2025 Pompeii Oven Build Log

[yoboseyo]

Update:

Completed the brickwork for the outer arch.

Also fixed up the inconsistent thickness of perlite.

I left a 20mm gap between the inner and outer arches for the door. There's going to be a landing slab placed on the bottom row of brick.

There's also a 25mm thermal break between the inner and outer arches, which I will fill with heat-resistant rope.



Put a wedge of brick to straighten the path of exhaust and support the vent

[yoboseyo]

Vent Design Questions:

Hey guys, time to design the vent and I am in need of assistance.

The vent construction is going to sit on top of the inner and outer arches. My idea is to stuff leftover pieces of CaSi in the thermal break (cut into L shapes so it fits not only the thermal break but sit between the vent construction and inner arch) to define the flue opening.

The arches were designed this way because the outer arch is normal brick (kilned) not firebrick. Insulating the vent as much as possible from the inner arch minimises heat leakage to the outer arch.

Illustration:


I've done extensive reading of this forum on vent design and this is my design so far:

Vent dimensions:

• Chimney size: 150mm/6"
• Size at opening: 400mm*200mm (more than double the chimney size)
• Reduction to chimney at a 60deg angle (I read that it should be 30deg - not sure whether that's 30 from horizontal or vertical - I assumed taller the better)

Sample Sketchup:


Questions:

1. Any advice/critique on the proposed vent design?

2. What should I construct the vent from? Options are:

• Brick - I may have trouble getting the ideal sloping shape using brick
• Castable
• Steel

3. Can I use this recipe for castable? These are the ingredients I have (castable refractory is not available here) Coarse sand:crushed firebrick:ciment fondu:fireclay 3:1:1:1

4. Should I insulate outside the vent?

5. If not insulated, can I apply the same rendering on the dome, which is mosaic tiles adhered using thinset to the flue? Or will it be too hot? Is there such thing as refractory tile adhesive?

[david s]

I think your solution is pretty sound, but it may lead to exposed cal sil at the sides of the entry and the stuff is not particularly strong so would be susceptible to knocks and abrasions there. Casting the vent transition is way easier than building that kind of form in brick. It also allows for a big reduction in weight and therefore heat sink.
i’ve found the 3:1:1:1 recipe leads to excessive shrinkage (you don’t want shrinkage cracks), from the high clay portion so I halve it, with much better results. 3:1:1:0.5, but you will need to add the very fine polypropylene burn out fibres for steam spalling protection. A casting thickness of one inch should be sufficient.

[yoboseyo]

exposed cal sil at the sides of the entry

I will seal all the edges of the transition using rope

I’ve found the 3:1:1:1 recipe leads to excessive shrinkage (you don’t want shrinkage cracks), from the high clay portion so I halve it, with much better results. 3:1:1:0.5

would it be better to omit the fireclay entirely?

fine polypropylene burn out fibres

I just looked up this stuff. It comes in different fibre lengths from 6mm to 18mm. which should I choose? how much of it do I need to add?

Do you have any experience with tiling on hot surfaces? Would I be able to lay tile directly on the vent? I read in another post of yours that you have used Monoflex, which I can get. That wasn't for a vent, though. I'm not sure how hot my vent will get.

[david s]

Don’t omit the clay in the recipe entirely, it adds refractory quality to the mix as well as making it more workable. The very fine polypropylene fibres I use are 19 mm long and added at the rate of 1% by volume. Be careful to disperse them well in the mix, they tend to clump. Mix for double the time you would normally expect.

[MikeD55]

What steps did you use to cut the inner arch like that? I tried that design and it didn’t work so well for me. Going with something
more Conventional

[yoboseyo]

What steps did you use to cut the inner arch like that? I tried that design and it didn’t work so well for me. Going with something
more Conventional

I followed this youtube video as a guide:

https://youtu.be/6aMd3RykSYI?si=vYuYgCaLLxRefZtt

My dimensions were different so I drew it up in sketchup, essentially intersecting the dome and arch bricks, and ensuring what was my 7th layer sits exactly on the inner arch.

You can see some pictures of the inner arch brick layout dry run in my 3rd post. See how i used the IT pivot with a 450mm piece of timber

[yoboseyo]

Cast the flue. Ended up running out of burnout fibres. I didn't know how to work with the stuff and I'm pretty sure I didn't distribute it evenly enough, buit whatever.

Attached the flue and the chimney. I was in two minds about whether to insulate the flue, and decided to do it anyway, as it would make it easier to finish. I wasn't sure whether render can withstand high temperatures

Also rendered the outside of the dome.

I think all but the cosmetic parts are done, and onto the curing fires!

[yoboseyo]

I'm up to the 4th set of curing fires, and I am noticing quite a bit of heat leakage through the bottom.

Remember that I have 3 layers of CaSi that is raised from the slab using tiles, and weep holes drilled into the slab.

On the 4th firing, I noticed that the underside of the slab was registering 30C, and it was a cold evening where the outside temperature was 10C. There's no heat leakage at all through the dome.

The top of the dome reached 400C. Yet the floor temperature was very uneven, even after a long period of firing, reaching only 250C at the hottest part.

I noticed that water was dripping out one of the weep holes.

I can't explain what's causing the water ingress. If water is leaking in from the outside somehow, that itself is not a big issue because the CaSi layer is suspended. I can't tell whether the CaSi layer itself is wet. I can't see how water could ingress and make its way up to the insulation layer, but the temperature leakage and hearth temperature suggests that moisture is a likely reason.

Also, it has been a few months where the oven has been air drying, so refractory materials/cement should be fairly dry.

Any ideas on how I can diagnose and rectify this issue?

Water dripping from weep hole in question: