After some feedback on my build thus far

[ChrisNazareth]

Hi All

Hi All
Im Chris from Australia and this is my first post
I probably should have reached out to the group before I started but anyway better late than never.
My build thus far.
120mm concrete slab on which i have 2 layers of 35 micron aluminium foil then 60mm ceramic blanket and then the fire brick floor.
the dome and arch are 65 mm refractory concrete over which I have 65 mm ceramic blanket then 2 layers of the same foil over which I have added 200 litres of perlite ( 5:1:1/4 perlite cement fire clay) I mixed it with a solution of water and 5% bondcrete.
It has now been sitting under cling wrap and plastic for 48 hours.
Should I start curing fires after 7 days or should I render first.
From what I have read the consensus seams to be cure before render, however I’m not sure if that only applies to brick ovens.
Has anyone tried to cure using a large lpg burner ?
Also the render mix I’m planning to use is 3:1:1:1/4 sand cement lime and fire clay with Sica 150 fibre mesh. Mixed with the water + bondcreat sol. Is this ok or is there a better recipe?
looking for any feedback on what I should be doing differently or what I should have done differently.
Thanks in advance and looking forward to hearing your feedback
Cheers
Chris
Dalmeny NSW Australia

[Pizzarotic]

120mm concrete slab on which i have 2 layers of 35 micron aluminium foil then 60mm ceramic blanket and then the fire brick floor.

Hi Chris, couple of observations from your post.
Ceramic blanket is not suitable for under floor as it will compress and loose a lot of its effectiveness. (perhaps you meant "board")
With the foil under the insulation, this could help trap water from above if the oven is exposed to weather so make an effort to waterproof the dome and also prevent water entry onto the oven floor.

Should I start curing fires after 7 days or should I render first. From what I have read the consensus seams to be cure before render,

The perlite will take some time to dry out depending on your temperature/humidity so I would suggest you do at least some curing before rendering. 7 days after casting the oven is too soon to start curing Portland cement mix in my opinion but refractory cement should be fine.

Has anyone tried to cure using a large lpg burner ?

Yes, it's a great way to do the initial drying. Some people worry about dying in a fireball explosion so if you are not familiar with basic propane safety then charcoal next best.

Also the render mix I'm planning to use is 3:1:1:1/4 sand cement lime and fire clay with Sica 150 fibre mesh. Mixed with the water + bondcreat sol. Is this ok or is there a better recipe?

That is more of a refractory type mix. A normal render mix will be fine or ideally purchase a waterproof acrylic render.

[Sixto]

would not recommend using fire clay in the render mix because it absorbs moisture, only use it as part of your home brew mortar or refractory casting mix to make it more workable.

[david s]

With 200 litres of perlcrete added, you will have mixed at least 60 litres of water with it. As that is more than double the amount that the hydration process will consume, you have a lot of free water to eliminate. So don't render until the perlcrete is dry. allowing the sun and wind to do much of the work for you is easiest, so unwrap it and leave it for at least a week. When you start your drying fires you should be able to feel the dampness on the perlcrete surface with your hand. If in doubt use a cheap garden moisture meter plunged into the perlcrete layer or throw a sheet of plastic over the dome to see if moisture is condensing under it.

Clay in render tends to increase shrinkage so omit it. Lime is a better addition as it imparts some elasticity as well as providing some self-healing to cracks.

The use of a gas burner, not designed for an enclosed chamber, to push out the water can be dangerous for two reasons. Firstly it tends to cook the flexible hose which can lead to a nasty accident. Secondly, as it doesn't have an integrated flame failure device, should the fame go out, you will have filled a hot oven with unburnt gas, a potential bomb. One of the big advantages of using wood is that it creates a coating of soot all over the inside of the dome. When that soot reaches around 300C it burns away. This is a great indicator of temperature for both the drying fires and also for cooking.

I suggest you go extra slow with the drying fires because adding foil layers, whilst doing a good job of reflecting radiant heat, provided the foil is not sitting against a dense conductive layer. However it also acts as a vapour barrier and traps water in, making it harder to escape and increasing the risk of steam pressure build up.

If it were mine I'd be taking a long skewer and plunging it deep into the perlcrete so lots of holes are made through the two foil layers.

I say this from my own experience of using foil outside the vermicrete layer in my own oven. Worrying about the vapour barrier issue I perforated the foil in two places, about 30cm2 with lots of holes, one at the top of the oven and another about two thirds up the dome. As we live in the tropics during the wet season in extreme humidity the whole oven can pick up moisture from the air even if it hasn't rained. On firing the oven in these conditions, the outside becomes quite hot to the touch in those exact places while the rest of the outside is only warm. This indicates to me that steam is passing through the perforations at those points.This has the effect of slowing the drying process.

[ChrisNazareth]

Hi Pizzarotic, Sixto and DavidS
Thanks for your feedback.
Unfortunately it is ceramic blanket under the floor. Guess I’m stuck with it unless you guys have any suggestions.
In relation to the foil , I’ll pierce some holes as DavidS suggested.
Also I forgot to mention that I put a 6 inch strip of aluminium flashing around the bottom edge between the perlite and the foil and sealed it with high temp silicon. I m hoping this will help prevent water ingress through the floor.
I will also hold off on the curing fires for a while longer till the perlite is dry . I will remove the cling wrap and let it dry out for another week at least.
Now that the potential dangers of the LPG burner have been highlighted, I will stick to wood or charcoal .
I will omit the fire clay from the mix so is 3:1:1 (sand cement lime) a good mix along with the AR fibre mesh and water & Bondcreat sol.
Thanks again your feedback I really appreciate it.
cheers
Chris

[ChrisNazareth]

Hi DavidS
Just wanted to clarify a couple of things in relation to the drying of the perlite.
when you said remove the cling wrap and expose it to the sun and wind to dry out. I still need to leave it under cling wrap for a week to wet cure right?
Also while I’m drying out the perlite do I have to cover it every time there’s rain predicted?
As per Pizzarotic’s suggestion Over the Stucco layer, I plan to put on a few coats of acrylic roll on render as a waterproof barrier, will this pose a problem if I puncture the foil dry the oven and then seal.?
Thanks
Chris from a wet and windy south coast of NSW

[david s]

There, so much free water in a vermicrete layer and as it’s strength is really low anyway, there’s not much point in trying to enhance its strength by damp curing. I think it’s more important to use the time to try to dry it out as much as possible. You only need sufficient strength in it to act as a firm substrate for applying the render.
I also coat the finished outer render with an acrylic render to waterproof it. But don’t apply it until you’ve had several decent cooking fires after completion, because a damp render layer can cause blistering under the acrylic layer. I use flexible pointing which is a really flexible UV resistant high build (sand) product used to coat and waterproof the oven. But for ease of application so it can be applied by brush, I water it down 20% so it can be painted on in about 3 coats giving you a thick membrane of about 1.5-2.0mm thick.

[ChrisNazareth]

Hi DavidS
That makes a lot of sense. I will remove the wrap and let it start to dry out as soon as it stops bucketing down.
I may have to wait for a while till this wet weather passes before I get it totally dry , cured, stuccoed and ready for the acrylic layer
Thanks again for all the help.
cheers
Chris

[david s]

Regarding the render mix 4:1:1 is probably a better mix. Cement and lime are both cementious materials and 3:1:1 makes it a bit too brittle.

[ChrisNazareth]

Thanks DavidS
Cant wait to start the curing fires. But I think it’s going to be a while with this weather we are having.
Cheers
Chris

[ChrisNazareth]

Hi DavidS
How much AR fibres do you add to stucco. The bag I have is 900gms.
Cheers Chris

[david s]

I usually work by volume sit's easier. For every 10 litres dry render mix, I add 0.4 litres AR glass fibres.

[ChrisNazareth]

Thanks David

[ChrisNazareth]

Hi

Just wanted to provide an update on the progress so far.
I pierced some holes in 2 places in the Aluminium foil then dried out the oven for about 10 days, 5 of which I had it under plastic with an old electric cupboard heater that kept the temp in the dome at around 45 degrees.
I then started the curing fires. Have done 6 days of fires all day with the door closed at night. 200 - 700.
only got one crack that I can see in the pearlcrete layer from the flue down to the arch where it meets the refractory mortar.
Looks very minor, not sure if I should try to fill it in or if the stucco will fill it. What do you think.
Also planning to stucco soon so just wanted to check if I should dampen the pearlcrete before applying the stucco.
cheers
Chris

[david s]

When rendering it is advised to dampen the substrate to reduce the suction of water from the wet render mix. Unfortunately because the vermicrete is so water absorbent it will disappear very quickly. Having gone to considerable trouble to drive out the water, why re-wet it again. There probably is a case to apply a sealer over the vermictrete layer, but I prefer to do the whole render layer in one go about 12-15 mm thick and then wet sponge the surface to remove any tooling marks and then immediately cover the whole oven with cling wrap so all the moisture is sealed in the outer rendered layer. The next day little beads of moisture can be seen under the cling wrap indicating there's plenty of moisture in the rendered layer. As the strength can be considerably enhanced if kept moist for a week, I leave the oven in this state for a week before unwrapping it.

Be sure not to render up against the pipe or you'll get problems with water entry there when the stainless flue expands and contracts against the render, probably cracking it and providing an easy entry for water at the join.Wrap some cardboard around the flue pipe, wrap some plastic around the card board (for easier removal) and then render up to that. Remove the cardboard in 24 hrs and fill the gap with high temperature silicone.

Those tiny cracks in your vermicrete are inconsequential. It doesn't matter what happens to the vermicretelayer as it will be encapsulated between the oven and render layers.