2 meter diameter New build oven with pumice reinforced with Basalt Rebar

[Alomran]

Help!
I am gutted!

I opened up the entrance to the oven and have found that all the wooden frame shaping the domes were soaking wet.
With the inside absolutely soaking wet from years of accumulating rain, and out of excitement I'd decided to start a small curing fire using a handful of twigs only to dry up the wood thinking which was soaking wet. This is since flexible plywood upon which the firebricks is sitting on was almost rotten from dampness as well as the structural ribs holding the sphere form were also very wet.
With plenty of water on the side in case of uncontrollable fire occurs,I started the small fire and for about an hour it was going OK with plenty of steam coming our of the timber frame. Then my family asked me to join them for a late snack leaving the small fire unattended thinking the wood was far to wet to be inflamed by fire.
After returning back to the curtilage, I have found that the structural wooden frame inside the oven was fully inflamed !
The dome is riddled with cracks, the cracks seem to run in both directions. IIt is evident the crack run vertically where the wooed structural frame were. I have counted 6 vertical cracks running along vertically and starting from the first third of the dome upwards towards the apex.
There are also horizontal cracks running in the last top quarter as a complete ring at the top of the dome!
How bad is this? Do I need to demolish it?

[david s]

The render does not have to be waterproof. In Italy typically their ovens were outdoors and finished with a lime based render. It allows the transmission of water both in and out. If the oven gets wet a waterproof exterior makes drying it out more difficult. I think the best solution is to have an unsealed dome under a roof which keeps out the rain yet lets the oven breathe. As a roof is expensive the next best is to build a dog kennel style or enclosure around the oven which both shields the oven from rain and allows it to breathe. Waterproofing the exterior and having the oven in the weather is probably the cheapest solution but not the best IMO. If waterproofing the exterior a vent that allows the release of moisture from the insulation layer is pretty much essential for a good functioning oven. We live in the tropics and during our wet season in 90% humidity conditions, the oven interior and insulation layers get wet even if it hasn’t rained. A few long slow fires restore normal function though.
I’d assume the Victas render is a calcium aluminate based product which means it will be expensive and also a waste of your $ because the outside never gets really hot.

[Alomran]

The swimming pool tile adhesive could work, but will probably be expensive also. Is it suitable to apply as a render?

Thanks David for the great info.si is having a lime render on a metal mesh without acrylic would be waterproof?
Or the acrylic is essential in making ir water proof?
Any idea if the Vitcas fireplace render is suitable for exterior stucco?

Cheers

[UtahBeehiver]

I moved this thread from Introductions to Other types of oven. I don't know why I did not do this earlier.

[david s]

The expensive stuff in the bucket is a 100% acrylic product and they’re all expensive. Commercial cement renders, dry in a bag, are much cheaper and have a degree of waterproofing added to them. If you mix some water with it it resists mixing and you have to work a bit to get it integrated. Some of them are labelled PM which stands for polymer modified, but they’re still essentially a cement based render. There are others that recommend the addition of an acrylic bonding agent usually 50/50 water/ acrylic.
You can make your own, but it won’t have any waterproofing qualities. The addition of clay in the mix increases the tendency for shrinkage cracks. A better recipe IMO is 4:1:1 sand,hydrated lime, Portland cement. The lime adds enough creaminess to make it nice and workable at the same time as providing some elasticity as crack healing properties. A non waterproof render can always be treated with a waterproof coating over it. The swimming pool tile adhesive could work, but will probably be expensive also. Is it suitable to apply as a render?

[Alomran]

Hi Alomran,

I used a premixed bagged stucco (render) brown coat mix..

I have done a quick search on exterior stucco and I'm gop smacked! a small bucket was reduced from £112 ($140) to only £68 ($100) for 960ml. https://www.directnine.uk/products/r...CABEgKeQvD_BwEhttps://www.directnine.uk/products/r...CABEgKeQvD_BwE

Given the oven is not sheltered in UK's wet weather, do you have any opinion on home made brew I have read using 3:1:1:1 sand, cement, lime, fireclay, if mixed with acrylic and added on a metal mesh? Any other viable solutions for approximately 5.6 sq m dome area?
Would I be able to use the VITCAS Fireplace Render -20kg-Fire Cement Render?
I have 12 buckets, or is this product is only for interiors?

Do you think if the dome is cladded with ceramic tiles using swimming pool adhesive,
https://www.dunloptrade.com/products...-tile-adhesive
https://www.dunloptrade.com/wp-conte...-adhesive1.pdf which is proven to be a superb water proof agentm would be an adequate water proof layer that can replace the waterproof stucco?
This tiles adhesive is a superb water proof as it is used in showers and swimming pools.
Any thoughts on this?

Thanks

[Gulf]

Hi Alomran,

I used a premixed bagged stucco (render) brown coat mix. I did not do the traditional 3 (scratch coat, brown coat, and finish coat) steps. I replaced about half of the water required for the stucco layers with a liquid acrylic fortifying / bonding agent for concrete. In the mix, it was used as a fortifier. The first coat was pressed firmly into the mesh wire which was uniformaly spaced from the underneath vcrete layer. I smoothed each layer with a wet sponge. On each layer, I painted the acrylic on full strength as a bonding agent. That includes also the outer layer to which the brick splits were applied. The acrylic is what seals against water intrusion.

[Alomran]

Alomran,
The last pic shows a mesh that I installed over the vermicrete with spacers. I then installed what I called at the time a stucco layer. It was not a coventional stucco application. More like water resistant ferrocrete. That layer is very strong and water resistant. It is not a breathable layer like true stucco. That is where a vent comes into play and is very necessary with a water proof outside shell.

Hi Gulf, Would you kindly elaborate on the contents and method you have carried out to do the water resistant ferrocrete?

[Alomran]

Do you advise to have a marble platform or landing plate outside the oven door to use for p[lacing food there? If the oven is approx 2m in diameter, adding the smoke chamber which could be around 46cm, won't that be adding more distance between the user and the centre of the oven. Also, The length of pizza peel stick is going to be a problem!

[Alomran]

I was going to say 2 days. But, looking back at my album, I can see 3 separate stages of drying. I'm sure that these weren't full days. Just, what time that my schedule would allow.

I have a question: How long does it take take for a dome with home-brew mortar to dry up prior to firing the oven for curing it in a wet country like the UK?
What is the formula that works out the flue diameter from the dome's diameter? My dome's diameter is 197cm, with the height of (approx.) 78cm any idea how to work our the flue's diameter?

[david s]

When you mix the correct amount of water with the vermiculite to make a vermicrete mix, because it is so water absorbent, you will be adding more than twice the amount of water that the hydration reaction requires. That means an enormous amount of free water left in the vermicrete layer. If the layer is thick it takes longer to eliminate the water. See my experiment on drying to give you an idea. You could fire the oven with just the blanket layers and it would function fine, the only reason to cover it with a more permanent outer shell is to prevent the blanket from getting wet with rain and to protect it from abrasion damage. The blanket is difficult to render against because it's somewhat springy and also a bit lumpy which then necessitates a very thick outer rendered shell. So, the usual method is to give the blanket a layer or two of lean (10:1) vermicrete which evens out the lumps and bumps and once set and dry is firm enough to render against while also providing an extra insulating layer. If it is made any thicker than an inch or so, then drying becomes difficult. When I first began making ovens the older generation of blanket was not classified as safe and the safe stuff now used was prohibitively expensive. Consequently I insulated entirely with 10:1 vermicrete and quickly found that it was best done in layers of around one and a half inches thick with a week of drying before proceeding to the next layer. I now use a blanket layer over which I do a vermicrete layer, then after a week of drying the drying fires and finally the rendered outer shell. Vermicrete insulating slab copy.doc.zip

[Gulf]

I was going to say 2 days. But, looking back at my album, I can see 3 separate stages of drying. I'm sure that these weren't full days. Just, what time that my schedule would allow.

[Alomran]

Alomran,
. Once that had set I installed the vermicrete. The last pic shows a mesh that I installed over the vermicrete with spacers.

Gulf: how long did it take you to finish of vermi-crete the entire dome?

[Alomran]

I have no idea about the food safe portion of your question. Soft insulated firebrick will shed small pieces when bumped with peels, pots, and or insulated doors. Those pieces getting on your food is probable.... Hazardous, I don't know? It is just not a best practice imo.

Gulf: I cannot thank you enough.. I honestly think if it was not for the advice I had from you and some others, my oven would have definitely been now a rubble....I am not saying it won't but less like since I am using firebricks and right approach. A great forum and a great entourage of oven builders. It has definitely been an experience. I have never realised how complicated building an oven properly.. Definitely far more complicated than any building I have designed.
Thank you for taking the time to answer my queries.

[Gulf]

I have no idea about the food safe portion of your question. Soft insulated firebrick will shed small pieces when bumped with peels, pots, and or insulated doors. Those pieces getting on your food is probable.... Hazardous, I don't know? It is just not a best practice imo.