42" Pompeii Oven in Jamaica

[MarkJerling]

Well I got polypropylene fibers today but can't find SS needles, nor any SS mesh wiring; so looks like I will be casting with just the fibres. I'm hoping that the slabs will be fine and wont break at any time now or in the future...

What will the polyprop fibres do when hot? I'd be looking for something that can handle the heat.

[wthwaites]

Well I got polypropylene fibers today but can't find SS needles, nor any SS mesh wiring; so looks like I will be casting with just the fibres. I'm hoping that the slabs will be fine and wont break at any time now or in the future...

[MarkJerling]

With mine, I managed to find some firebrick slabs, 390x260mm, which I used as the top of the chimney "roof" as it goes up over the dome. You can see what I mean if you look at my build photos. When I got to the top (highest point of the) dome, I set one of those slabs flat, and, before mortaring it in place, I carefully cut a round hole in that slab, only very slightly smaller than the stainless steel flue. The bottom of the stainless steel flue therefore rests on top of that slab. I then built up a short chimney using ordinary clay brick (4 courses high) which supports the steel flue pipe all round. This has turned out to be remarkably secure and holds the flue pipe securely, even with very high wind conditions. Have a look at my build drawings and photos and you'll see what I mean.

I think your castings should work well. If I did not have the large firebricks (the same type as I used for my oven floor) I would have had to come up with a different solution and casting a top cover in segments would have been my next option, I think. There's very little load on top, so I don't think much reinforcing is needed. Certainly, you don't want to reinforce with something that will contract and expand and crack the slabs. I think the better reinforcing would probably be some furnace blanket fibres, rather than steel, but I'm guessing - someone else may have a better idea.

[wthwaites]

Okay all...I'm starting my squirrel tail chimney.
Based on the design, the side walls are made from laying leftover refractory brick (no problem), and I am going to have to cast several small slabs for the top roofing part to close the channel going up to the actual chimney (tricky). Don't want whatever I am casting to break in the future.
From my research, I would do this cast out of the same refractory mix I have been making, with the addition of polypropylene fibers and stainless steel needles (melt extract SS fibers)? I may be able to source the fibers but definitely not the needles. is there an alternative to that, or is it not needed?

[wthwaites]

No, vent will not see temps to cause any melting problems, it just extends though the stucco layer to the top of the fiber blanket. It is placed at the top of the oven due to steam rises so I am not sure vents at the base give you any benefit.

This sounds like a great option. I'll definitely consider it david s. Thanks!

[wthwaites]

Sorry for my poorly worded reply. I did not intend to cause alarm or to question the squirrel tale design. My intent was only to figure out where would be the best place for a vent, if applicable.

Thanks Gulf !
Not worried at all, no need to apologize. Just want to follow best practices. Like i said, this is a first time for me; its just been a learning experience every step of the way...

[wthwaites]

No, vent will not see temps to cause any melting problems, it just extends though the stucco layer to the top of the fiber blanket. It is placed at the top of the oven due to steam rises so I am not sure vents at the base give you any benefit.

Great. Thanks for the advice UtahBeehiver !

[david s]

I like this idea. I think I would put one at the top of my dome, and two on either side at the bottom of the dome in line with the base (IFB and CaSi board). Or would this be overkill?
I also see in other threads that you've recommended a steam vent/breather cap which looks like plastic along with PVC bushing. Won't this melt? I assume copper or brass would be better?

The vent can prevent pressure build up like a hole in a saucepan lid. I don't think its placement really matters although as the top of the oven gets hottest first and also driest first and any steam generated moves away from the heat source hitting the cook outer shell, condensing there and running down the sides, it is the base of the dome that remains the wettest, so drains there would be more appropriate. In this case any pressure build up would actually force the steam/water out the drains in a more efficient manner just as drain holes in the supporting slab under the floor insulation do a similar task. My experience firing new kilns where water dripping from the lower front corners of the kiln, once it reached around 400C, taught me this. I have a drawing somewhere explaining this principle, but can't find it. Another rather elegant solution would be to drill some holes in the base of the flue pipe that is surrounded by insulation so steam can pass into the flue pipe. This would then eliminate any external vents on the outside of the dome. My system is something like this (drawing attached)

[UtahBeehiver]

No, vent will not see temps to cause any melting problems, it just extends though the stucco layer to the top of the fiber blanket. It is placed at the top of the oven due to steam rises so I am not sure vents at the base give you any benefit.

[Gulf]

.....If this isn't recommended, please guide me. I'm learning everything as I go along with the build.

Sorry for my poorly worded reply. I did not intend to cause alarm or to question the squirrel tale design. My intent was only to figure out where would be the best place for a vent, if applicable.

[wthwaites]

Water that works it way into the dome insulation, and it will, increases in volume by a factor of about 1500 times when it sublimates to steam. Unless you have a way to vent the steam out between the dome, insulation, and stucco the increase pressure in this area due to the volume increase has the potential to crack the stucco. Do a search on the forum for steam vents, are are a number of posts on this subject.

I like this idea. I think I would put one at the top of my dome, and two on either side at the bottom of the dome in line with the base (IFB and CaSi board). Or would this be overkill?
I also see in other threads that you've recommended a steam vent/breather cap which looks like plastic along with PVC bushing. Won't this melt? I assume copper or brass would be better?

[wthwaites]

I'm not sure how you will carry the weigtht of the flue above apex of the dome if there is a void ffilled with fiber blanket. But, if there is such a void, you may be able to elbow a pipe horizontal from the apex out to where the chimney takes off above the outer shell. It may help if you include a rough drawing of how your chimney/flue is designed.

Sorry, I meant that the chimney would sit on the apex of the dome, not the flue.
My original intention was to run the flue system directly on top of the thermal mass before insulation with the fiber blankets and finishing with stucco. Something similar to the design by MarkJerling

If this isn't recommended, please guide me. I'm learning everything as I go along with the build.

[wthwaites]

Water that works it way into the dome insulation, and it will, increases in volume by a factor of about 1500 times when it sublimates to steam. Unless you have a way to vent the steam out between the dome, insulation, and stucco the increase pressure in this area due to the volume increase has the potential to crack the stucco. Do a search on the forum for steam vents, are are a number of posts on this subject.

Thanks UtahBeehiver
I'll look search around some more...

[Gulf]

I'm doing a squirrel tail chimney so the flue will be sitting on top of the dome. You're referring to a separate vent? How would I do this?

I'm not sure how you will carry the weigtht of the flue above apex of the dome if there is a void ffilled with fiber blanket. But, if there is such a void, you may be able to elbow a pipe horizontal from the apex out to where the chimney takes off above the outer shell. It may help if you include a rough drawing of how your chimney/flue is designed.

[UtahBeehiver]

Water that works it way into the dome insulation, and it will, increases in volume by a factor of about 1500 times when it sublimates to steam. Unless you have a way to vent the steam out between the dome, insulation, and stucco the increase pressure in this area due to the volume increase has the potential to crack the stucco. Do a search on the forum for steam vents, are are a number of posts on this subject.