Mobile Community Oven Build In UK

[badrobot]

Hello all. I am looking for a bit of feedback around using/incorperating 10-25mm fire bricks (cut by me) to line the inside of a cast pizza oven self build. I am thinking of using the gym ball method.

I have a pretty reasonable understanding of the way I need to go about it for the desired result, yet have this developing idea that I would really like to incorporate, so thought I would ask for some input.

I would like to give the cast method some of the thermal mass qualities of the Brick oven method, though my project requires me to make it light yet robust (and costs to an absolute minimum) hence the gym ball/cast method, I already have most of the materials: including the ball, though I really would like to add the thermal mass and aesthetic quality of the inside of a brick oven to the project though with a total weight restriction of 650kg (Brenderup 1150s trailer) I can not afford the full weight of a full brick version but figure that in theory as long as the fire brick (tiles) are cut and packed neat and close together inc the keystone, the rest of the cast method should follow with no problem. Has anyone already done this? Any reason why it wouldn't work? Hope all this makes sense, does to me hah.

Any relevant thoughts/questions welcomed.


Many thanks

[david s]

I think you might be headed for trouble.
If it’s a trailerable project then any oven brick or cast is subject to bumps and vibrations that heavy brittle material does not like. An all brick oven is likely to rattle to bits. Some manufacturers offer a one piece rather than multi sections for trailerable ovens for this reason. Generally a 50 mm casting, well insulated is quite sufficient thermal mass. Whereas a 50 mm thick brick dome has insufficient thickness for the bonding of the bricks, stationary or mobile. I’d be concerned that a marriage between thin bricks and casting, whilst probably ok for a stationary oven, would have too much inherent structural weakness for longevity. But I’d love to see you try it and report back.
Regarding the gym ball idea, because it requires a fair amount of formwork and necessitates removal, lifting and replacement of the heavy cast dome, it will be way more trouble than using the sand mould method where the dome can be cast in situ.

[badrobot]

I think you might be headed for trouble.
If it’s a trailerable project then any oven brick or cast is subject to bumps and vibrations that heavy brittle material does not like. An all brick oven is likely to rattle to bits. Some manufacturers offer a one piece rather than multi sections for trailerable ovens for this reason. Generally a 50 mm casting, well insulated is quite sufficient thermal mass. Whereas a 50 mm thick brick dome has insufficient thickness for the bonding of the bricks, stationary or mobile. I’d be concerned that a marriage between thin bricks and casting, whilst probably ok for a stationary oven, would have too much inherent structural weakness for longevity. But I’d love to see you try it and report back.
Regarding the gym ball idea, because it requires a fair amount of formwork and necessitates removal, lifting and replacement of the heavy cast dome, it will be way more trouble than using the sand mould method where the dome can be cast in situ.

Hi Dave , Thank you for your feedback. Trouble ya say....

The casting of my own only intends to replicate the likes of which can be standard fare for any mobile set up really, 50mm for inner part -50mm for the insulating outer seems a reliable and an appropriate one in my case regards weight/mobility. And the shop bought ones are only pre cast in parts also btw.

I had planned to create a inner casing dome using refractory cement around 50mm thick + maybe melt extract fibers? I read on hear that you might be the person to ask about the latter. (60mm with said brick lining) and once cured another 50-60mm layer of sand/cement/vermiculite + something akin to straw to bind mix as the insulating layer to finish. Maybe then some sort of tiles if I can afford the xtra weight.

In response to the gym ball method, I am a carpenter so a bit of form work is no problem, and there is no need to lift, remove cast dome as you can just pull the nozzle out of ball to deflate once its at least half way to curing (around a couple of weeks) once it cures it doesnt seem to be an issue, and I can always get some help to manouvre. I am thinking placing all cut brick lining on gym ball then carefully building up cement from bottom. I also figure their should also be some intrinsic strength gained from the regulated shape of the hard round gym ball not disimilar to the residual strength of a well built (arch) seen in Pompeii brick ovens, Something inherant in a catanery arch which is partly why I chose it over the sand dome method, but I totally share a concern regards the possible vibration from trailer. (as I understand melt extract fibers help with thermal shock but will they add strength you think? I am no engineer :/

[david s]

I’m unsure of the finer detail of what you are planning. “ 10-25 mm firebricks” does that mean some are 10 and some are 25? and 50 mm inner dome thickness or 60 mm? You can’t be sure about the thermal expansion of the firebricks and your castable mix at different temperatures which could be problematic. What kind of castable are you considering? Will it be a proprietary castable refractory or refractory cement with added aggregate? Melt extract fibres are the recommended reinforcing for castable refractory and will increase strength if added a min of 2% by weight of dry castable. If you go above 5% it makes the mix less workable and as they’re called needles so you need to learn to handle them carefully.

"the shop bought ones are only pre cast in parts also" That is true and apart from making handling the pieces easier, making assembly easier it also provides for thermal expansion. It's also true that some manufacturers provide a one piece dome for mobile use as it's less likely to rattle to bits.

Adding sand to an insulation mix will add needless weight and drastically reduce its insulation value (see table attached).
Re moving the dome, you need to do a calculation to work out it’s weight. You don’t say the oven’s size, but a small increase in diameter produces a big increase in volume and weight.

The beauty of the sand mould is that you can cast the dome on top of the floor right where it’s going to be, no moving.

Concerning catenary arches, of which I am a big fan, unfortunately although their stability is superior to any other arch, to get one low enough for a pizza oven, produces unusable space at the floor level because of the walls being at a lower angle in that area.
The hemisphere, as 2000 years of Roman ingenuity has proved, is a far more suitable form.

[badrobot]

I should probably stay realistic on what I need to achieve here which is to establish a mobile wfo, (it may be that just buying one in might be the better option at the beginning)

That said I would still appreciate any advice, guidance on a developing a sound method as I will still explore my own build using a castable I make myself. 1 part refractory cement/5part vermiculite+SS Needles (which I am yet to purchase) I may forgoe the cut brick idea at this stage until I have the time/funds, it was just a thought. Definitetly something to experiment on at some stage. Do you think it is possible to find a proprietary castable refractory cement/brick combo that might work together?

The working space/ internal trailer sizes are: L1400xW900xH500 the top half of the trailer.


So to summarise, I figure the oven being around approx 850mm external at the base with a max height of 420mm, allowing space for plinth, with room for an arched entrance depth of around max 400mm.
the main dome of 60mm thickness, consisting:
1 part refractory cement
5 parts vermiculite
2-5% SS needles to add strength
ceramic fibre layer of 1-2inch
all tied in with wire/chicken wire
a scratch coat, to give final layer a key
then a final layer of render (not quite sure of most appropriate thickness of render for this build?) or what to best make this layer out of?

Do you think I am on the right lines?

This is an example:

This oven has the attention of detail I liked and would love to recreate (great open flue design/heat retention/herringbone brickfloor/build quality) sadly the spec/cost are too big for my requirements alongside shipping costs from states/australia.

https://www.melbournefirebricks.com....ired-oven-kits

I am totally willing to be educated/pointed to other places to swat up further. Do you know of a good reference point regards making a 'small' mobile oven. I had put those figures 10-25mm and 50-60mm as a guide initially as I was not set on sizes.

Hope this all makes sense. Thanks David

[david s]

A 5:1 vermicrete is not a dense refractory layer so will do virtually nothing in regards to contributing to thermal mass. As it is an insulating mix it will be resistant to absorbing heat as well as being unable to store it. If you look at the table I left in my last post you will see that a 5:1 vermicrete has very low strength (around 200 psi compared to a proprietary castable refractory of around 4-5000 psi see CCS cold crushing strength for the product I use, on attachment). Stainless needles will do little to strengthen such a weak mix. Other builders have replicated the gym ball/ vermicrete domes, but none have offered performance in the long term, even after invitations to do so. i suspect they all fail due to the poor strength of the inner dome which apart from thermal expansion tensions, is subject to knocks and abrasions. Do plenty of research before proceeding, or build a real cheapie with the intention of a rebuild once you learn its deficiencies. Suggest you start searching this site for mobile ovens and also look at home-brew castable. As you're in the UK also try their forum ukwoodfiredovenforum.

TDS - CASTFLO AG 1450 ARG.pdf

[badrobot]

Thanks Dave....It seems that many take this initial approach only to regret it later (5:1 vermcrete). I did see the uk forum you mentioned, I will check it out again thanks, I see you are also part of that uk forum? I love your set up and ingenius method of fixing it down/mounting to base and then trailer. Is that on rubber feet? Is refractory cement/hydrated lime and SS needles the way to go then you think? ifso what ratio would you recommend as aiming for the similar set up as yours, (the one with flames on side.) I thought adding lime was toxic for some reason. The research cont.

Regards Rob

[UtahBeehiver]

I agree with Dave on usage of V or Pcrete as your core dome cast material. There are some questionable YouTube videos out there advocating using this mix as the main dome structure. We see people posting to this web site who want to head this direction and we try to explain that this mix is not structurally sound enough and should be used as a insulation material only. We very rarely see follow-up from these builders on the status of the project.

PS Follow DavidS's advice, he does cast oven commercially in Aussie land.

[badrobot]

Hi, Yes I have just joined this, and another forum regards not falling prey to avoiding such shoddy application/s. There are so many strange videos. I have certainly taken on board what Dave has imparted for sure. I know sound advice when I hear it.

And freakin WOW, your project is dope. I mean really wow. I was so excited and impressed looking through album documenting process, And the copper icing on the cake! Real tight craftmanship. Have you got verdigris burns yet-lol. Had you intended/considered this? (I think that sort of patina will only add to the beauty btw!)

Regards Rob

[UtahBeehiver]

Thanks for the compliment. All the copper shingles were hand cut and bent by me using tin snips and hand bender. I am just now starting to see some green patina (built in 2012) and will be pushing up tulips by time the whole oven is green.....lol

[david s]

Russell, if you want that green look you can accelerate the reaction by painting it with white vinegar. Another trick to give concrete sculptures an aged look is to paint them with yoghurt.

[david s]

Thanks Dave....It seems that many take this initial approach only to regret it later (5:1 vermcrete). I did see the uk forum you mentioned, I will check it out again thanks, I see you are also part of that uk forum? I love your set up and ingenius method of fixing it down/mounting to base and then trailer. Is that on rubber feet? Is refractory cement/hydrated lime and SS needles the way to go then you think? ifso what ratio would you recommend as aiming for the similar set up as yours, (the one with flames on side.) I thought adding lime was toxic for some reason. The research cont.

Regards Rob

Rob,
I use castable refractory which is a blend of CAC (calcium aluminate cement), high temperature aggregates and burnout fibres which assist in water elimination during initial firing. The stuff is expensive and you need to do a calculation of how much is required. As it’s rated to 1450C it uses high temperature aggregates that are way in excess of what we require. The use of lime in conjunction with CAC is a no no because it acts as an accelerant reducing the already limited working time of the mix. The stuff is also very temperature dependant. There are recipes out there (including this site) that recommend CAC and lime together. Unfortunately those that attempt it will find their mix going off as soon as it’s been mixed.Another alternative that works (just) for our temperature range is what we call homebrew. It is used as a mortar for brick builds and can be used as a castable. It is 3:1:1:1 sand, hydrated lime, Portland cement and powdered clay. Very cheap and good working time, but the lime is vicious on the hands, wear rubber gloves. It also requires the addition of the burnout fibres which can be the really fine polypropylene ones used in reinforcing concrete. They are finer than human hair and take extensive mixing to disperse in the mix properly. I also use them for adding to mortar if brick building as they provide some measure of reducing steam spalling wher mortar joints can be extremely thick.
Regarding my mobile, it sits on a timber trolley that has 6 wheels that enable me to roll it on and off the trailer easily.
On the subject of information sharing, most of us here are old farts and well beyond wanting to keep secrets in order to build empires. Rather, we are excited by the enjoyment of cooking by fire in ancient yet highly efficient designs and we want to “spread the love”.

Dave

[UtahBeehiver]

I heard about both of those aging techniques as well as peeing on the copper too, neighbors would turn me in, haha

[david s]

I heard about both of those aging techniques as well as peeing on the copper too, neighbors would turn me in, haha

When an American is “pissed” he’’s usually pretty angry.
When an Australian is “pissed” he’s usually pretty happy.

[UtahBeehiver]

That's a good one, so I will remember that next time I am in Aussie land