600mm oven cast over sand - France

[david s]

Some people think a small oven will heat up faster, but as the fire in the chamber is smaller it usually takes around the same time. The wall thickness is a more determining factor. My ovens take around 1 hr to begin to clear. If roasting or baking that’s all I give it. For cooking pizza it’s completely clear in 1.5 hrs. If i’m cooking pizzas for a big crowd I give it 2 hrs.

For doors go to the “show us your door” thread.

[thargog]

hey david - from memory one of your ovens is roughly the same size as mine - 540mmm diameter - i know the materials are different but i just wondered... once fully fired and you are able to bring it up to pizza cooking temperature, roughly how long does it take for you to heat it up?

also, does anyone have any door-making tips..?? I've been resting some of the ply up against the outside whilst beginning small fires and it seems very efficient at helping the chimney to draw - am thinking a wrap of alu foil isn't such a stupid idea in the short term...

[thargog]

Got you. Thank you. Will report back...

In the meantime, thinking ahead, a quick question to anyone who has made a door... I have some lovely pieces of ready-shaped marine ply (from my formers for the moulds) which, with some sanding and cleaning up, would be a great base for a door. Is it completely stupid to say I could use these, lined with perhaps three layers of some really thick alu foil that I have, as a door?

Or I could make a temporary mould and face the ply with refractory mortar? or vermicrete? (although that may be too crumbly..!)

Or of course I could find someone to cut me a nice piece of stainless steel...

Any observations on appropriate solutions?

[david s]

If you render over damp vermicrete you will be locking in the moisture. Allow it a minimum of a week to dry before doing any drying fires. Render after the drying fires are completed.

[thargog]

Whilst I don't want to rush things (haha it's taken me YEARS to get this far!), I know the firing will be long and slow. Is it ok to render after two or three days when the vermicrete has firmed up and then wait the week..?

Or are you saying vermicrete, wait a week, render, wait a week, commence firing?

[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.

[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]

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]

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...

[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.

[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]

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.)

[thargog]

Cheers will report back

[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]

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..?