42" Pompeii Oven in Jamaica

[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.

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

[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?

[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]

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]

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.

[david s]

The polypropylene fibres will melt at 160C so are not suitable for reinforcing in this application. Once they have melted they leave behind a network of minute pipes that steam can pass through. This reduces the incidence of steam spalling provided heat up is taken slowly.
The recommended reinforcing for refractory is stainless steel needles, but their use is not mandatory. Their drawback is their expense and difficulty in handling when placing the castable.

[wthwaites]

So the fibers aren't needed?
I was able to find needles, but they weren't stainless steel and was only available in 55 lb buckets at $250 - way out of my price range! For the amount I would be casting (essentially a 2' x 4' x 3" triangular slab divided into three pieces), I also think that the purchase would be a waste.
I'm all ears for alternative solutions. I may be able to find stainless steel threaded rods. Will that suffice?

[david s]

The reason melt extract fibres (stainless needles) are recommended is that being a very small diameter they present a much larger surface area than thicker stuff. This enables them to dissipate their heat more easily to the refractory that surrounds them, thus reducing expansion issues of the two different materials. In normal ambient conditions eg red in a concrete beam the rate of temperature change is slow enough for the two materials to be quite compatible. But when extreme heat is applied > 200C/hr, then problems are likely to occur. You could try using some AR fibreglass fibres in the castable, glass has a melting point of around 900C so is well above our temp range.I use them as well as the stainless needles.

[wthwaites]

Ok Thanks David. I'll search around for fibreglass fibres.

[wthwaites]

I got Fibreglass fibres, so I'll be moving ahead with casting the slabs with both that and the polypropylene fibres. That's the best I can do with what I can source. Lets hope for the best!

[Gulf]

I'm rooting for you. I have a mental problem with needles of any kind lol. I'm looking forward to how it all turns out.

[david s]

Make sure they are the AR alkaline resistant ones.

[wthwaites]

Make sure they are the AR alkaline resistant ones.

I was told it was glass fibres for concrete; I will double check tomorrow. If they aren't, they can't be used?

[david s]

I was told it was glass fibres for concrete; I will double check tomorrow. If they aren't, they can't be used?

If they're not they will react with the cement and get destroyed. The AR fibres are coated in zirconium for protection against chemical reaction with the cement. Some have far more coating than others. How you tell, I don't know, but if they're pretty cheap, off eBay or similar, they may not have much zirconium. I pay around $200 for 20 kg. If they say suitable for concrete, then presumably you have the correct product. Use a reputable supplier and you should be ok.