Non-mortared hearth: Will it work?
The base of my oven is made of ciderblock, with concrete lintels spanning the void in between.
Around the perimeter of the base, I will lay a shallow wall of standard bricks, set on their narrow side. This will create a wall 4" or so high. It will be mortared to the base.
Inside the wall, on the bottom (directly on top of the slag lintels of the base) I will lay down a 2" layer of high density fiberfrax board. This eliminates the need for vermiculte cement. The board will be cut precisely so there is no gap. It fits in snug.
On top of this, I will place the firebricks for my hearth, without mortar. They are held in place by the standard brick wall. Again, they will fit in snugly (I may have to cut them to get them to fit properly).
Voila! Two and one half inches of firebrick, with two inches of fiberfrax insulation underneath. The shallow brick wall will keep everything from sliding side to side, and bricks are easily replaced if they break. They are not mortared in, so if they need to expand, they can.
Any problems with this approach? The only possible problem I can see is that the brick wall may not allow the firebricks to expand. To address that, I'll simply add a small gap (maybe 1/8")
Of course, I'll have to cover any exposed firebrick on the hearth that is not covered by the oven dome, but that's easy (I think).
I'm not sure that you should tightly constrict the cooking floor within a rigid brick wall. The bricks will expand and contract while the oven heats up and cools down (thermal dialation), which will either push the walls out, or crack the bricks.
In general, putting the brick cooking floor on a bed of sand and fireclay is easy, and it lets you make the floor pretty smooth. Most people build the Pompeii oven with the dome walls resting on the oven floor, which works fine.
The Forno Bravo ovens all have a round cooking floor, and the dome rests directly on the hearth. There is a gap between the cooking floor and the dome which allows for thermal expansion.
The dome around the cooking floor is a better design, as it does a better job of keeping heat inside the oven -- but it is more difficult to build.
Thanks for the excellent observation. What's driving this train is the fact that I paid (probably too much) a couple hundred bucks for a bunch of High Density Fiberfrax insulating board, which I MUST use as the hearth insulation (I can't send it back, and I want to get my money's worth). There's GOT to be a way to use it.
Seems to me that I could proceed with my plan as long as I allow some extra space between the bricks and the wall for expansion. I could also stuff some thin fiberglass insulation along the periphery which will give slightly as the bricks expand, thus keeping them snug yet allowing for expansion.
Just curious: why is thermal dilation not a problem with the bricks in the dome of the Pompeii oven?
Why does the floor of the Forno Bravo oven not expand and force the walls of the dome apart?
The good news is that you will have a really well insulated hearth, which is good. No problems there. Just use all of your board insulation in the hearth. The old adage holds. There is no such thing as too much insulation; though there is definitely such thing as too much thermal mass.
For the dome, remember that you need insulation that rests again the dome itself. Whether it's vermiculite, perlite or insulfrax, the dome has to be encased. Board insulation above the oven won't help. And sand is not good.
With a Forno Bravo oven, the round floor comes in pie shaped pieces, and the dome fits around the floor and rests on the hearth. There is a gap between the two that allows for expansion and contraction -- which gets filled up with ash. The gap is at the outside border of the cooking floor, so it doesn't become an issue with cooking.
Most Pompeii ovens have the dome resting on the floor. It isn't as efficient thermally, but the walls of the oven dome don't constrict the floor. Everything moves together...in harmony. That sounds like a song, or worse, a commercial. :eek:
James is correct, I think, in his observation that you can never have too much insulation. Last week, I added 2 inches of compressed fibreglass/foam insultation below my already 6 inch thick vermiculite layer. I took some contortionist photos during the install, which I will post (thanks to JE) when time allows.
Good luck with your work!
Please, be carefull with the fiberfrax temperature specification. I do not know yours, but be in mind that the hearth temperature will be over the 700°F.
Just some values: fiberglass is between 200/350°F and ceramic fiber around of the 600/700°F (over this values these fiber burns!)
In my oven I use fiberglass, however under and over the vermiculite layers.
No problem here. :D
4.5" of fire brick, covered with
1.5 " of refractory mortar, butted up against
1" of LD Fiberfrax Board, enclosed with
.5" of wonderboard,
The resultant temperature on the OUTSIDE surface of the wonderboard will only be 140°. Sweet!
That's my WORST case scenario, at the base of the dome with no vermiculite. In other words, the base of the dome will go straight up to the enclosure wall (This is due to space constraints due to my poor planning) As you go toward the top of the dome, the gap between the dome and the wall increases, and this gap is filled with vermiculite.
Sounds like I'm in business. I just gotta nail down a design and stick with it.
I don't think you need 1 1/2" of refractory mortar. Just enough to thinly coat the dome is good for structural support without adding too much mass. That will give you the space you need to add either 1"+ of vermiculite or even better, a 1" insulfrax blanket.
It will also give your oven the space it needs to expand and contract.
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