First build - query re single piece (no slab) WFO

[Tobylav]

Hi all

I had been oscillating for a while, and recently decided to make my first WFO. This is my first attempt at building anything by hand (not limited to masonry), so it has been quite a slow process.

Like many others, I had always wanted it to be as light as possible - and was prompted by the promises of the lightweight vermiculite ovens that gained a lot of traction on YouTube. However, somewhere along the time, I lucked open this forum, and have taken the time to read into what is required to build a more effective and quality WFO, and to try to minimise the number of beginner mistakes I make. However, I am at the point where I wanted to sense check with brighter minds than mine that my proposed plan may actually work, and I am not headed towards certain failure.

For background, what I am looking to build is admittedly still not an ‘ideal’ build, as I need something that is light enough to be movable (currently living in the CBD in a rental), but still retaining as many beneficial characteristics of a properly designed WFO (vis-à-vis a Youtube perlcrete oven) as possible. Importantly, I want the oven to be moveable (occasionally); but it need not be easily portable. Think moved once or twice between rentals over the course of a couple of years - without the use of a forklift.

So far, I have built the dome. The dome is a very small, raised ~19” internal diameter made from refractory castable. In essence, I used the gym ball method, but instead of casting around the ball from its widest point, I placed the ball higher (so the mid-point was above the ‘base’ level), created a straight form down from the widest point of the gym ball, and cast around that. Appreciate that explanation may be a bit poor, so see photo below for an idea of what I mean. The idea of this was that because my oven will be very small, having a raised bottom would mean more room for wood tighter in the corner without being blocked by the tight curvature at the edge of the dome.



After initially building the refractory castable element of the dome (approx. 1.5”-2” due to poor rendering/application by me), I let it set for a few days (a lazy ‘wet’ set, by spraying it a couple of times a day with a spray bottle), before doing a ~4 day slow ‘cure’ of the refractory castable dome only with small fires. My logic to this – rightly or wrongly – was to strengthen the refractory castable and get rid of any excess water before putting on the insulating layer(s), so that moisture didn't just seep into the ceramic blanket (and thus - in my mind - avoiding an even longer curing period). That said, I didn’t see any moisture come out of the outside of the dome during the curing. However, I noticed there were a couple of weak spots (and a couple of very small see-through holes, seemingly due to my refractory castable mix being too dry), so I filled these with a homebrew refractory mortar. At this point I could carry the dome by myself without too much difficulty, and would estimate (I did not weight it) it was between 50 and 60kg.

Earlier this week, I then put the isowool / ceramic blanket onto the outside of the dome, followed by the 10:1 perlcrete mix (more specifically, 5:5:1 perlite, vermiculite, Portland cement per a recommendation on this forum). I only had enough ceramic blanket for 1 coat (ie 1”) all over, but I had enough spare to put a second layer on the top third/half of the dome (ie 2”). There is then approximately 2” of 10:1 perlcrete on the outside of this (closer to 1” where I have put 2” of blanket). The perlcrete layer has now been setting for 5 days under a damp towel. I will continue this for at least 2 more days (although may do it for longer while the base cures if any material benefit to doing so).

I am now looking to build the base – and this is where my question arises. And apologies for the prolix background before this.

Set out below is what I am planning to do to ‘finish’ the build. In short (and the picture will likely describe it better than words), it involves putting the firebricks (only 1”, acknowledging this is probably not ideal) on top of ceramic board (again only 1”, acknowledging this is probably not ideal), on top of 2” of 5:1 perlcrete mix, on top of 2” of concrete with rebar. This 2” of rebarred concrete will be part of the hearth, which will be mortared to the dome. There will be no separate ‘base’ (suspended slab or otherwise), and it will likely just sit on-top of a table.

My current thinking with this build is that it will be ~150kg (complete guess!), so moveable by 2 or 3 people without a forklift. And, in a worst case scenario, I could try to cut through the mortar and separate the dome and base when it came time to move it.

Finally, the question(s): Do you think this will work? Do you think I am missing anything? Can you think of an alternative approach that would give me my desired approach (noting I have already built the dome)? Maybe a way where it is possible to have a separate dome and base (not mortared – although I assume the feet of the dome will not be perfectly flat, so this would result in a lot of heat escaping)? You can safely assume I know nothing.

I am conscious that this is not a perfect build by any means, and I accept that the results may not be perfect (and when I have purchased a home with a decent yard, I will likely try to build a more permanent fixture); however, I am keen to build as good a WFO as I can within the parameters I have. It (obviously) only needs to cook 1 pizza at a time, but it would be great if it could be used to cook other things with retained heat etc.

(please note that none of these images are to scale – or even shape… – as they were all drawn on MS Paint…) (Edit: photos look tiny - trying re-upload).



For completeness, I am looking to build the base effectively through a negative particleboard mould as follows (Edit: photos look tiny - trying re-upload; separate photos sorry):

[Tobylav]

Final photo (too many attachments):

[david s]

My 21” internal diam ovens weigh 250kgs, incl the precast supporting slab. They can be lifted with a couple of 4x2s placed under the oven with 4 blokes.
You have a very deep tunnel in front of the oven chamber which will make working the oven difficult. A 1” floor will not have enough thermal mass which will mean you’ll Cook 4 pizzas before requiring to recharge the floor. You could pour a 1” castable or homebrew over the floor insulation before loose laying the 1” firebrick tiles over it, but it will crack so some thin 6mm round rebar set into the middle of this layer is advisable. Any large sections of refractory, particularly if heated unevenly are prone to cracking.
I think a 6” diameter flue pipe is overkill for a 19” oven, a 5” would be adequate. The 6” may introduce too much heat loss. Did you provide a rebate in the oven chamber on the inside of the flue pipe so you can isolate the flue when doing retained heat cooking. The 10:1 vermicrete over the dome is equivalent insulating value to blanket but needs extended drying preferably in sun and wind. Being quite weak it is beast to render/stucco over it only after several decent cooking fires, to protect the vermicrete from damage.
Finally, did you cast or drill some holes in the supporting slab? This allows the escape of moisture from the underfloor insulation, which is the hardest to eliminate. If not it can be easily performed by drilling from underneath.

[Tobylav]

Thanks very much, David - that's really helpful.

You have a very deep tunnel in front of the oven chamber which will make working the oven difficult.

Agreed, although it's not as bad as my (poorly drawn) MS paint examples. I have attached a photo of what it actually currently looks like.

You could pour a 1” castable or homebrew over the floor insulation before loose laying the 1” firebrick tiles over it, but it will crack so some thin 6mm round rebar set into the middle of this layer is advisable.

Thanks, I might give this a try.

I think a 6” diameter flue pipe is overkill for a 19” oven, a 5” would be adequate. The 6” may introduce too much heat loss.

Unfortunately the only short ones I found (without an astronomical price) in Australia when I bought it were 3" and 6" - so I went with the bigger. Hopefully that doesn't cause a big issue.

Did you provide a rebate in the oven chamber on the inside of the flue pipe so you can isolate the flue when doing retained heat cooking.

Yes, but I am not sure it will be a 100% snug fit - closer to 97% (ie may be small gaps). This was done when I made the mould months ago at the very start of the process, and suffice it to say there were obvious areas for improvement at this stage. But the plan is to have a pre-flue door that closes fully (or as close as possible...) and a mouth door that sits on stands at ~1/3 of height of mouth (so the door covers the top 2/3 of the mouth, but allows cold air to come in the bottom 1/3). Unsure if that will work, but it is the plan!

The 10:1 vermicrete over the dome is equivalent insulating value to blanket but needs extended drying preferably in sun and wind.

Perfect, I'll let it out for the Australian sun to tear through!

Finally, did you cast or drill some holes in the supporting slab? This allows the escape of moisture from the underfloor insulation, which is the hardest to eliminate. If not it can be easily performed by drilling from underneath.

I haven't yet done yet (as I haven't done the slab), but I plan to. I have read a couple of threads on this, and understand that I am meant to drill a couple of small holes through the slab, but not perlcrete or ceramic board, and cover that with insect screen. Is that right? I recall reading somewhere about potentially a very small pipe, but I don't remember that (nor do I have the inclination to try to find such a small pipe if I can avoid it!).

[david s]

Thanks very much, David - that's really helpful.

Agreed, although it's not as bad as my (poorly drawn) MS paint examples. I have attached a photo of what it actually currently looks like.



Thanks, I might give this a try.

Unfortunately the only short ones I found (without an astronomical price) in Australia when I bought it were 3" and 6" - so I went with the bigger. Hopefully that doesn't cause a big issue.

Yes, but I am not sure it will be a 100% snug fit - closer to 97% (ie may be small gaps). This was done when I made the mould months ago at the very start of the process, and suffice it to say there were obvious areas for improvement at this stage. But the plan is to have a pre-flue door that closes fully (or as close as possible...) and a mouth door that sits on stands at ~1/3 of height of mouth (so the door covers the top 2/3 of the mouth, but allows cold air to come in the bottom 1/3). Unsure if that will work, but it is the plan!

Perfect, I'll let it out for the Australian sun to tear through!

I haven't yet done yet (as I haven't done the slab), but I plan to. I have read a couple of threads on this, and understand that I am meant to drill a couple of small holes through the slab, but not perlcrete or ceramic board, and cover that with insect screen. Is that right? I recall reading somewhere about potentially a very small pipe, but I don't remember that (nor do I have the inclination to try to find such a small pipe if I can avoid it!).

Re the rebate for the door to sit against, of course you don't use the door when cooking pizza. You need to have an active fire on the side, watch the pizza cooking and have decent access to feed that fire with small wood. You only require a door for retained heat cooking ie roasting or baking. The Italians used to use a wooden door and soak it in a bucket of water to reduce charring, before placing it against the oven mouth. They'd typically seal around the door with some excess bread dough. Most builders these days make an insulated door. I've gone with the wooden door faced with a cast insulating panel.

The second outer door you speak of is often referred to here as a "blast door". By reducing the air intake the atmosphere in the chamber achieves a more ideal fuel:air ratio and the oven will roar away with an extremely rapid rise in temperature. Personally I think it's a mistake to rapidly fire the oven. As every potter knows, too rapid a temperature rise is damaginging to not only the wares in a kiln but also to the refractory of the kiln itself. The difference in temperature between the top and the bottom of the oven as well as the inside and the outside leads to uneven thermal expansion. As a guide most potters restrict temperature rise to 100 C/hr, to prevent damage to the wares inside. By contrast most oven owners fire to around a rise of 300C/hr. Any minor fractures that will not be initially visible get gradually worse if too rapid a rise is continually used. As dense fire bricks or refractory take approximately one hour per inch for heat to travel through them most oven owners take a couple of hours to get the oven up to pizza temps, but only one hour for baking or roasting. Alternatively fire higher and wait for the temperature to drop.

If you haven't cast the supporting slab yet then it is better to cast in the holes which is easier than drilling later and avoids the possibility of drilling into the reinforcing. I use some 1cmx 1cm foam which I place in the slab after screeding it off. I dig a hole in the wet concrete with my finger, place the foam in position, then trowel neatly around it. It is very easy to remove when the concrete has set, I then glue a bit of insect screen on the upper side.