30" cast dome design

[sergetania]

You should be fine

No, David, it will take a few days before I feel fine again! My computer hands will feel this for a while! It looks easy in YouTube videos! It's HARD work!

Again, lots of respect for people who do this for living! I think if I do it again the result will be better. By the time I was done with it the render on the bottom got too hard to smooth out all gouges. I guess it will give the oven a character. I will feel bad about it but I really don't want to do another layer. I hope I am done. Covered with plastic wrap and plastic cover, it is curing. It is what it is...

Oh, btw, it is hairy in a few spots because of the glass fibers. It's especially bad around the facade. I need to shave it somehow

[sergetania]

Will do all that, David! Thank you! I made a float from polystyrene foam with the same radius as the dome. If using that with water doesn't smooth out the surface I will use a sponge. It's just scary to do this for the first time though I have watched a bunch of videos.

[david s]

All good then. I’d be dampening the edge of the concrete slab before rendering then. Start as usual from the bottom up.You should be fine, but when you’ve finished use a wet sponge to go over the whole surface. It will remove any tooling marks. Covering it then is important as holding the moisture in the outer layer for a week does increase its strength significantly.

[sergetania]

David, I did the seven fires. Last time the temp of the dome reached 850F. I roasted some fish and it tasted good! Now if the rains start the perlite will be just sucking water so I want to go ahead with render and then dry it gently again (I do have a breather tube through perlite).

Also,not worried about the crack. What I was trying to find out how I put the stucco over the bottom of the oven. I guess I will bridge the base and the dome and hope the stucco sticks even with nothing to hold it below the base. I will just round the bottom edge, right?

[david s]

A smal gap or crack like that is nothing to worry about, the render wil bridge it ok. Is the perlcrete layer thoroughly dry? It will contain wat more water than you think and how it appears. It is prudent to eliminate as much of this water as you can before rendering over it because steam expansion pressure can easily crack the outer shell. Do the 7 fires in 7 days getting progressively bigger before rendering over it. This will be enough to drive water out of the internal dome as well as the vermicrete layer. If unsure, throw some plastic over the dome during firing to make sure there’s no condensation on the underside.
Unfortunately this leaves a very dry substrate which will want to suck the moisture from your render. To counter this effect wrap the whole oven in clingwrap once the render is done. This will hold the moisture in that layer well. Do not uncover it for a week.

[sergetania]

Need help, PLEASE, rather urgent!

I am planning to render the dome today. I thought I would ask before making a mess. The picture of the side of the dome is below. The base and the perlite dome with a hairline crack between them. The question - how do I stucco that? I need 10-15 mm layer of stucco. Do I start at the bottom of the base or the dome? Will the stucco stick with nothing below (I will be using a bonding adhesive, the one you add to the mix instead of a part of the water)? Sorry, first time doing this. THANKS for the help!

[sergetania]

On my last curing fire, followed David's advice and roasted some fish. Turned out quite nice, especially for the first time! Need to tailor my cookware to the oven. 30" is somewhat of a tight fit but it will work.

[david s]

Carbon begins to burn off at around 600F (300C) so the top of the dome should be clear. I often use this indicator for my oven to stop adding wood and begin roasting or baking. For my oven this takes one hour to reach this temperature and saves time and fuel. All new ovens are smoky, the smoke escaping out the front will lessen as the oven dries more.

[sergetania]

With homebrew? Stopped homebrewing,too hard (for my back). I did pretty good for a few years though.

I am watching the curing fire #5, reaching 600F. Pretty cool. It seems above 500F the oven just takes care of burning anything without smoking much. The top of the front arch is pretty black though, lots of smoke was coming out of the oven opening.

[david s]

Those fibres look like mine and I’d say 1 lb/80lb dry render sounds about right. The roast chicken should go well with the .

[sergetania]

Are we talking about AR fiberglass fibers? The stuff in the picture below? That's what I got and I was surprised of how heavy they were for the volume.

Googled 1 liter of dry concrete = 5.3 lbs (LOTS of stuff on the Internets!). So 80 lbs need about 1 liter of fibers. I think one pound is about right for a 80-lb of dry stucco. Thanks!

Will wait, roast something else at 400-500F. Not the worst hobby I had in terms of waiting. Homebrewing was the hardest! I can do it! Chicken wings will sooth the pain! Thanks, David!!!

[david s]

I did say 700 ml (not grams) for every 10 litres dry render mix. The fibres are pretty light.
I also said keep covered for a week to damp cure. Depending on how thick you make your render layer it will still require drying. Even at 10 mm that will take more than a few days. Forget pizza for now, I know it’s hard, but you wouldn’t jump in a brand new car and yes it’s top speed on delivery. Be content with roasting some chickens or pork roasts for a few firings before going for pizza temperature.

[sergetania]

David, hope you are feeling better and out of the hospital!

It seems you use much more than I planned so I have ordered more. I will just throw 1-1.5 lbs of fibers into 80lbs of dry stucco mix. Hopefully, it is not easy to add too much. After curing the render for 2-3 days under the wrap and drying it for a 3-4 days should I wait much longer before I can make pizza?

[david s]

I’ve missed some of these posts, apologies, but would endorse what others have said. Sorry to disappoint, but I don’t have a precise measurement either, but it is pretty close to 700 ml AR glass fibres for every 10 litres of dry render/stucco mix.
As I’ve said many times there are many ways to skin a cat. To reduce labour I do the outer shell in one coat, around 10-15mm thick. Normally one would dampen the substrate before applying the render, but as the water removal is so important I prefer to cover it to hold the moisture in that outer layer instead. Anything containing Portland cement should be damp cured so maximum hydration takes place, generally for one week. Ferry concrete boat builders talk about 28 damp days. I think this is doubly important because the dry vermicrete is sucking moisture out from the inside.the presence of moisture beads under the clingwrap is evidence this is working.I also sponge finish the surface before wrapping which removes any troweling marks. The surface at this stage is quite hard and the plastic leaves no marks.
There is evidence to show that too high a cement content can produce a brittle render, also that each subsequent coat should be slightly weaker and that the addition of lime imparts some elasticity as well as self crack healing properties. I use a 4:1:1 sand, cement, lime mix combined 50/50 with a proprietary PM modified cement render. This works pretty well, the commercial render containing some waterproofing, imparting some resistance to moisture.
I’m a great fan of fibre reinforcement and use stainless steel fibres, macro and micro fibres and just recently added nano fibres to the armoury which work on a molecular scale using multi walled carbon nano tubes, expensive but increase strength by 50%

[fox]

I am a big fan of fibre reinforced cement for a base top layer, it is a hugely popular product where I live but I don’t see much evidence of its use on this forum.!

Basically a 2-1 mix of sand and cement with added fibres, it sets extremely hard, the high cement content makes is very weather proof and the fibres make it very crack résistent .
It sets fast and can be applied in thick layers up to 75mm (3’’) or more, however it still needs a top finish coat of 4-1 fine sand and cement and that need to be applied within 24 hours.
Fibre mix or rein fibre mix are both popular trade names and used extensively in the building trade.
My personal method to coat and weather proof the ovens i build (working on number 8 at the moment) is to apply a basic 4-1 sand cement mix in a thin layer over the ceramic fibre, not a heavy thick coat that will compress the insulation.
This is done in the morning so another thin coat can be applied in the afternoon.
(If you have used vermiculite mix then you dont need to do the two thin coats)
Then I apply a single fibre mix coat too get the shape I want, it is easy to build up a very thick single coat but not so easy to get a good finish so i then apply a top finish coat of 4-1 fine sand mix.
Rein fibre mix at 2-1 sets like sheet metal and you can watch rain bonce off it, you can make amazing shapes and build up a deep thickness, it sets very fast and hard.