600mm oven cast over sand - France

[david s]

The trick is to get the correct amount of water into the mix. I’ve found it to be 3 litres for every 10 of medium grade perlite/vermiculite. The fine grade requires more, the coarse grade a bit less. When mixed there should be a tiny bit of water starting to pool in the bottom of the barrow. When applying it on the dome, start at the bottom and build it up to the required thickness, going all around the base, but leave a flat top that can take the next lot to sit on top. You can leave this to set for 24 hrs then come back and build it up higher. You’ll swear it’s too crumbly and won’t work, but it does. Sand will reduce the insulation value of the mix. A little powdered clay added to the mix will give it some stickiness to make it more workable. It gets easier the higher you go because it’s leaning in towards the dome.

[thargog]

But now I've hit a problem. I attempted to mix up some vermicrete tonight and had an epic failure. I have read about it being quite crumbly but it was crumbly to the point where it just would not stand up at all. I tried adding some sand (which I understand reduces the insulation properties) and a whole load more water, and as you can see I finally got a trowel-load to look like it might stay in place. But I'm holding off pending some advice here - am I doing something wrong? I just can't see it holding in place. Any suggestions to make it a bit easier for a very inexperienced cement person..!?

The aggregate I have is a vermiculite/perlite half anf half blend. It's not fine but it's workable.

The first pic is after I cleaned away the first attempt that was just like breadcrumbs.
The second is with more sand and water but I'm really not that sure...

[thargog]

[thargog]

So an update on progress... followed by some questions.

Thank you so much to... well... loads of people here - various threads gave me the help and inspiration necessary to come as far as I have.

The last few weeks allowed me to get from where I had the insulating slab and firebrick base all the way to having the ceramic blanket on ready for a final insulation layer. A few observations of my journey -

• Creating a quarter-circle former for a smooth and accurate sand dome was a genius idea. I think I originally found it in a thread by oven-builder cnegrelli but lots of people have done it, and it's great.
• I found a purely decorative chimney base in a french DIY shop - it's not for connecting to a flue, it's for aesthetics apparently... but the good news for me was it was about 8mm wider than the standard flue (150mm), so I created a hole that was about 135mm in diameter, set the bolts for the surround in the top of the galley part, and now my chimney section is completely removable (it just sits on the castable refractory within the chimney base), or if I choose to purchase a longer flue I can. Being able to remove it could be great as I can throw a tarp over the whole thing for the short winter here...
• My galley is 'connected' to the oven with a stove rope breaker.
• I thought the SS needles were going to be difficult to work with but they weren't. A few stuck through the newspaper into the sand and so I trimmed them off inside the dome where I could but TBH it won't interfere with pizza cooking.
• My castable refractory slumped. And it's not pretty. But (I'm hoping!) it doesn't matter. It's probably closer to 75-80mm thick around the edge of the oven, and I ended up with a blobby ridgey dome - but I don't care. It's MY blobby ridgey dome. And the other layers can sort that out.
• My newspaper strips stuck to the refratory to the extent that I can't get the damned stuff off. But I reckon with an eventual temperature of 400C+ it won't last long..?
• I cut the ends off some spare firebricks to make the dome. Again, not pretty but it will do me for my first project.
• I drove some 75mm screws into the vermicrete base at an angle when applying the ceramic blanket layers. This really helped as I could first attach tie wire to these screw heads and then do the same with the chicken wire. Really helped keep everything taut.

Some problems/questions next but in the meantime -

Here are some photos...

[thargog]

Thanks for the reassurance regarding the size. Yes one speedy pizza at a time is just fine..! I hope one day to be as proficient as you (oh and to have that many friends..!).

Did you have any observations about my question regarding the firebrick floor? Is it worth trying to 'protect' it at all or do you just rely on sweeping it clean?

[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.

[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

[thargog]

Cheers. Watch this space...

[david s]

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

[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]

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

Vermicrete insulating slab PDF.pdf

[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]

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]

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 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.