600mm oven cast over sand - France

[thargog]

Bonjour encore...

I can't believe it's two years since I wrote my introduction here https://community.fornobravo.com/for...uild-in-france and received many helpful comments from other members. It's been an odd couple of years and progress has been slow to stopped for great chunks of time. Well I'm at the point where I am pretty much ready to start the actual build and thought I would document my successful (and not so successful) steps here.

I finally got a local ironmonger to make me a trolley/stand for my oven (pics). He also threw in a 3mm steel plate for me to use as a base. The idea of having a non-fixed base was so that if I ever needed to relocate it, I could employ a few friends to lift the top off and place in a trailer leaving the base separate. A more pessimistic addition could be that if I suffer COF (catastrophic oven failure) I could assemble a new oven and re-use my stand/trolley.

I am planning to build a small (600mm ID) dome over sand. 50mm castable refractory with ss needles, 50mm ceramic blanket, 75mm vermiculite cement coat, covering waterproof coat. To be built on a base of 3mm sheet steel, 100mm vermiculite cement, 50mm (or 55mm) refractory bricks.

I am the opposite of Mr DIY, so I have a number of starter questions - some are just seeking confirmation(!), for others I would welcome people's opinions.

1 - Base
Do you think this 3mm sheet metal base is usable/sufficient? I was planning to drill a number of holes to assist drainage/drying of a 100mm vermiculite cement base. Do you think I need rebar? or could I maybe drill extra holes and secure in a load of upward-facing 75mm bolts? (I want to do a few of these anyway so that the bolt heads below the plate ensure it is not possible for the whole to slip on the trolley, although with the weights involved this may be overkill!) Should I just create a temporary wooden form to cast the vermiculite or do you think it would be better to mount permanent wooden edges? (I was thinking temporary, and then maybe tile the cement faces afterwards.)

2 - Door, entry galley and flue
I am planning to cast the entry arch as a separate piece - 190mm high for the flue (63% of internal height). What width do you suggest for the entry? I guess it needs to be at least 350mm to be able comfortably to work a 30cm pizza - is 400mm too wide? The depth of the entry arch I want to keep as short as possible, mainly dictated by the width of the flue. Here in France we have readily available 80mm or 150mm flues. I recently picked up an extremely cheap 1000mm length piece of double walled flue - internal 80mm - (pic) at a car boot sale - if 80mm is sufficient for my small oven, I'd like to use this. I could then permanently mount a small section of 80mm flue to the oven, and make this 1 metre length removable, meaning I can throw a tarp over the structure in winter.

3 - Casting
Vermiculite (or perlite) to cement - recommended 5:1 for the floor and 10:1 for the dome - silly question but this is by WEIGHT isn't it..?

There will be more!

Any comments greatly appreciated.

Rik

[david s]

Hi Rik,

Your plans look pretty good, but those tiny wheels on the stand look somewhat inadequate to me. Your oven will weigh in the region of 300kgs and the very small wheel diameter may present problems running over tile joints. The steel plate for the base sounds adequate, although it will be subjected to heat and moisture which are major contributors to corrosion. I like your idea of the bolts sticking into the vermicrete insulation for location, although with so much weight on top it is hardly necessary.
It is best to cast the dome first, then cast the gallery ib front of it so you can create the rebate for the door. For a small oven there is more heat loss from the door opening, although a wide opening gives far better access. For my money a wide opening is better. You can easily retire a small oven but you can't increase the door width once it's built.
My door opening is 400mm gallery opening 440mm wide and actual door 430mm. I like it, good access.
80mm diam flue is too small, you'll have tons of smoke out the front, you need to go to 150mm. Mine has a 150mm but my oven diameter is only 540mm It works really well.
Your idea of vermiculite or perlite for the dome casting is a poor one. Even a small amount of either will drastically reduce the strength of the dome. Keep the casting dense and strong to store heat like a battery. Use vermiculite or perlite as insulation only. The ratios are by volume not weight. Interestingly even a !0:1 vermicrete mix has more mass from the cement than from the vermiculite.

Dave

[thargog]

Hi Dave - thanks for the comments.

The wheels do look a bit small in the photos but they should be fine. They're 100mm diameter heavy duty double bearing, and the set of four are rated to 700kg so I reckon they should cope. Luckily my terrasse tiles are reasonably well laid and level..!

Thanks for the comments re the door dimensions, flue and ratios - your door sounds wide for a 570mm oven but hey if it performs well, that's great news for me! Not such good news is I wasted a whole 10€ (!!) on the flue pipe. Oh well it was worth a try - I'll take it back to a car boot...! Re the ratios - cheers - it has reminded me there isn't such a thing as a silly question.

My casting question about vermiculite/perlite was indeed just for the insulation layer. (Here was my layer 'spec': "50mm castable refractory with ss needles, 50mm ceramic blanket, 75mm vermiculite cement coat, covering waterproof coat.") The castable refractory with needles will indeed be the inner ("dense/strong") layer.

Do you have any opinion on the subject of the need for strengthening the vermiculite base? Either with rebar or a good number of upward pointing bolts? I guess my worry was if I ever did get three burly blokes to help me lift it, whether the fact that the steel flexes ever so slightly would cause any issues or whether everything would be so glued together by that point that it would not be a concern.
And also whether it would be advisable to drill holes in the steel plate (from reading other threads I believe this would just aid the drying process rather than for any longer term gain...)

Cheers

Rik

[thargog]

Colder weather looks like it's over, so I'm ready to proceed.

Quick question - I've cut the firebricks ready to place on the vermiculite&perlite slab; am I better off placing these and settling them just after making the slab (i.e. whilst it has some 'give'), or should I let the slab go off and then place? On a bit of sand for levelling?

[thargog]

OK well I went ahead and cast the slab. After much further reading, I've decided to set the bricks on the slab with some 'mortier refractaire' to aid levelling... Pics when I have time.

[david s]

The floor bricks are better laid loose to allow for expansion as well as easy replacement if needed. However as you’ve already done it don’t worry it’ll be ok. You can build the casting on top of the floor or beside the floor. The cuts at the perimeter of the floor bricks can be a bit rougher if the dome sits on the top. This is easier but makes it almost impossible to replace a floor brick sitting under the dome. This is rarely an issue because any failure of the floor bricks always occurs near the centre.

[thargog]

Failure as in cracking? In my searches I thought I'd established that failure (and therefore replacement requirement) was extremely unlikely in domestic ovens but may occur over a long period of extended use in commercial ovens, hence I wasn't worrying too much. Which I think is what you're saying..!

My firebricks are disappointingly rough. I've seen pics of lovely flat snug bricks with an almost polished appearance - not available near me :-( So I'm planning on taking a belt sander to them after I've laid them. I'm going to lay bricks for the dome area and the beginning of the galley area but until I cast both (galley to be done separately) I'll wait for the final bit of floor because I'm thinking of putting something like a piece of granite as the landing area outside where the door ends up. I think it'll look good and be easier to clean than these rustic things.

As always, any comments much appreciated... I know these photos are only of a vermi/perli-concrete slab but it's the first time I've ever used cement so I'm happy it even looks slab-like!

[david s]

The two main problems of floor brick failure are spalling and cracking. A crack is not necessarily a problem, after all it’s just another joint which fills with ash anyway. Spalling is more of a problem if tiny bits start breaking away.
Be careful of using granite as it doesn’t like heat. Because of this I’ve stayed away from using it. Some others have and may report on how it’s holding up.
Try to expose your vermicrete slab to as much sun and wind as possible. As it’s on a trolley you will be able to wheel it around. You may also be able to weigh it periodically to measure the water loss in the slab.

[thargog]

Thanks david. Plenty of sun down here now. I read that you can start laying the bricks / building the dome a day or two after the slab has been poured, but surely longer exposure to the elements is better?

In any case it will be next week before I can do the next step.

Any granite users out there? Any comments on its longevity, or indeed any other materials successfully used for attractive, smooth, easy to clean landing spots?

[david s]

Read the attachment on vermicrete slab drying, it may help you understand.

Vermicrete insulating slab PDF.pdf

[thargog]

Wow ok thank you - so clearly longer is better - "patience is a virtue" - etc. My slab is 96mm thick and sitting on a metal plate (so no opportunity to expose the underside etc etc). It makes me think I should have cast the slab a month ago despite the colder and wetter weather. I will leave for a couple of weeks at least until the next step. Even when I lay fire bricks and build the dome, the slab will be exposed for a portion of its surface and all edges, at least for the time being, so will have to continue to dry out via those surfaces. At least we have some lovely sunny days ahead.

Do I need to be concerned about the risk of an odd overnight shower (none forecast but they do appear) - i.e. should I cover it up, or now it's been sitting there for three days, will it no longer absorb more than it's losing (if you see what I mean)?

[david s]

Yes, cover it if you think an overnight shower is possible.

[thargog]

Cheers. Watch this space...

[thargog]

I laid the firebricks on a bed of mortar réfractaire today. Looking OK. There are one or two minor high spots to take down with some 40 grit belt sanding or maybe a disc.

I had a couple of questions (one definitely for david s)...

1 - when I come to cast the dome, i know to lay thin strips of wet newspaper over the sand dome to aid release (and possibly to stop sand adhering to the dome top and falling in my pizza). Is there anything similar recommended to protect the firebrick floor or do you just sweep it / vacuum it afterwards?

2 - david s - you mentioned in a post above that your oven is just 540mm internal diameter. The width of my base is 900mm (limit of the width of a trolley I can wheel around the side of the house). Originally I was planning a 600mm ID oven, but by the time I add 50mm castable refractory (700mm), 50mm ceramic blanket (800mm), that leaves me just 50mm for the outer vermiculite/perlite concrete insulation plus any waterproofing.
The only reason I was planning 600mm was it was smallish, and a round number. However I am thinking I could go smaller and increase the insulation... may I therefore ask what you use your 540mm oven for and how successful it is? I just want to be able to cook a 30cm pizza, one at a time - does your oven do that well?

Cheers

Rik

[david s]

First up, it is better to lay the floor bricks loose because the get heated unevenly and it allows them to expand individually. It also makes the replacement of any brick far easier. However don’t rip them up, what’s done is done, many builders have laid theirs in mortar as you’ve done.
The strips of wet newspaper are to prevent sand sticking to the castable as you describe. If done on a breezy day they may dry off and blow away. Some watered down PVA or flour and water might prevent this as well as a spray bottle of water to keep them damp once placed.
My oven sits on a 910 x 910 precast supporting slab and the oven internal diameter is 540 mm (21”) I generally cook one pizza at a time, but as it only takes 2 mins to cook I’m working at top speed on my own to prep, cook, cut and serve in that 2 min time. Parties up to 50 guests are doable, but it takes practice.