Hearth Floor

[ThisOldGarageNJ]

Hello All,,
I poured my vermiculite slab today and hope to start the actual oven next week... I was wondering how many people lay their hearth bricks flat and how many stand them on edge and if you have your reasons for doing so ?

Thanks
Mark
TOGNJ

[DrakeRemoray]

Re: Hearth Floor

Most lay them flat so the floor will stay nice and hot during pizza making.

[Les]

Re: Hearth Floor

Mark, if you lay them on edge you are adding mass. That may not be a bad thing. In hind sight (as I am trying to cook coconut shrimp) I would consider a longer heat up time for more retained heat. Baking is becoming the best feature of my oven - we are getting burned out on pizza (you guy's don't know where I live so put out the torches ) My reason for laying them flat - it was in the plans...
Les...

[MK1]

Re: Hearth Floor

Les,

My question concerns the thickness of the floor vs the dome. I know from reading a lot on this forum that this design is very well thought out and enough have been built and tested to refine and prove them. I think I understand why most of the features and details are established but I wonder why the floor can be less thermal mass than the dome.
Is it because the fire and coals conduct directly, heating the floor to a higher temperature? Would a 3" or 4" thick floor burn the bottom before the top is done? I assume there's some kind of thermal release from the floor to the pizza or bread because direct contact and might suppose that a 2" floor has just the right amount of retained heat.
You stated that you're using your oven more for baking. Is it just the floor that needs additional mass or the whole oven?
I have started on my supports for the slab, who ha! Materials are converging on my unworthy domicile at this very moment. I'm excited. Damn, I love a project!

Mark

[Les]

Re: Hearth Floor

You stated that you're using your oven more for baking. Is it just the floor that needs additional mass or the whole oven?

I think the whole oven if your goal is baking. The Allen Scott design is all about mass and it's for baking. I don't think you will go wrong with adding more to the floor - it will require more fuel and time to get to temp. This design lets the dome reflect the heat back down to re-charge the floor. I have heard of some, that while cooking a lot of pizza's, move the coals over the spot to speed up the process. If I were to do it over, I think I would put them on edge - you will end up with more edges, but it time it will smooth out. Be interesting to see what others think about this.

Keep having fun!

Les...

[dmun]

Re: Hearth Floor

The 2 1/4 inch floor developed through trial and error. The early builders thought that the oven should have additional mass on the floor, and the original plans called for the insulation layer to be below the support slab. This was not a success, with long heat-up times and the floor too cool for pizza cooking. Then there was experimentation with an "island hearth", with a slug of concrete thermal mass just below the cooking floor. Finally it was determined that just the single layer of firebricks did the job fine, directly on top of the insulation.

So, in retrospect, why does the oven have so much more mass on top of the oven than on the floor? The simple answer is that heat rises. The oven gets MUCH more hot on top than on the bottom, to the point where the WFO can work like the broiler in a gas stove, using the heat radiating down from the top. A thick floor is a cool floor, and a thinner floor will more easily get and keep pizza heat.

[MK1]

Re: Hearth Floor

So, let me see if I have this right. We place the item (pizza or bread) on the floor. The bottom of the item gets most of it's heat from conduction, the top from convection and radiation. While cooking the floor is giving up heat and cannot recharge because it's shaded by the item. When removed, the thinner floor recharges quickly. So it's a fast floor.
A floor with greater mass after heating up would have greater heat reserves, not need to recharge as soon, but would then take longer to recharge, so it's a slow floor.
If I'm understanding this thing right, sizing the components based on their conductivity and heat retention capabilities is a bit of a trick. An oven with greater mass might heat up quicker with a more conductive material, behave more like a lower mass oven but would require more insulation. I might be way off base here, and I'm not sure what compromises material properties would impose.
I see some builders use the "island hearth" with a layer of splits added to the floor but I don't think I saw more insulation specified. If my thinking is correct, this gives greater heat reserves but slows the floor down. I would be very interested to know how the 3" island hearth performs.

Mark

[dmun]

Re: Hearth Floor

I would be very interested to know how the 3" island hearth performs.

There were very few of these built, it was a transitional idea, and was soon abandoned. A forum search for "island" didn't return anything, perhaps James has a better memory about this than I do.

Why do you feel you need more floor mass? Am I missing something?

[MK1]

Re: Hearth Floor

dmun,
The reason I was contemplating hearth mass was because of the quote from Les below. He implies the oven is tuned for pizza over all around baking. I'm very hesitant to change anything in the basic design for the reasons mentioned in my previous post. The only change I have in my build is a copper foil vapor barrier between the dome and it's insulation.
I know I'm being a PIA but I am compelled to turn this thing over in my mind until I think I understand the thermal dynamics of the oven, always trying to reinvent the wheel.
I don't want to prototype an oven. I've done that with too many other things and properly done the process involves too much effort, expense and iterations.
Thanks,
Mark

Mark, if you lay them on edge you are adding mass. That may not be a bad thing. In hind sight (as I am trying to cook coconut shrimp) I would consider a longer heat up time for more retained heat. Baking is becoming the best feature of my oven - we are getting burned out on pizza (you guy's don't know where I live so put out the torches ) My reason for laying them flat - it was in the plans...
Les...

[james]

Re: Hearth Floor

Dmun has it right. We thought about the island hearth as a transition design, back when there was a larger number of folks considering building the Scott oven. I even built an oven with the island hearth myself, but experience has shown that you don't need the "island" mass under a standard 2"-3" cooking cooking floor. More than that, the extra mass can be negative.

The question is whether a thick floor will give you more retained heat for baking, and in a vast majority of situations, the answer is no. Don't forget that your oven will only retain heat that you put in it in the first place. The longer the fire, the more heat you have to work with - regardless of how thick the floor is. Also remember that in order to fully heat your floor, you would have to heat the entire mass (top to bottom) to 800?F+ -- which is an incredible amount of heat. That is why you don't need to go thicker on the floor. You can just fire your oven longer with the standard floor, and you will never completely "fill it up".

Hope this helps.
James

[Les]

Re: Hearth Floor

You can just fire your oven longer with the standard floor, and you will never completely "fill it up".

James - isn't there a point where the bricks get saturated? I know I've seen where I slid out a dutch oven that the bricks under it were cooler. It just seems adding a "little" more mass might not be a bad thing. Wasn't someone building their floor with the bricks on edge - be interesting to see his numbers.

les...

[james]

Re: Hearth Floor

Hi Les,

They will definitely eventually get saturated, but I think it's at a much hotter and longer burn than we are typically doing when we either bake pizza or fire our ovens for baking. I used the term "never" too liberally -- but my point is that we aren't "filling" the floors that we have. I have never even fully fired a Primavera -- which holds a huge amount of heat.

In terms of floor mass, we've seen it all:

9" Scott oven -- bricks on side, set on 4 1/2" concrete slab
5" 2" Casa floor set on 3" firebrick island
4 1/2" bricks on side
2 1/2" - 3" firebrick floor
2 3/4" Modena commercial oven floor
2" precast residential floor
1 1/2" firebrick split floor

James

[DrakeRemoray]

Re: Hearth Floor

MK1,

Check out this thread.
http://www.fornobravo.com/forum/f17/...temp-6528.html
It discusses the problem of burned bottoms on bread. I think a cooler floor is acutally a benefit when baking. I do a lot of baking (see this thread) - and have had more problems with burnt bread bottoms than anything else. I think of the dome as the battery that charges the oven. A little extra mass in the dome (I added 1inch of mortar) may help, but others without that extra mass have reported very similar oven temps over time. I think the real key is insulation.

Hope that helps,
Drake

[MK1]

Re: Hearth Floor

Thanks to all for the replies. I have a clearer picture of what's happening in the oven. The thread Drake refers to is somewhat telling in that each quadrant has a slightly different temp, particularly the fire quadrant.
I'm assuming that when James says we're not filling the floors as it is, the dome is 1000F near the end of firing, the floor is somewhere below that (800?) and as the initial "plasma" phase dies out the respective temps equalize.
I have my four piers done and in the next few days I'll pour the slab. I'm going to insert an .015 X 3" stainless flashing edgewise into the concrete (vertically) that will describe the outer perimeter of the insulation and be inside the final render (igloo design) I'll soon post a few pictures as there is more to look at. I will also mention that I've looked at almost all the builds on this forum and you SOBs have set the bar pretty high!

Mark

[dmun]

Re: Hearth Floor

I know I'm being a PIA but I am compelled to turn this thing over in my mind until I think I understand the thermal dynamics of the oven,

Not at all. We would a LOT rather have people ask questions before they build, rather than after.

always trying to reinvent the wheel.

I'm in no position to criticize any one for that.