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Frances 09-17-2007 07:13 AM

Vermiculite - help!
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I put down the Vermiculite layer a week ago. 5:1 ratio with cement, seemed to be the right amount of water, kept it damp etc. As I posted before, the Vermiculite I got was kinda chunky (4-8mm diameter or up to 0.3 inches). I thought I'd read that you need the chunkier kind because it insulates better, but now I can't find that reference anywhere.

I stood on the layer to test it, and while it is firm, you can compress the Vermiculite particles with your thumb. Or pull them off. Is this really the material I should entrust the whole weight of my oven to?? Is it going to hold up? Has anyone made an oven on a layer of Vermiculite this chunky before or did I get the wrong sort? Or is Vermiculite always like this?

I'd really like to know now, before I build the oven and it all sinks in and falls down...

maver 09-17-2007 10:20 AM

Re: Vermiculite - help!
That's chunkier than the perlite I had but not by much. When mine had set it did not have any real compression when I stood or pushed on it, but you could definitely scrape it. Mine was not ideally level either, so I put a thin skim coat of mortar to level it before applying sand/fireclay, and after that it was very solid. I think the vermiculcrete (or perlcrete, as I used) is fine for compression, but any shearing force will tear it apart. I bet you are ok. If you read back through the forum, almost every builder raises the question you are asking (will this stuff really work?).

nissanneill 09-17-2007 06:03 PM

Re: Vermiculite - help!
Hi Frances,
this is a very common problem with most ove builders.
The vermiculite I used was around half the size of yours and set fine with a ratio of 1:5. With yours being much courser, I would have been tempted to reduce the ratio to 1:4 or even a little less to ensure that sufficient portland cement will bind the vermiculite particles together better.
Chech out my pics at:

permalink #2.
I would be tempted to mix a higher cement ratio batch and put another thinner maybe 40-50mm layer on. You might also scrape off some of the older vermiculite cement from the edges where the dome bricks would be mortared so that a thicker stronger layer would support better the dome weight. The extra insulation would also help your hearth/oven to hold the heat better as it looks a little thin to me.

Good luck.


wlively 09-18-2007 08:11 AM

Re: Vermiculite - help!
Don't worry about pulling it off, that is not what it is there for.

As for compression... Looks like you may have used rather large chunks. The bags I bought were on the large side, which is why I added a bit of sand to my mix, also made is easier to mix. Before you go tearing it up you may want to try the stress test. I don't remember who first said it on this forum, but place a brick down and stand on it with all your weight. That should approximate the weight per sq inch area of the dome. You can't use your thumb because that is too small an area. If the brick does not compress, then you have no problems. You may also want to wait 2 weeks after the pour before testing to assure that it firmly set.

Frances 09-18-2007 10:17 AM

Re: Vermiculite - help!
Hey, thanks guys! Good trick with standing on the brick - now, would that be my weight, or my hasband's?

I really don't want to add any more height to the stand, so I'll pass on adding more Vermiculite - it is a full 10 cm layer so it'll just have to be enough.

If I add a thin layer of mortar, I'll have to wait for at least another week before starting the oven floor, right? But that would give the Vermiculite a full two weeks, so maybe that will be for the best.

Thanks again for the advice,

maver 09-18-2007 09:15 PM

Re: Vermiculite - help!
I don't think you'll need to wait a full week if you add a thin layer of mortar - 2 days ought to be plenty. And 10cm ought to be plenty of underoven insulation. For the standing on the brick question, obviously it should be whoever has better balance!

Frances 09-19-2007 12:51 AM

Re: Vermiculite - help!
Made me laugh (no mean feat this early in the morning...)
Cool, I'll get on it right away, then I can start the dome this weekend after all!

nissanneill 09-19-2007 04:09 AM

Re: Vermiculite - help!
don't waste time and get right on into it. The sooner you get it done, the sooner you get to enjoy it!


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