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KINGRIUS 08-19-2009 08:37 PM

Hearth Slab, no lime...
Hello all, I am building my first oven and just poured the hearth slab last week. Halfway through the pour I realized I forgot to mix in the lime. I used 15 80lb bags of premix quickcrete concrete. According to Quickcrete engineering, their concrete with withstand up to 300 degrees before breaking back down to a powder form. I guess that's where the hydrated lime comes in, it holds the conc. together. My question is, will the slab get over 300 degrees with about two hours of fire before cooking with 4.5" of fire brick in between? I have received advice from some that say I should tear down and re-pour, to "don't worry about, it would take years for that slab to break down, and it would be just a few bricks settling over time..." I am leaning towards wanting to believe the latter, for the prospect of busting up all my concrete it disheartening. I have reinforced the dickens out of it with rebar and wire mesh. On the other hand, I don't want my oven to cave in on me. Any advice?

Tenne 08-22-2009 04:09 PM

Re: Hearth Slab, no lime...
You are going to pour either an insulating layer of portland cement and vermiculite(see the forum for the ratio) or use the ceramic fiber board available from FB. This will go between your brick floor and the concrete(no lime needed here, just plain old cement) hearth. That is exactly what the plans calls for. If you don't use the insulation layer you will not be happy with the result as the oven will not heat up properly.

KINGRIUS 08-26-2009 08:20 PM

Re: Hearth Slab, no lime...
The ceraminc board is available from "FB", is that Thank you for the information. I will need the board soon because I am planning on starting the hearth next week.

dmun 08-26-2009 08:24 PM

Re: Hearth Slab, no lime...
You can get the board from refractory dealers, or ceramic suppliers, as well as FB. There are several different kinds. You can also use a four inch layer of vermiculite or perlite (the ordinary soil conditioners) mixed with portland cement to solidify it.

DO NOT lay your oven floor directly on your hearth slab. Insulate! Insulate! Insulate!

DrakeRemoray 08-26-2009 08:25 PM

Re: Hearth Slab, no lime...
King Make sure you download the FREE plans from the fornobravo online store...

KINGRIUS 08-27-2009 06:13 AM

Re: Hearth Slab, no lime...
1 Attachment(s)
I have placed a 3" layer of perlite/portland mix at the BASE of my hearth slab. I have Rado Hand's cd and it appears this is what he does on his tutorial. Have I misunderstood the instruction?

RTflorida 08-27-2009 06:35 AM

Re: Hearth Slab, no lime...
To be honest, the Rado Hand method is "old school". Unless you are running a commercial bakery, the "heat bank" that Rado touts will be nothing more than a time wasting and wood consuming frustration.
Think of the mass you will have to will literally take SEVERAL hours to get up to pizza temps (trust me, that is A LOT OF WOOD).
If your plans are to bake multiple batches of bread, you may have chosen wisely, but you will still use a ton of wood at every firing.

The Pompeii plans call for the insulation to be on top of the hearth slab, directly under the firebrick. This allows for quicker heat ups (much less wood) and you still have plenty of retained heat for bread or roasting (even a day or 2 later) - The best of both worlds.

If it is not too late.....I would change it now, you will be much happier with performance later.


DrakeRemoray 08-27-2009 08:58 AM

Re: Hearth Slab, no lime...
What RT said is true. If you are a home pizza make or baker (as opposed to a professional making huge quantities) follow the free fornobravo pompeii plans...


KINGRIUS 08-27-2009 10:23 AM

Re: Hearth Slab, no lime...
1 Attachment(s)
Thanks all for your advice. No I am not building for commercial use, just home pizza/breads, etc. I have been building this oven piece-meal, depending on when I get the funds for the next stage. I will need to save for a few weeks before I can purchase the insulating board, but it sounds very well worth the money and having to wait a little longer before I get to begin the most exciting part of the oven.
I just set my lower arch yesterday afternoon. This crap is really fun!

Thanks again,


Neil2 08-27-2009 03:07 PM

Re: Hearth Slab, no lime...
The bottom of my 4 inch 1:6 vermiculite/portland cement layer (top of structural slab) gets up to about 120 F degrees after a full firing.

(I have an ash drop outside the door through the structural slab so I was able to take a measurement at this interface.)

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