Hearth Design Philosophy

[CanuckJim]

Re: Hearth Design Philosophy

Les,

Small hairlines shouldn't be a problem, but get yourself some burlap, cover the slab with it, wet it and cover it with a tarp to reduce shrinkage as the concrete cures. Keep it wet for a week. What sort of ratio did you use in your mix? Also, was it straight Portland, Type S, Type N, or something else?Sounds like you might have mixed it on the dry side.

Jim

[Les]

Re: Hearth Design Philosophy

While we are on the subject of hearth design. I poured mine yesterday with concrete. I plan on using the superisol product so I did not use any vermiculite. About three hours after the pour and finish, I noticed hair line cracks that seem to follow the pattern of the rebar underneath. Do you guys think that this should be a concern in regard to strength? I've poured many yards of concrete and never seen this before. The weather conditions were perfect for the pour.

Any insight would be appreciated.

Thanks,

Les...

[dmun]

Re: Hearth Design Philosophy

Yes, you absolutely need insulation under the island hearth. Otherwise all the heat you need to cook pizza will bleed into the support slab and be lost. The original pompeii had insulation under the support slab, which went out to the side of the enclosure. It was found that this caused too much heat loss, hence the decision to switch the two. Whether you use insulation board rather than vermiculite concrete is mostly about having room for the extra thickness for insulation. I think the two both work well.

[stuart]

Re: Hearth Design Philosophy

Yes I understand the current thinking , but for sake of discussion this Easter morning...would you then have a full layer of the Super Isol beneath the slug and surrounding insulation?

[dmun]

Re: Hearth Design Philosophy

If you wanted a concrete slug for thermal mass under your brick floor, you could make a ring out of the insulating material (which would keep the mass where you needed it) and fill it with concrete, using the ring to level it.

It seems the current thinking is that the additional mass in not necessary.

[stuart]

Re: Hearth Design Philosophy

2. Insulation -- keeping heat in the oven and floor. That's the insulating concrete layer, with vermiculite held together with portland cement.

3. The cooking floor -- which is thermal. You want it to heat up, hold heat, and efficiently re-charge the heat in the floor from your live fire. The firebrick on its flat side gives you enough mass for typical backyard cooking and baking, and doesn't have too many seams. The Forno Bravo ovens have 2" floors that come in pie-shaped pieces. If you really want a little more mass under your floor (don't forget that commerical pizza ovens only have 3"-4" floors), you can add more mass under the floor, either with a second layer of bricks, or a poured disk of refractory mortar. That's the Island hearth (the extra mass under the cooking floor is surrounded by vermiculite concrete).

That's why we have formally changed the recommended hearth design. For most installations it is good to put the cooking floor directly on the insulating layer, and if you want more mass you can add it, while taking advantage of a more efficient design.

James can you address these two points again now that the use of Super Isol is being recommended for insulation in lieu of the vermiculite/portland mix? I'm curious how one would incorporate an Island hearth design with a Fornor Bravo oven using Super Isol.

[james]

This is a sticky posting that describes the logic behind the Forno Bravo hearth design.

I think of the hearth and cooking floor assembly as serving three functions:

1. Structural support -- holding the oven up and not sagging. Nothing thermal here, which is why you can use standard concrete (cheap) with rebar.

2. Insulation -- keeping heat in the oven and floor. That's the insulating concrete layer, with vermiculite held together with portland cement.

3. The cooking floor -- which is thermal. You want it to heat up, hold heat, and efficiently re-charge the heat in the floor from your live fire. The firebrick on its flat side gives you enough mass for typical backyard cooking and baking, and doesn't have too many seams. The Forno Bravo ovens have 2" floors that come in pie-shaped pieces. If you really want a little more mass under your floor (don't forget that commerical pizza ovens only have 3"-4" floors), you can add more mass under the floor, either with a second layer of bricks, or a poured disk of refractory mortar. That's the Island hearth (the extra mass under the cooking floor is surrounded by vermiculite concrete).

That's why we have formally changed the recommended hearth design. For most installations it is good to put the cooking floor directly on the insulating layer, and if you want more mass you can add it, while taking advantage of a more efficient design.