Concrete hearth--Vermiculite on top or bottom??

[meco211]

I am in the process of building my WFO and have found this fourm the other day and saw on here that some put the Vermiculite base on top of the hearth. I was planning on putting mine at the bottom in a Vermiculite / Portland Cement mix of 2" them pour the other 4 1/2 part of the slab in conrete and then putting my firebrick on the side for the bottom of the oven. Not sure the differnece? Wondering if anyone had any advice on this

[Tscarborough]

If you do it like you plan, you will be using the slab as a heat sink. You will have to get that mass to temperature and that could take a long time. Done as shown on the FB plans, the floor mass is just the firebrick, and that alone is hard to get hot enough.

[meco211]

Was just wondering the difference. Would that be better if I am also planning on baking bread in the oven. My guess is more mass, less cool down time?

[eprante]

Toby,
My oven is constructed using the FB plans. My oven is 550 degrees the next morning after cooking pizza ( that is without a proper insulated door, just covering the opening with fire bricks cut to size). If you are planning on baking large quantities of bread, ie multiple oven loads then the 4" concrete slab will hold heat longer, but it will also take a long time ( and a lot of wood) to get up to temp. Most people on this site use the perlite/portland insulating layer with the firebrick set on top of that so the cooking floor is 2.5" thick. If you want more thermal mass you could set your floor with the bricks 4.5" thick.
Eric

[dmun]

Yes, the support slab is a HUGE heat sink if you don't insulate between the slab and the floor. The only reason to build an oven with massive extra thermal mass is if you are planning to bake commercial quantities of bread. For home use, there is no reason to make the oven any thicker than the thickness of a firebrick on the floor, or the width of one on top.

Many who have built the oven according to plan note that it takes far too long to cool the oven off from pizza temperatures to bread baking temperatures to do both in the same evening. Insulation is the key.

On that topic, two inches of vermiculite concrete isn't nearly enough. We recommend four. If your design limits you to two inches for insulation, use a refractory insulation board.

Good luck with your project.