vermiculite still damp

[SimonWoodfield]

Hi all,
I built am completed my oven about 18 months ago, using 4" of vermiculite concrete and rendered it (stucco?). It worked well, but after a few firings cracks appeared in the render which was getting very hot. So I removed all the render and vermiculite and started again. This time I used 6-7" of vermiculite concrete, I didn't render it. It has been under a tarp for around 8 months, with occasional usage. Again I noticed at some points it was very hot. So yesterday I had a 5 hour fire, marked all of the hot points with paint, most of them were cracks. I figured I would angle grind around the cracks and re fill. Then put an extra inch or 2 all over. When I cut into the vermiculite, it is still damp! and there is steam comming from the cracks. I know the weather in the UK is rubbish, but should it still be damp, and is this why it is cracking. Is this why it isn't retaining heat properly, is the moisture conducting it? The fire was still burning at 11:00 last night door was shut with the embers still glowing, and it is 100 C 10 hours later. I have read on here of much better heat retention. I can use it for Pizzas, which are excellent, but I had hoped to do bread and all of the other exciting things that seem to go on here!!

Any thoughts appreciated.

[SpringJim]

Re: vermiculite still damp

if your vermiculite was properly mixed with the cement, it should set up without being wet, ...particularly after 18 months and using the oven.

It sounds like you have used it enough to drive out the moisture?? Is it in the rain? or does the tarp keep it dry? Did you cure it properly to begin with?

What about just keeping it hot now for a few days to see if you can drive out the moisture before fixing the cracks?

[dmun]

Re: vermiculite still damp

I suspect leaks in the stucco dome. You may want to consider a proper roof at some point. It really isn't going to cook well unless it's completely dry

[SimonWoodfield]

Re: vermiculite still damp

I cured it for about 3 weeks, the outside was certainly completely dry. I thought that the tarp was keeping it dry, maybe not as well as I thought. I will try to drive the moisture out. Dmun, there is no stucco at the moment, I took off the first attempt completely and have left it with just the vermicrete but covered with a tarp. I would put a roof on, but the wife likes the dome shape and she is the boss obviously. I think fires until I get no more steam then fix the cracks then stucco.
Thanks for the thoughts

[egalecki]

Re: vermiculite still damp

What was the ratio of portland to vermiculite you used? You'll get better insulating value if you use less portland to more vermiculite. I'm not sure why it's still wet, though. It certainly shouldn't be. I'm thinking your tarp isn't doing the job.

When I was finishing up my dome, I would leave the tarp off during the day and put it on at night or when it rained. That way it dried better and didn't just stew in its own condensation.

I also think that if you've got 4 inches of vermicrete, you may need a couple more for the best insulation if you haven't used any of the insulating blanket. I think the plans called for 6 inches. You could conceivably just put more on the outside. Then, when it's good and dry, you can stucco it again.

[Neil2]

Re: vermiculite still damp

No stucco is ever water proof. Even acrylic stucco is, at best, "water resistant"

You will have to cover it with a waterproof tarp in the wet months or when it rains.