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Old 07-09-2011, 03:45 AM
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Default hearth material ?

I am ready to pour the insulating slab , that my hearth slab will sit on.My question is, what material is used for the hearth slab, that will absorb and stand up to the heat of the oven . I want the slab 6" thick and then lay the fire brick on top of that to make the hearth 10" thick. what should this 6" hearth slab be made of ?
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Old 07-13-2011, 04:07 AM
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Default Re: hearth material ?

Why are you putting you insulation under your hearth? What I did and I think most people do is a cement slab, then insulation (vermiculite in my case, about 3 inches), then your fire brick for a cooking surface.
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Old 07-13-2011, 07:01 AM
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Default Re: hearth material ?

Sounds like you've been following plans for a barrel vault bread oven. That ten inches of thermal mass means that your oven will take massive amounts of time and wood to heat up, and may never reach pizza temperatures. Some of those plans also have the concrete slab suspended from a few exposed lengths of rebar, which is such bad practice as to make your head spin.

I'd stop now and take the time to read the forno bravo oven plans (Pompeii Plans Ready for Download).
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Old 07-13-2011, 01:13 PM
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Default Re: hearth material ?

Quote:
Originally Posted by kirk View Post
I am ready to pour the insulating slab ,
If you are doing a barrel here is my take on the hearth insulation.

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Old 07-13-2011, 01:26 PM
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Default Re: hearth material ?

Indeed, I am building A vault oven that will actually be a hybrid ,allowing for pizza (with the fire still in the oven) and bread baking after the fire is removed.I understand that the thicker the mass the more wood it will take along with the extra time needed to bring the oven up to the correct temp.I'm going to scale down the thickness of the hearth and dome to end up with an oven that fits my needs.I am not suspending the oven .it will be setting on a solid bed of sand contained by the foundation.The question still remains. What material should be used for the hearth slab , that will both absorb and retain heat .Please share your knowledge with me. Thank you , Kirk
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Old 07-13-2011, 02:51 PM
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Default Re: hearth material ?

I think your best bet would be to tell us what you want out of your oven. The hybrid you describe sounds a lot like the concept behind the Pompeii oven, and if my notion is correct there would be little reason to reinvent the wheel, even if you decided to go with a barrel vault shape built like a Pompeii. The Pompeii plans even cover adding additional mass to the hearth for more retained heat for bread if you need it.
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Old 07-13-2011, 03:17 PM
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Default Re: hearth material ?

Hi Kirk,

The question seems odd because most of us don't use a hearth slab to absorb heat. It seems an unconventional approach. In my view, the top slab should be made of concrete and rebar. This slab supports insulation, hearth and oven. You don't want it to get hot. You can choose from several types of insulation and you can use two layers of fire bricks if you want to hold heat in the hearth or you can stand fire bricks on their narrow side to make a very substantial, heat holding, hearth.

Your options for insulation include ceramic fiber board (2 or 3 inches) or a mixture of vermiculite and Portland cement or perlite mixed with Portland cement, in a ratio of 1 cement to 5 of vermiculite or perlite. You can make the home made insulation 3" to 5" thick.

The short answer to your specific question "What material should be used for the hearth slab , that will both absorb and retain heat". There is none that I would recommend, except for additional fire brick.

Check out the free Pompeii oven plans for additional ideas on insulation and construction.

Cheers,
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Old 07-13-2011, 03:25 PM
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Default Re: hearth material ?

In the classic Alan Scott design the structural hearth is directly bellow the firebrick cooking hearth and used for additional thermal mass with the insulation then located under the structural hearth. I'd assume that is where this design is stemming from.
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Old 07-13-2011, 03:32 PM
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Default Re: hearth material ?

If it were my oven I would consider laying the standard 2 1/2 inch thick firebrick hearth on the insulation layer. I would then build the vault. An additional 2 1/2 inch thick firebrick layer, laid loose, would bring your total up to 5 inches which is a lot of thermal mass for the floor. Three layers would be 7 1/2 inches.

If these upper layers are designed to be removable, this will give you lots of options to modify the thermal mass once you gain experience with the oven.

Last edited by Neil2; 07-13-2011 at 03:34 PM.
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Old 07-13-2011, 03:36 PM
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Default Re: hearth material ?

My floor thermal mass is 40mm of clay paver with 40mm of fire tile on top and it is plenty, the oven stays hot for a week with 1 firing. (see pic above)
All sitting atop of a 40mm calcium silicate board.
I am at a loss as to why you would need more mass unless you plan to run a bakery.
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