obtain in small quantities and also present problems when handling in a wet mix applied by hand (they’re not called needles for nothing). A much easier and more readily available method is to use the strength enhancing fibres used by the concrete countertop manufacturers who use GFRC glass fibre reinforced concrete. Add 2% by volume. Also referred to as AR alkaline resistant glass fibres. These fibres don’t melt until around 900C so are quite suited to a WFO (but not for a kiln).

The 10:1 mix refers to the insulation mix used over the dome, not the dense castable mix for the inner casting. It uses 10 parts (by volume) of perlite, vermiculite or a combination of both, with one part OPC and a little powdered clay to give the lean mix some added stickiness.
The sand used for the mould works best if it’s pretty fine, but any should work ok. It’s pretty easy to form a perfect hemisphere if you place a stick in the centre, half the diameter then just shape it by eye. I read somewhere that the correct ratio is 10% water, but experience as a kid on the beach building sandcastles, we never measured the water content. You could add a little powdered clay to the mix to assist it to hold together, but not really necessary.cover the sandcastle with wet strips of newspaper, adding some PVA to the water to help the strips stick.There are plenty of cast homebrew builds in the “other ovens” section worth reading for tips.

[SoCalZa]

Thanks for the response! Confirmed on the feedback regarding less clay content, and the need for burnout fibers...

I will also have a browse through "other ovens" as suggested.



Meanwhile, went to pick up the frame and thought I'd share as this is the first meaningful update I have to offer. It's 38"W x 36"D x 40"H, on 8" caster wheels. It's a beast!

David, in your first response to me you indicated that 2" of calsil board would be superior to 5:1 being poured in as the base. Now that you can see here my intent was to form a "pan" to cradle the base so that the calsil or 5:1 would not be load baring in the usual sense, do you still feel 2" calsil is the way to go?


I'm currently sourcing materials and had a question on the right type of firebrick. The Forno Bravo guide on firebrick has a grid that suggests Medium Duty, the ideal firebrick, has Aluminum Oxide content of roughly 38%. The firebrick I'm finding for purchase is called Medium Duty but the Aluminum Oxide content is usually lower, around 30. I'll attach an example here so you can see the decision I need to take:

The first option is called Medium Duty and has Alumina content of 31; the second option is Heavy Duty with Alumina of 37. Is this really the primary driver of the decision? Does it make much a difference on the cooking surface? They are roughly the same price...






Many thanks for the continued support and I look forward to posting updates as I progress. It's starting to get fun...

[david s]

2” of cal sil is adequate, but some would recommend 4.
Medium duty are what you want. Light duty a bit too soft and high duty a bit too dense (which therefore burns the bases more easily) and are very hard to cut., wearing out expensive diamond blades faster. The Al oxide content is only one factor to consider, their density is the more important factor. Any medium duty bricks should be fine. Your stand looks terrific.

[SoCalZa]

Much appreciated, as always David! I am locking in orders for things now.

Since I am going with your suggestion of CalSil as my insulating layer, I am guessing I don't want the CalSil board exposed to the elements - What should serve as a topping layer to cover the CalSil, for all the unused space surrounding the oven itself, from external diameter to the perimeter of the steel frame?

To be clear in what I'm planning now, we have a steel pan serving as the base, then 2" of calsil, then firebrick. What should come between the calsil and firebrick, to cover the entire surface?

Thanks in advance.

[david s]

Usually nothing. The cal sil board can easily cope with the service temperatures of an oven. Some folk cover it with a layer of foil to prevent water getting into the board, but this is not a good idea because it also prevents moisture from escaping. If the surface is a bit uneven where there are joins in the board or slight differences in floor brick thicknesses, a dry levelling mix of 50/50 fine sand and powdered clay about 1/2” thick can be used, but this may not be necessary if the brick floor looks pretty flat.

[SoCalZa]

Thanks David. I follow what you're saying but since I haven't seen this on a prior build perhaps I haven't been clear in what I'm asking about (or I'm missing a critical element in my design).

Here's a rudimentary top-down in hopes of illustrating what I can't articulate!



Put another way: I imagined using the firebrick just for the shape of the oven to avoid excess, but hadn't considered that this would leave the top of the calsil board exposed. Are you imagining that I'm using firebricks across the entirety of the surface?

[david s]

The exposed calsil will be covered and sealed by the layer of render/stucco. You have two choices with the floor bricks. Build the oven dome on top of the floor or build the dome beside the floor. There are advantages and disadvantages of each method.