#11  
Old 07-06-2006, 04:32 PM
james's Avatar
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Hi Mike,

Vermiculite has great compression strength and will easily hold up your oven, arch, etc. As someone once noted, vermiculite concrete is used under swimming pools, that are a lot heavier than your oven!

On the Super Isol compression numbers, I will have to check. It has lots of little air holes, but it has good compression strength -- I just tried to push my thumb into it, and cannot make an impression. I had to push hard with the cap of a pen to make an indentation.

James
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  #12  
Old 07-07-2006, 08:01 AM
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Mike - definitely go for it. I am just nit picking and trying to keep my brain thinking. I probably should have made my post a private post to James. Doing a Google search I think that the units are wrong (ft^3 instead of in^2). that is also supported by quick experiment that James did.

If you search the threads you will find that the compressive strength of vermiculite has been discussed and approved.
http://www.fornobravo.com/forum/show...ssive+strength (Question about isolated hearth slab)

Or you could read my ramblings below:

POOL COMPRESSION CALCULATION (Simplified)
1 gallon of water equals 8.33 lbs.
A 5 gallon pail has a diameter of 11.125" and a height of 13.25"
5 gallon pail/bucket contains .75 cubic ft.


take a column of water in a pool that has the dimensions of the 5 gallon bucket and a pool depth of 12 feet. What is the compressive equivalent pressure?

the pool is 12 feet high or 144 inches and the bucket is 13.25 inches : 144/13.25 = 10.9 buckets call it 11

there are 5 gallons/bucket and you have 11 buckets so you have a total of 55 gallons

there are 8.33 pounds/gallon of water and you have 55 gallons : 8.33 X 55 = 458 lbs round it up to 460 lbs.

that weight is on a diameter of 11.25 inches or on a surface area of 99.4 in^2 (Area = pi d^2 / 4)

so you have 458 lbs pushing down on 99 in^2 or 458/99 = 4.6 psi

vermiculite & cement = vermiculite concrete, at 6:1 mix has an average compressive strength of 155 psi

POMPEII COMRESSION STRENGTH
I am going to make some gross assumptions (lean toward conservatism) here.

Assume the weight of the oven is evenly spread about the circumference of your first ring.
Assume that each brick weighs 10 lbs (conservatism since they really are closer to 8).
Assume that the dome takes 200 bricks (a 42 inch dome is 180 bricks)
Assume you decide to use a façade of bricks, say 350 and instead of bearing the weight on their own circle you manage to make the dome bear the weight.
Assume the width of the weight bearing brick is 3 inches and the inner diameter of the oven is 42 inches thus the outer diameter is 45 inches (that will give you the area that bears the load)
Assume the opening for the door is 15%


Area of a 45 inch circle is 1590 in2
Area of a 42 inch circle is 1385 in2
Subtract the inner circle from the outer to get the area of a Closed Oven is 205 in2.

Remove 15% for the door and your bearing area is 174 in2

that area is loaded up with 550 bricks at 10 pounds each or 5500 lbs

How much pressure is it made to bear? 5500/174 ~ 32 psi way below the compressive strength of 155psi

je

Last edited by jengineer; 07-07-2006 at 10:50 AM.
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  #13  
Old 07-07-2006, 08:42 AM
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Default Now I get it

je

Thanks for expanding the logic on this. I haven't had any doubts about the perlite or vermiculite concrete strength because of the information that's been presented so far. Now that you've done the math it is reassuring but more than that it's fun to see the math problem worked out. I just couldn't see where you were going at first.

Thanks
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  #14  
Old 07-08-2006, 07:57 AM
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Location: Ontario Canada
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Default glue for insulating board

If I were to glue the new whizzy insulating board to the vermiculite layer under the hearth what type of glue would you suggest Would silicon work? Is it worth the trouble re heat retention in the hearth ?
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  #15  
Old 07-08-2006, 08:48 AM
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Default Glue

Fud,

I'd go for Bulldog Premium construction adhesive. It gets stronger with heat. Not much of a chance the board would come down. Alternately, try to find Bond Lock. We use it for stone, and it's even stronger.

Jim
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  #16  
Old 07-14-2006, 12:01 PM
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Default Under Oven Insulation

James,

Just to be clear. If I use the new insulation board do I not need the insualtion layer on top of the concrete hearth?

Thanks,

Ryan
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  #17  
Old 07-14-2006, 12:30 PM
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Default Exactly!

You attach the SuperIsol directly to the concrete layer, and you do not need the vermiculite layer. They serve the same purpose, so you don't need the vermiculite layer. That would be overkill, and you don't need to go to the expense of hassle.

We sell a high temp caulk to ahere the panels, which I will put into the Forno Bravo Store latery today. It isn't expensive, so you don't have to waste time looking for a safe product to use.

Cool stuff.
James
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  #18  
Old 07-15-2006, 05:50 AM
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Join Date: Feb 2006
Location: Eastern NC
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Talking Looking for an update on the Super ISO board

Has anyone got an update on the ISO board?

How is it working? Any lessons learned?

I plan on using this product.

Was not sure from the previous posts how hard the material is - Can you poke a hole in it with your finger? I bet not, but was concerned.

Thanks

Christo
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  #19  
Old 11-03-2006, 08:17 PM
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Default CalSil thickness ?

Hello James,

on the order sheet I just received from my supplier in Melbourne, Australia, there are two sheets of calcium silicate board listed for under the cooking tiles (2 in) of the 1000 mm (ID) dome - but these are only 25 mm thick (1 in)! Is that sufficient, or will I still need to pour a vermiculite/ciment fondu insulation layer under the hearth?

Thanks again for your help!

Rgds,
Carioca
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  #20  
Old 11-03-2006, 08:32 PM
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Default Cal sil.

My calclium silicate insulation board is one and a half inches. The dude who donated it to us said it was rated to twelve hundred degrees. We'll see, but I'm pretty confident it will insulate sufficiently.
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