#21  
Old 06-28-2005, 03:54 PM
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Here's the post I did on the same subject on the brick-oven Yahoo group (for someone building an Alan Scott version but it works for the Pompeii too):

It's dependent on your specific installation -- hanging hearth, not
hanging, more custom stands (like fieldstone, etc.) change the size of
an individual installation. But, it's easy enough to calculate. Take
the width multiplied by the length of your forms (inner dimension),
multiply that by the height in those forms you're using for your
insulating hearth (e.g. forms are 2x6 which are really 5 1/2" tall but
you're going to have a 3" insulating layer & a 2 1/2" hearth layer --
you'd use 3" for the height). Convert to cubic feet & round up. That's
the total cu ft of the slab. Divide by 7 for the number of bags of
Portland and the rest is vermiculite or perlite.

Example:
Hearth slab of 6'x7' with a 3" insulating layer

Calculation is: 72" x 84" x 3" = 18,144 cu in
Divide by 1728 cu in per cu ft = 10.5 cu ft
Round to 11 cu ft (insulating hearth volume)

Divide by 7 (1/7th of the slab is Portland) = 1.6 cu ft of Portland or
1.6 94lb bags of Portland
Multiply this by 6 (6/7ths is vermiculite) = 9.6 cu ft of vermiculite
or 3 bags of vermiculite in 4 cu ft bags

So, you'd buy
2 94lb bags Portland
3 4 cu ft bags Vermiculite (or Perlite)

You'll have some of both left over. The extra Portland can be used for
the brick mortar when you build the dome and the extra vermiculite can
be used for insulating the dome. Adjust the calculations for your
specific hearth size.

Jim
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  #22  
Old 06-28-2005, 07:21 PM
paulages's Avatar
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Default thanks, jim!

okay, now that you posted that, i feel really lazy, like i was trying to cheat my way out of a 6th grade math test. thanks anyway.

here's today's labor:
note that i am creating a 3 1/2 inch overhang on the front (though this may be almost negligable when i veneer the wall underneath it...in any case, it gives me a bit more hearth space, where i couldn't quite afford it in the footprint.)
also note that, despite the foundation and first course of cinderblocks being level, somehow it didn't end up that way by the time i reached the top of the block stand. i decided it would be easier to fix with the top forms for the hearth, rather than adding mortar in the block stand to make the top course level, so that's what i did. i filled the large gaps between the block stand with duct tape. the exposed tape will never be seen, as the back three sides of the hearth will be covered by the veneer walls in the end. i caulked the crack on the front side, as this will possibly be the only place you will actually see the edges of the hearth, so i wanted the bottom edge to be smooth.
-paul




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  #23  
Old 06-29-2005, 06:52 PM
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LOL! No cheating.

Bob (I think) did a cantilevered design with his hearth so the 3 1/2" overhang you've got going should work very well. No issues about the leveling of the block walls - just get the top of the hearth forms level and the hearth will be spot on.

Jim
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  #24  
Old 06-30-2005, 12:21 AM
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Default woohooo!

the insulating perlite concrete layer...

and the thermal/structural concrete layer.

i sprinkled in a red concrete dye before the final trowel, just in case i end up not tiling the front shelf. i had it laying around anyway.
by the way, i used steel wire ties between each 2 x 6 and it's opposite to keep the forms from pushing outward with the weight of the concrete and it worked perfectly. the corners are just screwed together. oh yeah, and i staked 2x4's underneath the top form all the way around so that the form would sit flush with the edge of the block stand.
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Last edited by paulages; 06-30-2005 at 12:36 AM.
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  #25  
Old 06-30-2005, 10:40 AM
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Default hearth with stain

(and bird poop!!!!)
damn robins.
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  #26  
Old 06-30-2005, 02:22 PM
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Everything to excess. Moderation is for monks.
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  #27  
Old 06-30-2005, 06:15 PM
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Default except for the belgian monks...

who make great beer.
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  #28  
Old 07-01-2005, 06:45 AM
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Paul,
It looks as though you're off to a grat start! Now the fun stuff begins.
Good work!

Bob C.
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  #29  
Old 07-01-2005, 10:33 AM
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Default thanks bob,

yeah, i'm trying to round up some firebricks today, so i can work on this over the weekend. otherwise i'll have to wait until tuesday. i'm thinking of incorporating a BBQ into the design, just because it seems like there will be a lot of wasted space, even with a 42" interior.
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  #30  
Old 07-01-2005, 03:33 PM
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Default firebrick types

i've been unable to find a low duty firebrick here in portland. the closest i've been able to find is a medium duty made by carter (info found@ hwr.com, under the product datasheets). they are $1.50 a pop. other than what's listed in the firebrick primer, anybody have any advice? i'll keep looking, but wonder whether i should settle for the medium duty at that price...
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