Homebrew Castable Oven

[ebbbe]

Hi

Previously I made this thread about my upcoming pizza oven build:https://community.fornobravo.com/for...-oven-planning

Since the thread is named "planning" and I have actually started my build now, I'll make a new one.

My oven is a 75cm ID homebrew oven (with burnout fibres, but no SS needles). It's on a wooden table with a 8 cm concrete slab and 10 cm perlcrete insulation under the floor and 7 cm on the dome.

So far I have made the table with the slab (curing now) and I have precast the underfloor insulation in 4 quarter circles and a piece for the opening. These have now dried for 4 weeks or so, so they should be good and dry when I'm going to cast the dome.

Now for the questions:

1. I'm having trouble finding burn out fibres. I've found these (https://www.bauhaus.dk/fibre-sikacim...E&gclsrc=aw.ds), but I'm not exactly sure they are suitable for the purpose. They are 6mm in length and 225 m^2/kg if that helps.
2. I'm also unsure about the dimensions of the oven opening. The height will be 24 cm (~0,36*37,5 cm) and I've planned on a 45 cm width and 25 cm debth. However now that I have made the drawings I think 45cm is a lot compared to small diameter of the oven, and I'm worried I'll have to much heat loss. On the other hand I think a smaller opening will make it very difficult to get stuff in and out of the oven. Is this just an inherrent problem with small ovens? Alternatively I'd probably be able to go up to 80cm ID and then just 5cm of insulation.

Best Regards

ebbbe

[ebbbe]

Just some pictures.

[david s]

1. These are the correct fibres to use. Although they are designed for a different purpose, they will melt at 150 C leaving a network of 0.018 mm pipes that can help eliminate steam. Add about half a handful for every 10 litres of castable and mix them really well. They tend to clump and not disperse easily so use more mixing time.

2. Good thinking about the entry dimensions, you are correct, it is an inherent problem with smaller ovens. A larger entry means greater heat loss but a smaller one makes the oven hard to work. My own oven (540 mm diam) oven mouth is 400 mm wide x160 mm high, with the outer decorative arch internal dimensions 440 x 180 mm, but still large enough to fit a 4 kg turkey and provides good access for working the oven.You can play with the width but the height should be approx 63% of the ovens internal height. As a hemisphere works best the internal height should be half the diameter of the oven interior.

The oven stand is best made from masonry because there is no movement. If you plan to have your stand out in the weather, I'd be adding some more diagonals. The oven will be heavier than you think.

[ebbbe]

Hi David,

Great and fast response as always. Thank you.

I'll go ahead and buy the fibres.

Regarding the oven opening. I think I'll make the outer arch 45 cm and the inner arch 42 instead. Hopefully that'll work for me.

As for the masonry stand - I would if I could. I don't have that opportunity and I can't weld so wood will have to do. I see that I actually uploaded the wrong picture. It does have som diagonals and in addition all the joints are half laps, which also adds to the strength. I did discuss the design on a woodworking forum before making it. But I'll keep your advice in mind.

Another question. When I have cast the dome, I'm supposed to cure it under wet towels, correct? For how long? would a week be sufficient?

[david s]

Remove sand mould after 24 hrs to reduce shrinkage pressure on casting and to allow inspection and filling of any voids on the inside. Then damp cure for one week.

[ebbbe]

Alright, I've made some progress now. I've cut the bricks for the floor and cut the chimney to size this weekend. Today I put in the insulation slab, the floor, made the form for the oven entry and made the sand dome.

I'll post some more details and pictures soon, but since I'll cast the inner dome tomorrow I just want to be completely sure about one thing. For the first 24 hours I'll cure the dome with the sand mold inside then remove it. Should the first 24 hours be damp curing with towels over it as well?

Also does it make sense to put tarp over the towels to prevent them further from drying up?

Thanks in advance!

[david s]

It depends a bit on your weather conditions, but wet towels plus tarp is probably better. Apart from gaining strength you get a better bond if the mix is still damp when you fill any voids on the inside after sand removal.

[ebbbe]

Last week i got cracking on the floor, the chimney, the sand dome and the inner dome. The floor was cut with my fathers 230mm angle grinder. He paid for a new diamond blade with the promise of pizza when he visits, so now it has to work.

For the chimney i bought a 1m Ø15 cm stainless steel pipe with a socket and spigot in either end. I then cut it into 1/3 with the spigot on it and 2/3 with the socket. I figure I can then put the short one in the oven and the long piece will then be removable so I can disassemble the chimney if I want to.

For the entrance I wanted to make an opening 0,63 x oven height and 42 cm wide and then with a 45 cm decorative arch. I'm not to strong on oven terminology but I wanted the part of the arch that led to the cimney (the vent? The flue?) to be taller than the rest of the arch. The way I went about this was to make a form in 3 pieces, which are seen in the x picture, which is sort of an exploded shot of it. The assembled form is seen in picture x. The part closest to the camera is the actual entrance to the dome. Then there is a front (furthest away from the camera) that is lower, but has the wall for all of the arch and two pieces of wood at the bottom to hold up the middle part. The middle part, which should be taller sits on top of this. It was all held togenter with a few screws that can be taken out from the front, so it could be disassembled from the front. Or at least that was the plan. It took a little more "disassembley" with a drill, a hammer and a chisel to get it out but it worked!

[ebbbe]

Okay I had to split it up into several posts due to 6 picture restriction.

For the sand dome I made a wooden quarter circle with the correct radius and then used this to make sure I got a nice hemi circle. For the concrete dome I made a tool to make sure I had the right thickness. Both are seen in picture x.

[ebbbe]

The actual dome-making went well although it took some time. After 24 hrs i removed the mould and patches the holes on the inside. The dome didn't have many cavities on the inside, but because the newspaper dried up a bit in someplaces and started flapping I had to rewet it and pad it down. This caused wrinkles which are actually very visible on the inside. There are some pictures below both before and after, but it's hard to really see anything. You'll just have to take my word that I improved it a bit.

[ebbbe]

The last pictures show the results so far.

It has now been damp curing for a week today so I'll start the insulation asap. For that I plan on making 50 mm 1:10 perlcrete on the arch, with a taller section around the chimney to hold it in place. Then for the dome I plan on 70mm of 1:10 perlcrete. I remember reading a comment from David S on here about adding it in two thinner layers to shorten the drying. I figure I'll do the 50mm on the arch in one go and then two times 35mm on the dome itself. Does this sound like a good plan?

[UtahBeehiver]

You can start out with 10 to 1 but you may have to drop down to 8 to 1 if the mix is too hard to handle. Vcrete holds lots of water so be sure to let it dry out well before any curing fires.

[david s]

The two layer method assists greatly in drying the stuff. Also a cheap garden.moisture meter will tell you when it’s dry. If not using blanket insulation against the dome this is quite important because any wet vermicrete sitting hard up against the dome can create a large amount of steam very quickly. I find the 10:1 brew about as lean as you can go without it becoming difficult to apply. Remember that the more cement you add the denser and less insulating you make the resulting layer. A handful of powdered clay for every litre of cement helps enormously to make the mix more workable. When mixing the water content is also very important. Too much water will wash the cement off the grains of vermiculite so water pooling in the bottom of the barrow is a little too much. The finer the grade of vermiculite the more water is required, but generally the ratio is about 3 litres water to every 10 litres vermiculite.

[david s]

I currently have 17 students making these carvings from 7:1 vermicrete (plus some sand) that we cast. Very easy to carve. Pretty sick of mixing because guess who ends up doing most (all 40 litres each casting) of it?

[ebbbe]

Alright thanks both of you. I'll try with the 10:1 with some clay in it in two layers of 35 mm.

I think my brother has a moisture meter from when he bought his house last year. Does the moisture level have to go to 0% or how low can I expect it to go before I add the next layer or start with the curing fires?