#1  
Old 03-06-2007, 01:26 PM
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Default Splits in the hearth..

Was on the phone with a refractory supplier down here and he said it would be a good idea to double up on a the brick in the hearth and put it in running bond so that seams overlap...

He mentioned using soaps which are he said are standard brick cut down the 4.5" side, so you'd have 2.5 x 2.25 x 9... I'm thinking you would just use them on the sides to set the offset...

However instead of doubling up on the standard brick and increasing the hearth mass, you could just use splits... and cut splits in half lengthwise to get the affect of the soaps to offset your seams on the bottom layer... I imagine you could also do 2 reverse herrring bones as well..

Anyway, he said, as much as you can, you want create a fire break and the seams in a single layer give heat more direct access to your insulation layer than if you overlap seems... I hadn't heard of this before, so I thought I'd throw it out there for everyone...

The downside of using 2 layers, whether they be splits or full size is the additional cost for the hearth floor.. my understanding is that splits costs about the same as full size bricks. Haven't run the numbers, but I wouldnt' think it would be significant in overall project cost terms...

Any thoughts?

JB
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Old 03-06-2007, 03:33 PM
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Default Re: Splits in the hearth..

I don't understand the point of this. Two layers of firebrick will be too much thermal mass. Two layers of "splits" will only make a weaker and more fragmented floor. Any cracks in the floor, and I haven't heard of this being a problem, will just fill with wood ash.
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Old 03-07-2007, 06:32 AM
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Default Re: Splits in the hearth..

I can only imagine the seams being a problem if you have an isulation layer that is not up to the temperatures, but that's why we don't use loose fill paper to insulate the floor . I agree with DMUM.
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Old 03-07-2007, 09:42 AM
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Default Re: Splits in the hearth..

Hmmm... remember.. I'm only the messenger here.. but it did get me thinking... it is clear to me that a single firebrick layer works fine... there are just too many ovens in use and with no issues...

On the other hand, from an academic point of view, could it be better? Could you potentially require less insulation OR have more heat staying in the floor than bleeding into the underlying insulation? It would seem to make sense... just like I think it make sense to run 2 layers of 1" insulation board and overlap seams than just one layer of 2" with the seams.. Sure the 2" single layer works, but...

Anyway, the purpose of the topic is to consider alternatives.. improvements.. refinements... not to say that the current process is wrong, just throwing something out there for consideration.

I do disagree with the splits creating a weaker more fragmented floor... the splits are pretty damn strong.. it's not like we're driving a car over them or anything.. in any event, at one to two bucks a brick, it's not rough to replace one if it cracks.. be it a full size or a split...

On second thought though, doesn't the space shuttle have a single layer of heat shield tiles? Mavbe that's substantial enough reason to leave it alone...
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Old 03-07-2007, 11:25 AM
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Default Re: Splits in the hearth..

I understand your question and the idea that layering may have value, but from the insulating standpoint I don't think the idea holds up. The idea of firebrick is to have a material that conducts heat well to help with handling the high heat and to form an even "heat well" to avoid hot spots. By layering bricks you may lose some of that type of beneficial conduction (it's the job of the insulation to put a stop to that) and may have more risk of uneven heating. You also add more potential uneven spots (raised areas) in your hearth which would be annoying with cooking. Layering whole firebricks would make your heat well very deep, maybe ok for retained heat cooking but much harder to bring up to pizza temps.
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Old 03-07-2007, 02:15 PM
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Default Re: Splits in the hearth..

Maver,

I think it would make sense to hear from someone with an engineering background... Perhaps an engineer could shed some light in absence of any real testing... Otherwise, it's all conjecture.

Any engineers out there care to weigh in?

JB
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Old 03-07-2007, 08:33 PM
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Default Re: Splits in the hearth..

Quote:
Originally Posted by johnrbek View Post
from an academic point of view, could it be better? Could you potentially require less insulation OR have more heat staying in the floor than bleeding into the underlying insulation? It would seem to make sense... just like I think it make sense to run 2 layers of 1" insulation board and overlap seams than just one layer of 2" with the seams.. Sure the 2" single layer works, but...
Hey, I'm not an engineer, but I took the same physics courses in college as my engineering friends. If we are only talking conceptually please allow me to play. I'll sit out if someone wants to crunch numbers or create a computer model. As James has pointed out before, there are two types of materials in an oven, insulators and conductors. Could you cut down on insulation if you increase the conducting layer in your oven by doubling the firebrick? Yes, because even firebrick (which is purposed in your oven lining with conducting and retaining heat) has some insulating value. Do you increase your insulating value by doubling up on splits or full bricks? The very small air interface between the splits (assuming an imperfect mating top to bottom) should have better insulating value than the brick itself (air is a pretty good insulator).

For pizza cooking, I think you really want your oven's refractory layer to allow heat through to the insulation fairly quickly, then you want the insulation to stop the heat in it's tracks. The balance is in having enough refractory layer (thermal mass) to keep the heat even without incurring excessively long heat up times. Makes me wonder if anyone has made a hearth with a single layer of splits - would it heat up faster (than an hour which is where a lot of us are with our full brick hearths)? Can you maintain an even temperature or does it fluctuate?

I don't think the downside of using double layers of splits is loss of conduction (nor is there much upside in increased insulation) so much as error propagation in creating a very smooth hearth. If you start with a very smooth surface like superisol and brush the first layer clean before applying the second layer this would likely be ok. But how would it be better? Overlapping an insulator as you plan for your insulation board makes more sense than overlapping conductors where junctions might impede heat transfer.

Hopefully someone with real engineering background weighs in.
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Old 03-08-2007, 01:34 AM
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Default Re: Splits in the hearth..

Good points... now you're talking!

To respond to your last two sentences, according to doug, the idea of overlapping the "seams" in the conductors (the splits in our case) would be to increase the "firebreak" between your open fire and your insulator. Sure, some ash will fall in their, but 1.25" of firebrick just has to be better at impeding access of the fire to the insulation layer than a minute amount of ash in a crack. The heat that might otherwise be lost directly into the insulation might then be retained in the firebrick itself. That was the idea anyway. I have nothing practical to go on other than his experience with these materials.

I don't get your last sentence... we want to impede heat transfer by overlapping these seams.. that way the heat from the fire stays in the "conductor" layer(s) and doesn't get lost as easily into the "insulation" layer (through the seams). I do think however, that there has to be some inefficiency in conduction created by the minute air layer introduced by the second splits layer. Significant enough to affect heat up times? No idea.

Thanks for your response...

JB
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Old 03-08-2007, 03:59 AM
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Default Re: Splits in the hearth..

This post comes at a most opportune time, as I wrestle with the question of my cooking floor materials.

Due to the height of my (now cast in the hearth slab) thermowells, I have no option but to go for a 3” floor. My refractory supplier has urged me to consider 12” x 12” x 2” or 12” x 12” x 1½” tiles, rather than standard 3” high bricks, so I am now thinking of two layers of the 12” x 12” x 1½” tiles. Further, I am also thinking of laying the first tile layer, putting the first (half-height or split) ring on top of them, and then the second tile layer inside the first ring. Normal build-up from there.

So I reckon I would get the best of both worlds – spreading the load of the dome over a greater area on my Cal Sil boards, yet having the ability to replace the floor (well the top half of it) in the future should I need to. I hadn’t thought of staggering the joins, but see no reason why this should not be done. As for the issue of air gaps between layers, I would be looking at laying the top layer on a very thin bed of refractory mortar, as long as this wouldn’t prevent future top tile removal, otherwise a sand/fireclay mix might be better. Or I might just mortar the joins of the bottom tile layer and not worry about the space between the two layers.

Comments?
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Old 03-08-2007, 06:23 AM
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Default Re: Splits in the hearth..

Quote:
Originally Posted by johnrbek View Post
according to doug, the idea of overlapping the "seams" in the conductors (the splits in our case) would be to increase the "firebreak" between your open fire and your insulator. Sure, some ash will fall in their, but 1.25" of firebrick just has to be better at impeding access of the fire to the insulation layer than a minute amount of ash in a crack.
I'll comment here from experience with my oven hearth (and regular brushing of the hearth) that the vertical seam between bricks get packed with ash (which has fairly high insulating value relative to the firebricks). As I work with the oven there is regular need to brush the hearth to keep the surface free of ash and (during baking) embers - this drives ash into the cracks, 'sealing' them.

Quote:
Originally Posted by johnrbek View Post
we want to impede heat transfer by overlapping these seams.. that way the heat from the fire stays in the "conductor" layer(s) and doesn't get lost as easily into the "insulation" layer (through the seams). I do think however, that there has to be some inefficiency in conduction created by the minute air layer introduced by the second splits layer. Significant enough to affect heat up times? No idea.
Here's where I think you and I are looking at it differently, JB. I would direct heat from my oven chamber through to the outer layers of my firebrick (right up against the insulation, perhaps with copper heat pipes?) if it were practical. Why? To improve heat up time (reducing wood use). As you fire the oven, the surface of the firebrick begins to heat but the conductive effect is to suck that heat through to the cooler brick close to the insulation. I didn't build my oven with thermocouples, but you can read Aravelo's experience with observing hearth surface temperatures and the time it takes to thoroughly saturate the bricks with heat.

http://www.fornobravo.com/forum/atta...pizzaexcel.jpg

I don't think there is any benefit from slowing the conduction of heat through the firebrick - in the ideal cooking situation you are starting cooking (after heat up) with an oven that has equal heat at the hearth side and insulating side of the firebrick. Let the conducting layer heat up to a steady state, then let the insulating layer do it's job of keeping the heat there.

You and I are in complete agreement that the air interface between two layers of splits may not be significant. Hendo, you could answer this. I suggest you build your oven with one side of the oven composed of 3" firebrick and the other side made of a double layer of 1 1/2" splits. Be sure to install thermocouples in both sides of the hearth at the insulation layer. Build your heat up fire in the middle of the oven. I look forward to your data .
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