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Rocko Bonaparte 03-24-2011 08:26 AM

Extending front entrance with perlcrete
I wanted to check if this is a bad idea or not. I wanted to bring forward the front entrance to my cob-like oven a little bit while I'm setting up all my perlcrete insulation. It will move something like 5 inches forward. I first thought I might plop down some more cob made from my sand/clay mix around a sand mold right in front. However, this region isn't really a cooking surface and I don't necessarily want it heating up either. So I thought instead I'd form up the sand support like I was doing cob, but instead lay some perlcrete. This would form a perlcrete entrance, and one that would be unsupported overhead for that ~5 inches of archway. Will that hold up okay?

tusr18a 03-24-2011 07:09 PM

Re: Extending front entrance with perlcrete
As I am not an engineer, I will not comment on whether it will hold up. On a practical level, I see other issues. First, perlcrete is relatively soft. You are going to be moving a lot of wood into this oven. While you will understand how important it is to carefully place the wood in the oven to ensure that the perlcrete does not get damaged, you will inevitably ask someone else to stoke the fire who does not understand how soft the perlcrete is. That person will accidently throw a log against the perlcrete causing bad things to happen.

My inner arch is very narrow where the flu drops down through it. When designing this inner arch I originally did not see this as a big problem. After hitting the inner arch with just one log, the arch collapsed. While I was able to rebuild the arch, it remains very fragile. If I ever decide to rebuild the oven, I will definitely beef up the entrance area to allow for the errant log to bounce off of and not damage any of the masonry as it passes through the arch area to the oven chamber.

david s 03-25-2011 05:46 AM

Re: Extending front entrance with perlcrete
Perlcrete won't be strong enough IMO and it's made from Portland cement so can fail in high heat situations. I think you'd be better to use a castable refractory.

Rocko Bonaparte 03-28-2011 04:38 PM

Re: Extending front entrance with perlcrete
I extended out the walls with more of my cob mix this weekend. It was a bit tough; it was drying very fast and cracking all the way through. But I fixed that up and it seems to be holding true. The bit of the floor I extended is with perlcrete that I had smoothed out though. I think I'll have to just see how well that works after a few uses. I have more sand and clay that I can replace it later if I need to.

Tscarborough 03-28-2011 07:39 PM

Re: Extending front entrance with perlcrete
Pictures or it never happened.

Rocko Bonaparte 03-28-2011 09:24 PM

Re: Extending front entrance with perlcrete

Originally Posted by Tscarborough (Post 110336)
Pictures or it never happened.

Dunno what the skepticism is. I had said the walls were the cob mixture; only the floor is perlcrete. But if you're really curious, it will have to be Wednesday. The National Weather Service doesn't think anything's going to happen here tomorrow, but everybody else is flirting with the idea of anything from drizzle to a thunderstorm, so I have the whole mess covered. It's still drying out and would wash away if it rained. The temperature drop is welcome since it's curing a lot better.

Tscarborough 03-29-2011 06:01 AM

Re: Extending front entrance with perlcrete
No skepticism, I would just like to see your creation. And I sure hope it does rain, we need it badly.

Rocko Bonaparte 04-07-2011 07:41 PM

Re: Extending front entrance with perlcrete
3 Attachment(s)
OK pictures finally. Here you can see it while it's still just the cob mixture, but with the extension I said I did:

I had made a looser mix to try to fill in the transition from the old front to the new front. So you see some discoloration in there where I plopped in some stuff. I expect most of it to flake away but I am hoping any gaps from the original entrance end up plugged up.

Here you can see it insulated and covered in perlcrete. It's a little uneven because I ran out of light. I plopped as much as I could before then but I didn't get it quite even. I did a cure fire yesterday. We ended up trying to dry the next batch of wood overnight and came out to see it was smoking away at it. We got those out and started doing another one, that wanted to start smoking up too. Tonight we threw some more in after leaving the mouth open and empty. It really does carry its heat really well now.

Here you can see the problem I'm facing right now:

I kept some lip and it looks like I needed it for the insulation. I wonder if I have the room necessary to put on the finishing coats with what I have here. Also, I am trying to figure out what to do with the cement board lip. Can I expect to just be able to finish that fine or will I have to fill in that space?

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