600mm oven cast over sand - France

[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?

[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]

[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...

[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]

OK Thanks david. Do you just mix by hand? I bought a sweet little cement mixer that was wonderful for the castable refractory but I'm thinking it might be overkill for this layer..?

[david s]

No, don’t use a mixer, it abrades the grains too much and it sticks to the sides and blades of the mixer. Better to mix in a barrow with a spade so you can inspect the mix as you go. Mix ingredients dry first then add 1/3 of the water at a time.

[thargog]

Cheers will report back

[thargog]

OK feel a little bit happier... I applied a low wall according to David's instructions (thanks again) and will wait for it to set before building higher. Didnt bother with any trowel or whatever - just put a glove on and carefully worked it through the chicken wire a bit and shaped it by hand. As reported elsewhere it feels so crumbly but time will tell... Should I wrap it in clingfilm or is that not necessary? Touching it at all feels dangerous..!

(On a side note, and again as covered elsewhere, don't do what I did and buy yourself a lovely new smoothing/skimming trowel in the hope of emulating the guy from Melbourne Firebrick Co - his vermiculite mix is smooth and sticky. Not like this at all.)

[david s]

If you want the mix to be strong and easy to apply you can make it denser, but that reduces its insulating capacity as well as increasing its cost. At 10;1 it is a pretty good insulator and it will set up firm enough to be a reasonable substrate to render against.

[thargog]

Well I got back tonight and it still looks very crumbly. I'm worried it's because it's about 32-35deg C here at the moment. I am shielding the build with a parasol but the ambient heat is still pretty high. I shall try a second level tomorrow morning.

[thargog]

This morning I found that the first batch (and therefore half of my first layer) was far too crumbly, it just fell apart when approached by a fingertip. The other part seems OK. So I've redone the first part. Think I'm getting the hang of the consistency - I don't want to have to keep redoing it all...

My next question - what is the 'strong render' to put on top? Just a sand/cement mix? I guess I can use 'enduit' - the external wall covering popular here in france...

[david s]

You can purchase a commercial sand/cement render mix or make your own(cheaper) using sand /cement/hydrated lime. I use 5 parts sand to 1 part hydrated lime and I part cement by volume. I also add alkaline resistant fibreglass fibres as random reinforcing, then trowel this mix on around 12 mm thick. An alternative to the AR fibres is chicken wire reinforcing but it takes ages as you are applying the mix over a compound curve. Keep the rendered coat covered to hold in the moisture for a week. This enhances the strength enormously. I wrap the whole oven in cling wrap.

[thargog]

OK thanks david. Did another wall of vermicrete this morning. A bit higher as it's beginning to slope away from vertical. And all clingwrapped up. One more day (possibly two)...

How long should I leave the vermicrete before doing the render? I trust not too long as I guess it's pointless firing the oven until that last wet layer is on and has cured for the week you suggest.

[david s]

Because vermicrete takes so much water in the mix there’s plenty of free water available for the hydration process, so there’s little need to keep it covered. What’s more important is to dry it out because fast escaping steam can be a problem. Depending on the weather it may look dry (white), but it will still be moist deeper in. You can get a cheap garden moisture meter to test the water content. I usually leave it a week of good sun and wind exposure.