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dkoenig 05-19-2008 07:45 AM

2- layer Hearth question
In the pdf, on pages 25 and 26 it describes the 2 layer hearth, structural and then insulating. on pictures 4.3 and 4.4 the 'oatmeal' layer is very rough. Is this what that layer should look like when done, or does it need to be smoothed out? Or does the underfloor paste that goes on next take care of that?

Thank you,


dmun 05-19-2008 08:27 AM

Re: 2- layer Hearth question
Yes, If you use the vermiculite concrete, you will end up with a rough surface, and some kind of layer is used to form a smooth surface for your fire brick. The current thinking is to put a smooth layer of DRY fireclay/sand or refractory mortar on the rough surface, trowel it smooth, lay your floor, and when everything is as smooth as you want it, then you wet the floor to consolidate the dry layer underneath.

If you use insulating sheets of cal-sil or other refractory insulation, you should be able to lay your bricks directly on top of the insulation.

dkoenig 05-19-2008 08:56 AM

Re: 2- layer Hearth question
How do you wet it after the fact. mister? light garden hose spray?

Thank you,


dmun 05-19-2008 12:29 PM

Re: 2- layer Hearth question
Yep. Or pretty much anything short of throwing a bucket of water on it. Sopping wet sponge would work too. Firebrick is pretty porous. The water will get where it's needed. This was Alf's idea, and he's a professional oven builder.

dkoenig 05-19-2008 02:26 PM

Re: 2- layer Hearth question
thank you very much. We got it into our heads to do a mud oven this summer. i have browsed these forums for years, so we are building the base to take a brick oven.

Thank you again,


Ed_ 05-20-2008 01:48 PM

Re: 2- layer Hearth question
Doesn't this make it awfully difficult to replace a chipped floor brick down the road? Not that this would ever be needed, of course. ;)

nissanneill 05-20-2008 04:56 PM

Re: 2- layer Hearth question
Hi Don,
I poured my concrete foundation and topped it with vermiculite cement. I agree with the others but does not need to be rough. You can smooth it our perfectly flat and level with a little care, but it seems porous with a surface resembling cork tiles.
See my Posting at:
which will show and explain my experiences throughout the whole build process.


dkoenig 05-27-2008 01:05 PM

Re: 2- layer Hearth question
Thank you (especially for the pictures). Helps a lot.


james 05-27-2008 01:36 PM

Re: 2- layer Hearth question
One other thing. It's OK that the insulating layer (FB Board, vermiculite, perlite) is not smooth. You will make a bed of sand (or sand and clay, or high heat mortar) that will let you level your cooking floor. The insulating layer has to be level, but not smooth.

Jim Bob 06-23-2008 07:51 AM

Re: 2- layer Hearth question
Just wanted to know how long you should let the vermiculite/concrete layer to harden or cure before starting on your oven? Is one day enough or does it require more time to harden?

Jim Bob

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