28" homebrew cast oven in walled enclosure Belgium

[david s]

The rated payload is overstated and includes the tray body. My Mazda one tonne utility truck is severely down on its springs with 1/2 a m3 of aggregate or sand which weighs around 750kg. 1/4m3 which weighs around 380kg is a comfortable load.

That roll of AR fibreglass mesh could be the answer, but being flat will not conform to a compound curve. You would need to do lots of cutting and ensure it’s embedded . The beauty of random fibres is that you just mix them into the wet mix, then the whole job can be done in one go, rather than two or three coats. It’s a huge time saver.

[MarvinG]

1. I see - I'll go with rather washed sand then. I can have washed (black) or unwashed yellow here.

2. There are silicones out there wich (they say) hold up to 1200 degree celsius. But yeah, mortaring it to the floor makes sense.

3. I think you might be wrong. These things are quite tough. Official payload is 350kg - so that will do. I will not be able to race - but it only needs to move - how fast, not very relevant in my case.
But its a good thought - maybe its smarter to tow - and have a trailer - then I do not need to compromise in weight, especially I want a thick 6cm oven floor.


regarding the AR resistant fibre mesh/strips. I was just thinking: is that not what builders use on the walls - to reinforce concrete plasters - for them not to crack. that's very cheap... :-)

[david s]

1. I use silica sand because it has sharper grains which should give it better grip in the matrix surrounding it. River sand tends to have much rounder grains. The downside is that silica sand has higher purity, being nearly all silica, but as all sands are pretty much all silica I don’t think it makes too much difference. Perhaps the river sand has impurities that could cause problems. I took my cue from the brick builders who all swore by the homebrew as a mortar, using really thick joints on the outside with apparent problems and years of trial with little error. I therefore could see no reason why the same recipe couldn’t be used as a castable mix.

2.There’s no heat resistant silicone that will withstand the sustained oven temperature that I’m aware of. I use some between the flue pipe and a terracotta weather cap near the base of the flue pipe. It is rated to 371 C (intermittent) certainly not enough for the hot oven interior. You may find something more suitable, but dampening the floor bricks a little before applying the castable against the sand mould is sufficient to get a decent bond.

3.That little HiJet truck will not hold or pull the weight of e 28” oven. My mobile oven is only 21” internal, weighs 250kg and my 2600cc Mazda truck pulls it comfortably but a 3 cylinder700cc certainly wouldn’t.

[MarvinG]

Great answers - super informative. Thanks so much!

summing up...

- I will try to find the AR resistant fibres. (otherwise buy them online abroad)
- I will take homebrew - use the insitu ball/sand dome way. (its not worth to drive to turkey 500 miles) just for expensive concrete. ;-)
- I'll add the PP fibres
- Sure - the tires are made for this... durable but still flexy.
- I will try not to overdue the heating - however i will def. have 500°plus where the flame hits the ovens. (since I will use gas as well)

Therefore two question I still have:

1. Since sand seems to be the "bad guy" when heated up above 571°. My thought would then be - is there an alternative product instead of sand to avoid this issue? Does it matter which sand we use?
2. When I do the casting - Ill do the dome and I guess the vent separately. I intend to "glue" to dome to the oven floor with a heat resistant "silicone". makes sense, right?

I just had a look to find some pics of my second build. Unfortunately unsuccessful. they must be on an old mobile phone.

For my new project I will def. make a new thread when I get started and share the progress. Now I'm looking tu buy one of these cute things :-)

[david s]

Because I manufacture I use a proprietary castable refractory, but I believe the extra durability it might give is not really worth it as the material is really expensive and contains ingredients that are designed for much higher temperatures, in excess of the temperature range to which we fire. The homebrew is almost as good as the far more expensive products. All reports on the homebrew longevity say it holds up very well. So cheap and easy to rebuild anyway. The vulnerable temperature range when thermal expansion of most materials peaks, is from 500C-650c should be avoided to prevent damage. Quartz (silica) does undergo significant and sudden thermal expansion in the 571-576C range and as all sand is primarily silica this is either a reason not to use sand as an aggregate or alternatively avoid the higher temperatures. So never fire an oven as hot as you can just to see how high you can get it. Only go as hot as you need.But this advice also applies to all ovens as a variety of different materials are used and the different rates of expansion can be incompatible. See attached graphs as an example of this. Mineral composition.docx

Interesting about your tyre solution to mount the oven on. I have been using that system for years when freighting oven kit parts. I place a motorbike tyre on a pallet, then assemble oven components on top. I’ve never had any breakages in transit.

On using fibreglass, you must get the AR (alkaline resistant) ones. They are coated in zirconium which is expensive, but the coating prevents chemical attack from the cement. They are widely used by the concrete countertop manufacturers so search there to find them. Glass doesn’t melt until sround 900C so fibreglass fibres are suitable for our temperature range as long as they are the AR type.
The other type of fibres that are probably more important to add are the very fine polypropylene fibres that melt at 160C leaving behind a network of fine pipes through which moisture can escape. This provides a measure of protection from steam spalling whe the oven is completed and firings begin


I love your photos, post more please.

[MarvinG]

Thanks for this input David! Highly appreciated as always.

Yes, the oven (with a similar bottom construction as done before) will probably sit on (lying) old tires which are placed on the pickup. To constantly shock absorb.
The floor will be bricks, sitting on cermaic boad.
So if I got you right - you recommend not to use homebrew - but use refractory cement instead, right? (cause its nearly impossible to buy this here) I'd have to go over the boarder - which is ok, but take some time.
And yeah, I read most of your threads of course where you explain plenty of details! :-)

Does anyone have experience with this fibreglas to make things stronger? I was thinking of doing this:



with this material. either Glass fibres or Carbon fibres.

[david s]

Some manufacturers make one piece cast domes specifically for mobile application. The problem is that any large cast section is prone to cracking com uneven thermal expansion. My first mobile oven built in 2007 was a one piece dome sitting on a one piece cast floor, both castings were of proprietary castable refractory. I lasted around 8 years. The floor started spalling and the dome developed a vertical crack at the back, right opposite the oven mouth. You could fit a plying card in the crack, but it never got any more severe and did not affect the oven’s performance. I have seen similar cracks in two other one piece cast ovens in exactly the same location. I’m actually now on my third mobile oven. There have been reports of brick mobiles rattling to bits very quickly, so don’t go down that route. A 3 piece cast is probably a good compromise and what I now have. All mobiles don’t like being moved. Make sure you fit shocks on the trailer and always avoid unsealed roads. Regarding the floor, it is best to stick to fire brick because the floor takes a real beating and subject to a lot of uneven thermal expansion as well as extremely rapid temperature rise.

[MarvinG]

Hi there!

What a great thread and build! Some might remeber, I built two dome ovens (on the picutres) and run now a small pizza place in tbilisi.
I am about to build an oven on a key truck.

Is the assumption correct that the oven will be better/more durable on the ride to cast 1 piece in situ with ball/sand instead of building a brick oven?
I know that a cast with 3 pieces is probably best, but that is not yet reachable with the time available.
So, is it not possible to use some more reinforcements? What I was therefore wondering is, if one cannot reinforce the homebrew with glass fibre strips.
or would it be smarter to use a thin refractory cement base and then homebrew on top?
or could you even soak a thin theramic fibrewool planket? just for strength of course.
What do you think?

[AJH]

I also wanted to get more insight in my fuel consumption, so I weighed all my firewood before and after use on 2 occasions: I use 10kg of oak for 12 pizzas, and 9kg for 10 pizzas.
This includes starting the fire and getting the oven up to temp obviously.

I can see myself doing this after I get my oven built =) At least enough to get a sense of how much wood it takes to heat up and keep making pizza's for a few hours...

[david s]

I have a couple of finger sized sticks preheating on either side of the entry so when thrown in behind the flame deflector they burst into flame almost immediately. This makes it easy to keep the fire going continuously.

[Kris S]

Dear David, i used the pictures of your heat deflector at the top of this page as inspiration to make mine.
The SS is at least 1mm thick.

[david s]

I also use one of these, it is particularly useful for small ovens and maximises oven floor space and reduces burning the pizza edge that’s too close to the fire. For years I used a prototype made from 0.5mm stainless, but it did warp badly from the heat, so I had a bunch laser cut using 0.9mm stainless with holes in the front face to allow some heat through.

[Kris S]

The heat deflector works like a charm no more fire rolling onto pizzas, and less quick burning of the crust as well !



I also wanted to get more insight in my fuel consumption, so I weighed all my firewood before and after use on 2 occasions: I use 10kg of oak for 12 pizzas, and 9kg for 10 pizzas.
This includes starting the fire and getting the oven up to temp obviously.

I have no idea if that's much or little. anyone else ever try to do this?

[Kris S]

It’s more a case of just wanting to try and see if it makes a big difference. I’m not having any draw issues either, (no smoke ever escapes through the oven mouth, except for 5 minutes when igniting the fire) but I think it may help prevent small logs rolling off the fire and into my pizza!

I might make a smaller version, as my internal diameter is only 28” – 70 cm.

Rebar should do, okay – Thanks!

[JRPizza]

It should hold up OK. I made a fireplace grate out of rebar that has survived several seasons. I was going to build a log holder like you are showing, but the Pompeii designed ovens draw so well (at least mine does) that I can't justify the space it would take up when I was not firing the oven.