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gemunden 04-05-2009 10:43 AM

Problems with the Cooking Floor and Hearth
Hello Everyone, I was wondering if someone could help me out with a couple problems that have set me back a bit. First of all, the Hearth that I poured for my oven did not dry properly. It did in a few places but for the most part the vermiculite and concrete remain wet or at least dry and squishy. Then, I made the oven floor out of fireclay and sand all just like it said in the plans I downloaded and that mixture did not harden either. I waited about 2-3 weeks hoping that maybe my mixture was just a little slow or something but its been 4 months now and im preparing to start up construction. Before I start though, I want to make sure I know what went wrong and also if someone could explain very simply how the heart and cooking floor mixtures should be constructed. Thank You.


Jed 04-05-2009 05:35 PM

Re: Problems with the Cooking Floor and Hearth
Hey Lou,

The vermicrete mixture should have cured to a solid structure. This stuff is not as strong as a concrete mix, and it takes several days to get 'hard', but it should get hard. Can you describe how you mixed the vermicrete, and what your mix formula was?

I used perlite, but the two materials should function very similar. I mixed anywhere from a 5 to 1 mix (perlite and Portland cement) to an 8 to 1 mix. In every case, it took several days, but the stuff got 'hard'; If I smacked it with a hammer, it would dent, but not very much. There are two theories on how to mix this stuff. I loaded 5 shovels full of perlite into my wheel borrow. In a five gallon bucket, I put in 1 shovel full of Portland cement, and water. I had to experiment with the water to get enough, but enough water that I could stir the Portland in to a slurry. I used a drill with a paint mixing attachment to mix this Portland and water slurry. I poured the slurry in on top of the perlite and mixed this concoction as complete as I could with a garden hoe, coating all of the perlite and working it to a consistent mix. This mix went onto the structural slab up to the top of the pre-built form, was leveled and left to cure. It took several days to cure to hard, but it did get hard. The alternate strategy to mix the stuff is to put the Portand and perlite into a wheel barrow, and mix till a uniform color and all of the perlite is covered, then add water and mix to a uniform consistency.

The fireclay and sand mixture doesn't get 'hard'. It's job is to provide a surface that will be adjustable... meaning you can level one brick on the oven floor relative to the next brick on the oven floor. You install the fire clay with a tile mastic trowel, or near equivalent. You install just a thin coat of this stuff and as you install the bricks, you can tap the bricks into place and get the floor more uniformly flat by adjusting the elevation of the brick by just a little bit. This mixture installs at the same time you are installing the firebrick floor of you oven. You want this stuff squishy when you set the brick in place, so you can tap the brick level.

Hope this helps...


Xabia Jim 04-05-2009 11:08 PM

Re: Problems with the Cooking Floor and Hearth
I used a rubber hammer to finely adjust my firebrick hearth into the sand/fireclay mixture (but I used a dry mix) it should work with the squishy mix too I'd think.

Xabia Jim 04-05-2009 11:09 PM

Re: Problems with the Cooking Floor and Hearth
Oh, and my perlcrete was like styrofoam when it cured, but dry.

Neil2 04-10-2009 12:20 PM

Re: Problems with the Cooking Floor and Hearth
I found that perlite makes it more "workable". Use a large granula perline, not the fine stuff.

For under the hearth I used; 4 vermiculite, 4 perlite, 1 Portland cement. i.e 8:1 mix. This will provide plenty of compressive strength for your hearth and dome.

For the dome insulation I used; 6 vermiculite, 6 perlite, 1 Portland cement. i.e 12:1 mix. A weaker mix has a higher insulating value.

I mixed it gently in a wheel barrow dry than added water. Just enough water to make it workable. If you mix it a machine, you will tend to break down the vermiculite/perlite particles increasing the density and reducing the insulation value.

Keep it shaded and let set for four to six hours. Thereafter keep covered and damp for at least seven days. Do not allow it to dry out.

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